Patent Publication Number: US-8121828-B2

Title: Detecting conditions for transfer of execution from one computer instruction stream to another and executing transfer on satisfaction of the conditions

Description:
This application claims priority, as a continuation of U.S. application Ser. No. 09/626,325, filed Jul. 26, 2000, which issued as U.S. Pat. No. 7,065,633 on Jun. 20, 2006, which is a continuation-in-part of U.S. application Ser. No. 09/385,394, filed Aug. 30, 1999, which issued as U.S. Pat. No. 8,074,055 on Dec. 6, 2011, which is a continuation-in-part of U.S. application Ser. No. 09/322,443, filed May 28, 1999, which issued as U.S. Pat. No. 6,941,545 on Sep. 6, 2005, which is a continuation of U.S. patent application Ser. No. 09/239,194, filed Jan. 28, 1999, which issued as U.S. Pat. No. 7,275,246 on Sep. 25, 2007. The entire disclosures of the parent applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     The invention relates to computer management. 
     Each instruction for execution by a computer is represented as a binary number stored in the computer&#39;s memory. Each different architecture of computer represents instructions differently. For instance, when a given instruction, a given binary number, is executed by an IBM System/360 computer, an IBM System/38, an IBM AS/400, an IBM PC, and an IBM PowerPC™, the five computers will typically perform five completely different operations, even though all five are manufactured by the same company. This correspondence between the binary representation of a computer&#39;s instructions and the actions taken by the computer in response is called the Instruction Set Architecture (ISA). 
     A program coded in the binary ISA for a particular computer family is often called simply “a binary.” Commercial software is typically distributed in binary form. The incompatibility noted in the previous paragraph means that programs distributed in binary form for one architecture generally do not run on computers of another. Accordingly, computer users are extremely reluctant to change from one architecture to another, and computer manufacturers are narrowly constrained in modifying their computer architectures. 
     A computer most naturally executes programs coded in its native ISA, the ISA of the architectural family for which the computer is a member. Several methods are known for executing binaries originally coded for computers of another, non-native, ISA. In hardware emulation, the computer has hardware specifically directed to executing the non-native instructions. Emulation is typically controlled by a mode bit, an electronic switch: when a non-native binary is to be executed, a special instruction in the emulating computer sets the mode bit and transfers control to the non-native binary. When the non-native program exits, the mode bit is reset to specify that subsequent instructions are to be interpreted by the native ISA. Typically, in an emulator, native and non-native instructions are stored in different address spaces. A second alternative uses a simulator (also sometimes known as an “interpreter”), a program running on the computer that models a computer of the non-native architecture. A simulator sequentially fetches instructions of the non-native binary, determines the meaning of each instruction in turn, and simulates its effect in a software model of the non-native computer. Again, a simulator typically stores native and non-native instructions in distinct address spaces. (The terms “emulation” and “simulation” are not as uniformly applied throughout the industry as might be suggested by the definitions implied here.) In a third alternative, binary translation, a translator program takes the non-native binary (either a whole program or a program fragment) as input, and processes it to produce as output a corresponding binary in the native instruction set (a “native binary”) that runs directly on the computer. 
     Typically, an emulator is found in a newer computer for emulation of an older computer architecture from the same manufacturer, as a transition aid to customers. Simulators are provided for the same purpose, and also by independent software vendors for use by customers who simply want access to software that is only available in binary form for a machine that the customer does not own. By whatever technique, non-native execution is slower than native execution, and a non-native program has access to only a portion of the resources available to a native program. 
     Known methods of profiling the behavior of a computer or of a computer program include the following. In one known profiling method, the address range occupied by a program is divided into a number of ranges, and a timer goes off from time to time. A software profile analyzer figures out the address at which the program was executing, and increments a counter corresponding to the range that embraces the address. After a time, the counters will indicate that some ranges are executed a great deal, and some are barely executed at all. In another known profiling method, counters are generated into the binary text of a program by the compiler. These compiler-generated counters may count the number of times a given region is executed, or may count the number of times a given execution point is passed or a given branch is taken. 
     BRIEF SUMMARY 
     In general, in a first aspect, the invention features a computer with an instruction processor designed to execute instructions of first and second instruction sets, a memory for storage of a program, a table of entries corresponding to the pages, a switch, a transition handler, and a history record. The memory is divided into pages for management by a virtual memory manager. The program is coded in instructions of the first and second instruction sets and uses first and second data storage conventions. The switch is responsive to a first flag value stored in each table entry, and controls the instruction processor to interpret instructions under, alternately, the first or second instruction set as directed by the first flag value of the table entry corresponding to an instruction&#39;s memory page. The transition handler is designed to recognize when program execution has transferred from a page of instructions using the first data storage convention to a page of instructions using the second data storage convention, as indicated by second flag values stored in table entries corresponding to the respective pages, and in response to the recognition, to adjust a data storage configuration of the computer from the first storage convention to the second data storage convention. The history record is designed to provide to the transition handler a record of a classification of a recently-executed instruction. 
     In a second aspect, the invention features a method, and a computer for performance of the method. Instruction data are fetched from first and second regions of a single address space of the memory of a computer. The instructions of the first and second regions are coded for execution by computer of first and second architectures or following first and second data storage conventions, respectively. The memory regions have associated first and second indicator elements, the indicator elements each having a value indicating the architecture or data storage convention under which instructions from the associated region are to be executed. When execution of the instruction data flows from the first region to the second, the computer is adapted for execution in the second architecture or convention. 
     In a third aspect, the invention features a method, and a computer for performance of the method. Instructions are stored in pages of a computer memory managed by a virtual memory manager. The instruction data of the pages are coded for execution by, respectively, computers of two different architectures and/or under two different execution conventions. In association with pages of the memory are stored corresponding indicator elements indicating the architecture or convention in which the instructions of the pages are to be executed. Instructions from the pages are executed in a common processor, the processor designed, responsive to the page indicator elements, to execute instructions in the architecture or under the convention indicated by the indicator element corresponding to the instruction&#39;s page. 
     In a fourth aspect, the invention features a microprocessor chip. An instruction unit of the chip is configured to fetch instructions from a memory managed by the virtual memory manager, and configured to execute instructions coded for first and second different computer architectures or coded to implement first and second different data storage conventions. The microprocessor chip is designed (a) to retrieve indicator elements stored in association with respective pages of the memory, each indicator element indicating the architecture or convention in which the instructions of the page are to be executed, and (b) to recognize when instruction execution has flowed from a page of the first architecture or convention to a page of the second, as indicted by the respective associated indicator elements, and (c) to alter a processing mode of the instruction unit or a storage content of the memory to effect execution of instructions in accord with the indicator element associated with the page of the second architecture or convention. 
     In a fifth aspect, the invention features a method, and a microprocessor capable of performing the method. A section of computer object code is executed twice, without modification of the code section between the two executions. The code section materializes a destination address into a register and is architecturally defined to directly transfer control indirectly through the register to the destination address. The two executions materialize two different destination addresses, and the code at the two destinations is coded in two different instruction sets. 
     In a sixth aspect, the invention features a method and a computer for the performance of the method. Control-flow instructions of the computer&#39;s instruction set are classified into a plurality of classes. During execution of a program on the computer, as part of the execution of instructions of the instruction set, a record is updated to record the class of the classified control-flow instruction most recently executed. 
     In a seventh aspect, the invention features a method and a computer for the performance of the method. A control-transfer instruction is executed that transfers control from a source execution context to a destination instruction for execution in a destination execution context. Before executing the destination instruction, the storage context of the computer is adjusted to reestablish under the destination execution context the logical context of the computer as interpreted under the source execution context. The reconfiguring is determined, at least in part, by a classification of the control-transfer instruction. 
     In general, in an eighth aspect, the invention features a method of operating a computer. Concurrent execution threads are scheduled by a pre-existing thread scheduler of a computer. Each thread has an associated context, the association between a thread and a set of computer resources of the context being maintained by the thread scheduler. Without modifying the thread scheduler, an association is maintained between one of the threads and an extended context of the thread through a context change induced by the thread scheduler, the extended context including resources of the computer beyond those resources whose association with the thread is maintained by the thread scheduler. 
     In a ninth aspect, the invention features a method of operating a computer. An entry exception is established, to be raised on each entry to an operating system of a computer at a specified entry point or on a specified condition. A resumption exception is established, to be raised on each resumption from the operating system following on a specified entry. On detecting a specified entry to the operating system from an interrupted process of the computer, the entry exception is raised and serviced. The resumption exception is raised and serviced, and control is returned to the interrupted process. 
     In a tenth aspect, the invention features a method of operating a computer. Without modifying an operating system of the computer, an entry handler is established for execution at a specified entry point or on a specified entry condition to the operating system. The entry handler is programmed to save a context of an interrupted thread and to modify the thread context before delivering the modified context to the operating system. Without modifying the operating system, an exit handler is established for execution on resumption from the operating system following an entry through the entry handler. The exit handler is programmed to restore the context saved by a corresponding execution of the entry handler. 
     In an eleventh aspect, the invention features a method of operating a computer. During invocation of a service routine of a computer, a linkage return address passed, the return address being deliberately chosen so that an attempt to execute an instruction from the return address on return from the service routine will cause an exception to program execution. On return from the service routine, the chosen exception is raised. After servicing the exception, control is returned to a caller of the service routine. 
     Particular embodiments of the invention may include one or more of the following features. The regions may be pages managed by a virtual memory manager. The indications may be stored in a virtual address translation entry, in a table whose entries are associated with corresponding virtual pages, in a table whose entries are associated with corresponding physical page frames, in entries of a translation look-aside buffer, or in lines of an instruction cache. The code at the first destination may receive floating-point arguments and return floating-point return values using a register-based calling convention, while the code at the second destination receives floating-point arguments using a memory-based stack calling convention, and returns floating-point values using a register indicated by a top-of-stack pointer. 
     The two architectures may be two instruction set architectures, and the instruction execution hardware of the computer may be controlled to interpret the instructions according to the two instruction set architectures according to the indications. A mode of execution of the instructions may be changed without further intervention when execution flows from the first region to the second, or the mode may be changed by an exception handler when the computer takes an exception when execution flows from the first region to the second. One of the regions may store an off-the-shelf operating system binary coded in an instruction set non-native to the computer. 
     The two conventions may be first and second calling conventions, and the computer may recognize when program execution has transferred from a region using the first calling convention to a region using the second calling convention, and in response to the recognition, the data storage configuration of the computer will be adjusted from the first calling convention to the second. One of the two calling conventions may be a register-based calling convention, and the other calling convention may be a memory stack-based calling convention. There may be a defined mapping between resources of the first architecture and resources of the second, the mapping assigning corresponding resources of the two architectures to a common physical resource of a computer when the resources serve analogous functions in the calling conventions of the two architectures. The configuration adjustment may include altering a bit representation of a datum from a first representation to a second representation, the alteration of representation being chosen to preserve the meaning of the datum across the change in execution convention. A rule for copying data from the first location to the second may be determined, at least in part, by a classification of the instruction that transferred execution to the second region, and/or by examining a descriptor associated with the location of execution before the recognized execution transfer. 
     A first class of instructions may include instructions to transfer control between subprograms associated with arguments passed according to a calling convention, and a second class of instructions may include branch instructions whose arguments, if any, are not passed according to the calling convention. One of the execution contexts may be a register-based calling convention, and the other execution context may be a memory stack-based calling convention. The rearrangement may reflect analogous execution contexts under the two data storage conventions, the rearranging process being determined, at least in part, by the instruction classification record. In some of the control-flow instructions, the classification may be encoded in an immediate field of instructions, the immediate field having no effect on the execution of the instruction in which it is encoded, except to update the class record. In some of the control-flow instructions, the classification may be statically determined by the opcode of the instructions. In some of the control-flow instructions, the classification may be dynamically determined with reference to a state of processor registers and/or general registers of the computer. In some of the control-flow instructions, the classification may be dynamically determined based on a full/empty status of a register indicated by a top-of-stack pointer, the register holding a function result value. The rearranging may be performed by an exception handler, the handler being selected by an exception vector based at least in part on the source data storage convention, the destination data storage convention, and the instruction classification record. Instructions of the instruction set may be classified as members of a don&#39;t-care class, so that when an instruction of the don&#39;t-care class is executed, the record is left undisturbed to indicate the class of the classified instruction most recently executed. The destination instruction may be an entry point to an off-the-shelf binary for an operating system coded in an instruction set non-native to the computer. 
     The operating system may be an operating system for a computer architecture other than the architecture native to the computer. The computer may additionally execute an operating system native to the computer, and each exception may be classified for handling by one of the two operating systems. A linkage return address for resumption of the thread may be modified to include information used to maintain the association. At least some of the modified registers may be overwritten by a timestamp. The entry exception handler may alter at least half of the data registers of the portion of a process context maintained in association with the process by the operating system before delivering the process to the operating system, a validation stamp being redundantly stored in at least one of the registers, and wherein at least some of the modified registers are overwritten by a value indicating the storage location in which at least the portion of the thread context is saved before the modifying. The operating system and the interrupted thread may execute in different instruction set architectures of the computer. During servicing the entry exception, a portion of the context of the computer may be saved, and the context of an interrupted thread may be altered before delivering the interrupted thread and its corresponding context to the operating system. When the thread scheduler and the thread execute in different execution modes of the computer, the steps to maintain the association between the thread and the context may be automatically invoked on a transition from the thread execution mode to the thread scheduler execution mode. The thread context may be saved in a storage location allocated from a pool of storage locations managed by a queuing discipline in which empty storage locations in which a context is to be saved are allocated from the head of the queue, recently-emptied storage locations for reuse are enqueued at the head of the queue, and full storage locations to be saved are queued at the tail of the queue. A calling convention for the thread execution mode may require the setting of a register to a value that specifies actions to be taken to convert operands from one form to another to conform to the thread scheduler execution mode. Delivery of an interrupt may be deferred by a time sufficient to allow the thread to reach a checkpoint, or execution of the thread may be rolled back to a checkpoint, the checkpoints being points in the execution of the thread where the amount of extended context, being the resources of the thread beyond those whose resource association with the thread is maintained by the thread scheduler, is reduced. The linkage return address may be selected to point to a memory page having a memory attribute that raises the chosen exception on at attempt to execute an instruction from the page. The service routine may be an interrupt service routine of an operating system for a computer architecture other than the architecture native to the computer, the service routine may be invoked by an asynchronous interrupt, and the caller may be coded in the instruction set native to the architecture. 
     In general, in a twelfth aspect, the invention features a method and a computer. A computer program executes in a logical address space of a computer, with an address translation circuit translating address references generated by the program from the program&#39;s logical address space to the computer&#39;s physical address space. Profile information is recorded that records physical memory addresses referenced during an execution interval of the program. 
     In general, in a thirteenth aspect, a program is executed on a computer, the program referring to memory by virtual address. Concurrently with the execution of the program, profile information is recorded describing memory references made by the program, the profile information recording physical addresses of the profiled memory references. 
     In general, in a fourteenth aspect, the invention features a computer with an instruction pipeline, a memory access unit, an address translation circuit, and profile circuitry. The instruction pipeline and memory access unit are configured to execute instructions in a logical address space of a memory of the computer. The address translation circuit translates address references generated by the program from the program&#39;s logical address space to the computer&#39;s physical address space. The profile circuitry is cooperatively interconnected with the instruction pipeline and is configured to detect, without compiler assistance for execution profiling, occurrence of profileable events occurring in the instruction pipeline, and cooperatively interconnected with the memory access unit to record profile information describing physical memory addresses referenced during an execution interval of the program. 
     Embodiments of the invention may include one or more of the following features. The recorded physical memory references may include addresses of binary instructions referenced by an instruction pointer, and at least one of the recorded instruction references may record the event of a sequential execution flow across a page boundary in the address space. The recorded execution flow across a page boundary may occur within a single instruction. The recorded execution flow across a page boundary may occur between two instructions that are sequentially adjacent in the logical address space. At least one of the recorded instruction references may be a divergence of control flow consequent to an external interrupt. At least one of the recorded instruction references may indicate the address of the last byte of an instruction executed by the computer during the profiled execution interval. The recorded profile information may record a processor mode that determines the meaning of binary instructions of the computer. The recorded profile information may record a data-dependent change to a full/empty mask for registers of the computer. The instruction pipeline may be configured to execute instructions of two instruction sets, a native instruction set providing access to substantially all of the resources of the computer, and a non-native instruction set providing access to a subset of the resources of the computer. The instruction pipeline and profile circuitry may be further configured to effect recording of profile information describing an interval of the execution of an operating system coded in the non-native instruction set. 
     In general, in a fifteenth aspect, the invention features a method. A program is executed on a computer. Profile information is recorded concerning the execution of the program, the profile information recording of the address of the last byte of at least one instruction executed by the computer during a profiled interval of the execution. 
     In general, in a sixteenth aspect, the invention features a method. A program is executed on a computer, without the program having been compiled for profiled execution, the program being coded in an instruction set in which an interpretation of an instruction depends on a processor mode not expressed in the binary representation of the instruction. Profile information is recorded describing an interval of the program&#39;s execution and processor mode during the profiled interval of the program, the profile information being efficiently tailored to annotate the profiled binary code with sufficient processor mode information to resolve mode-dependency in the binary coding. 
     In general, in an seventeenth aspect, the invention features a computer with an instruction pipeline and profile circuitry. The instruction pipeline is configured to execute instructions of the computer. The profile circuitry is configured to detect and record, without compiler assistance for execution profiling, profile information describing a sequence of events occurring in the instruction pipeline, the sequence including every event occurring during a profiled execution interval that matches time-independent selection criteria of events to be profiled, the recording continuing until a predetermined stop condition is reached, and is configured to detect the occurrence of a predetermined condition to commence the profiled execution interval after a non-profiled interval of execution. 
     In general, in a eighteenth aspect, the invention features a method and a computer with circuitry configured for performance of the method. During a profiled interval of an execution of a program on a computer, profile information is recorded describing the execution, without the program having been compiled for profiled execution, the program being coded in an instruction set in which an interpretation of an instruction depends on a processor mode not expressed in the binary representation of the instruction, the recorded profile information describing at least all events occurring during the profiled execution interval of the two classes: (1) a divergence of execution from sequential execution; and (2) a processor mode change that is not inferable from the opcode of the instruction that induces the processor mode change taken together with a processor mode before the mode change instruction. The profile information further identifies each distinct physical page of instruction text executed during the execution interval. 
     Embodiments of the invention may include one or more of the following features. The profiled execution interval is commenced at the expiration of a timer, the recorded profile describing a sequence of events including every event that matches time-independent selection criteria of events to be profiled, the recording continuing until a predetermined stop condition is reached. A profile entry is recorded for later analysis noting the source and destination of a control flow event in which control flow of the program execution diverges from sequential execution. The recorded profile information is efficiently tailored to identify all bytes of object code executed during the profiled execution interval, without reference to the binary code of the program. A profile entry describing a single profileable event explicitly describes a page offset of the location of the event, and inherits a page number of the location of the event from the immediately preceding profile entry. Profile information records a sequence of events of the program, the sequence including every event during the profiled execution interval that matches time-independent criteria of profileable events to be profiled. The recorded profile information indicates ranges of instruction binary text executed by the computer during a profiled interval of the execution, the ranges of executed text being recorded as low and high boundaries of the respective ranges. The recorded high boundaries record the last byte, or the first byte of the last instruction, of the range. The captured profile information comprises subunits of two kinds, a first subunit kind describing an instruction interpretation mode at an instruction boundary, and a second subunit kind describing a transition between processor modes. During a non-profiled interval of the program execution, no profile information is recorded in response to the occurrence of profileable events matching predefined selection criteria for profileable events. The profile circuitry is designed to record a timestamp describing a time of the recorded events. The profile circuitry is designed to record an event code describing the class of each profileable event recorded. A number of bits used to record the event code is less than log 2  of the number of distinguished event classes. 
     In general, in a nineteenth aspect, the invention features a method. While executing a program on a computer, the occurrence of profileable events occurring in the instruction pipeline is detected, and the instruction pipeline is directed to record profile information describing the profileable events essentially concurrently with the occurrence of the profileable events, the detecting and recording occurring under control of hardware of the computer without software intervention. 
     In general, in a twentieth aspect, the invention features a computer that includes an instruction pipeline and profile circuitry. The instruction pipeline includes an arithmetic unit and is configured to execute instructions received from a memory of the computer and the profile circuitry. The profile circuitry is common hardware control with the instruction pipeline. The profile circuitry and instruction pipeline are cooperatively interconnected to detect the occurrence of profileable events occurring in the instruction pipeline, the profile circuitry operable without software intervention to effect recording of profile information describing the profileable events essentially concurrently with the occurrence of the profileable events. 
     In general, in a twenty-first aspect, the invention features first and second CPU&#39;s. The first CPU is configured to execute a program and generate profile data describing the execution of the program. The second CPU is configured to analyze the generated profile data, while the execution and profile data generation continue on the first CPU, and to control the execution of the program on the first CPU based at least in part on the analysis of the collected profile data. 
     In general, in a twenty-second aspect, the invention features a method. While executing a program on a computer, the computer using registers of a general register file for storage of instruction results, the occurrence of profileable events occurring in the instruction pipeline is detected. Profile information is recorded describing the profileable events into the general register file as the profileable events occur, without first capturing the information into a main memory of the computer. 
     In general, in a twenty-third aspect, the invention features a computer that includes a general register file of registers, an instruction pipeline and profile circuitry. The instruction pipeline includes an arithmetic unit and is configured to execute instructions fetched from a memory cache of the computer, and is in data communication with the registers for the general register file for storage of instruction results. The profile circuitry is operatively interconnected with the instruction pipeline and is configured to detect the occurrence of profileable events occurring in the instruction pipeline, and to capture information describing the profileable events into the general register file as the profileable events occur, without first capturing the information into a main memory of the computer. 
     In general, in a twenty-fourth aspect, the invention features a computer. The instruction pipeline is configured to execute instructions of the computer. The profile circuitry is implemented in the computer hardware, and is configured to detect, without compiler assistance for execution profiling, the occurrence of profileable events occurring in the instruction pipeline, and to direct recording of profile information describing the profileable events occurring during an execution interval of the program. Profile control bits implemented in the computer hardware have values that control a resolution of the operation of the profile circuitry. A binary translator is configured to translate programs coded in a first instruction set architecture into instructions of a second instruction set architecture. A profile analyzer is configured to analyze the recorded profile information, and to set the profile control bits to values to improve the operation of the binary translator. 
     Embodiments of the invention may include one or more of the following features. At least a portion of the recording is performed by instructions speculatively introduced into the instruction pipeline. The profile circuitry is interconnected with the instruction pipeline to direct the recording by injection of an instruction into the pipeline, the instruction controlling the pipeline to cause the profileable event to be materialized in an architecturally-visible storage register of the computer. An instruction of the computer, having a primary effect on the execution the computer not related to profiling, has an immediate field for an event code encoding the nature of a profiled event and to be recorded in the profile information, the immediate field having no effect on computer execution other than to determine the event code of the profiled event. Instances of the instruction have an event code that leaves intact an event code previously determined by other event monitoring circuitry of the computer. The profiled information includes descriptions of events whose event codes were classified by instruction execution hardware, without any explicit immediate value being recorded in software. The instruction pipeline and profile circuitry are operatively interconnected to effect injection of multiple instructions into the instruction pipeline by the profile circuitry on the occurrence of a single profileable event. The instruction pipeline and profile circuitry are operatively interconnected to effect speculative injection of the instruction into the instruction pipeline by the profile circuitry. A register pointer of the computer indicates a general register into which to record the profile information, and an incrementer is configured to increment the value of the register pointer to indicate a next general register into which to record next profile information, the incrementing occurring without software intervention. A limit detector is operatively interconnected with the register pointer to detect when a range of registers available for collecting profile information is exhausted, and a store unit is operatively interconnected with the limit detector of effect storing the profile information from the general registers to the main memory of the computer when exhaustion is detected. The profile circuitry comprises a plurality of storage registers arranged in a plurality of pipeline stages, information recorded in a given pipeline stage being subject to modification as a corresponding machine instruction progresses through the instruction pipeline. When an instruction fetch of an instruction causes a miss in a translation look aside buffer (TLB), the fetch of the instruction triggering a profileable event, the TLB miss is serviced, and the corrected state of the TLB is reflected in the profile information recorded for the profileable instruction. The profile control bits include a timer interval value specifying a frequency at which the profile circuitry is to monitor the instruction pipeline for profileable events. The profile circuitry comprises a plurality of storage registers arranged in a plurality of pipeline stages, information recorded in a given pipeline stage is subject to modification as a corresponding machine instruction progresses through the instruction pipeline. 
     In general, in a twenty-fifth aspect, the invention features a computer with instruction pipeline circuitry designed to effect interpretation of computer instructions under two instruction set architectures alternately. Pipeline control circuitry is cooperatively designed with the instruction pipeline circuitry to initiate, without software intervention, when about to execute a program region coded in a lower-performance one of the instruction set architectures, a query whether a program region coded in a higher-performance one of the instruction set architectures exists, the higher-performance region being logically equivalent to the lower-performance program region. Circuitry and/or software is designed to transfer execution control to the higher-performance region, without a transfer-of-control instruction to the higher-performance region being coded in the lower-performance instruction set. 
     In general, in a twenty-sixth aspect, the invention features a method and a computer for performance of the method. At least a selected portion of a computer program is translated from a first binary representation to a second binary representation. During execution of the first binary representation of the program on a computer, it is recognized that execution has entered the selected portion, the recognizing being initiated by basic instruction execution of the computer, with neither a query nor a transfer of control to the second binary representation being coded into the first binary representation. In response to the recognition, control is transferred to the translation in the second representation. 
     In general, in a twenty-seventh aspect, the invention features a method and a computer for performance of the method. As part of executing an instruction on a computer, it is recognized that an alternate coding of the instruction exists, the recognizing being initiated without executing a transfer of control to the alternate coding or query instruction to trigger the recognizing. When an alternate coding exists, the execution of the instruction is aborted, and control is transferred to the alternate coding. 
     In general, in a twenty-eighth aspect, the invention features a method and a computer for performance of the method. During execution of a program on instruction pipeline circuitry of a computer, a determination is initiated of whether to transfer control from a first instruction stream in execution by the instruction pipeline circuitry to a second instruction stream, without a query or transfer of control to the second instruction stream being coded into the first instruction stream. Execution of the first instruction stream is established after execution of the second instruction stream, execution of the first instruction stream being reestablished at a point downstream from the point at which control was seized, in a context logically equivalent to that which would have prevailed had the code of the first instruction stream been allowed to proceed. 
     In general, in a twenty-ninth aspect, the invention features a method and a computer for performance of the method. Execution of a computer program is initiated, using a first binary image of the program. During the execution of the first image, control is transferred to a second image coding the same program in a different instruction set. 
     In general, in a thirtieth aspect, the invention features a method and a computer for performance of the method. As part of executing an instruction on a computer, a heuristic, approximately-correct recognition that an alternate coding of the instruction exists is evaluated, the process for recognizing being statistically triggered. If the alternate coding exists, execution of the instruction is aborted, and control is transferred to the alternate coding. 
     In general, in a thirty-first aspect, the invention features a method and a computer for performance of the method. A microprocessor chip has instruction pipeline circuitry, lookup circuitry, a mask, and pipeline control circuitry. The lookup circuitry is designed to fetch an entry from a lookup structure as part of the basic instruction processing cycle of the microprocessor, each entry of the lookup structure being associated with a corresponding address range of a memory of the computer. The mask has a value set at least in part by a timer. The pipeline control circuitry is designed to control processing of instructions by the instruction pipeline circuitry as part of the basic instruction processing cycle of the microprocessor, depending, at least in part, on the value of the entry corresponding to the address range in which lies an instruction processed by the instruction pipeline circuitry, and the current value of the mask. 
     In general, in a thirty-second aspect, the invention features a method and a microprocessor chip for performance of the method. The microprocessor chip has instruction pipeline circuitry; instruction classification circuitry responsive to execution of instructions executed by the instruction pipeline circuitry to classify the executed instructions into a small number of classes and record a classification code value; lookup circuitry designed to fetch an entry from a lookup structure as part of the basic instruction processing cycle of the microprocessor, each entry of the lookup structure being associated with a corresponding address range of a memory of the computer; and pipeline control circuitry designed to control processing of instructions by the instruction pipeline circuitry as part of the basic instruction processing cycle of the microprocessor, depending, at least in part, on the value of the entry corresponding to the address range in which the instruction address lies, and the recorded classification code. 
     In general, in a thirty-third aspect, the invention features a method and a microprocessor chip for performance of the method. The microprocessor chip includes instruction pipeline circuitry; an on-chip table, each entry of the on-chip table corresponding to a respective class of event occurring the in the computer, and designed to hold an approximate evaluation of a portion of the computer machine state for control of the circuitry; and pipeline control circuitry cooperatively designed with the instruction pipeline circuitry to control processing of instructions by the instruction pipeline circuitry as part of the basic instruction processing cycle of the microprocessor, based on consultation of the on-chip table. 
     In general, in a thirty-fourth aspect, the invention features a method and a microprocessor chip for performance of the method. The microprocessor chip includes instruction pipeline circuitry; an on-chip table, each entry of the on-chip table corresponding to a class of event occurring the in the computer and designed to control consultation of an off-chip table in a memory of the computer when an event of the class occurs; pipeline control circuitry cooperatively designed with the instruction pipeline circuitry to consult the on-chip table as part of the basic instruction processing cycle of the microprocessor, as the classified events occur; and control circuitry and/or software designed to cooperate with the instruction pipeline circuitry and pipeline control circuitry to affect a manipulation of data or transfer of control defined for the event in the instruction pipeline circuitry based on consultation of the off-chip table after a favorable value is obtained from the on-chip table. 
     Embodiments of the invention may include one or more of the following features. The transfer of execution control to the higher-performance region may be effected by an architecturally-visible alteration of a program counter. The region about to be executed may be entered by a transfer of control instruction. The first image may be coded in an instruction set non-native to the computer, for hardware emulation in the computer. Instructions of the second binary representation may be coded in a different instruction set architecture than instructions of the first binary representation. The second image may have been generated from the first image by a binary translator. The binary translator may have optimized the second image for increased execution speed, while accepting some risk of execution differing from the execution of the non-native program on its native instruction set architecture. A decision on whether to transfer control from the first image to the second may be based on control variables of the computer. The classes of events may be memory references to corresponding respective address ranges of a memory of the computer. The address ranges may correspond to entries in an interrupt vector table. The recognition may be initiated by consulting a content-addressable memory addressed by a program counter address of the instruction to be executed. The content-addressable memory may be a translation lookaside buffer. The off-chip table may be organized as a side table to an address translation page table. The on-chip table may contain a condensed approximation of the off-chip table, loaded from the off-chip table. The lookup structure may be a bit vector. Bits of the entry corresponding to the address range in which the instruction address lies may be AND&#39;ed with corresponding bits of a mask associated with the instruction pipeline circuitry. Error in the approximation of the on-chip table may be induced by a slight time lag relative to the portion of the computer&#39;s machine state whose evaluation is stored therein. The pipeline control circuitry may be designed to control processing of instructions by the instruction pipeline circuitry by evaluating the value of the entry corresponding to the address range in which the instruction address lies and the recorded classification code, and triggering a software evaluation of a content of the memory addressed by the microprocessor chip. The control of instruction processing may include branch destination processing. 
     In general, in a thirty-fifth aspect, the invention features a method and a microprocessor chip for performance of the method. Instructions are executed on a computer, instruction pipeline circuitry of the computer having first and second modes for processing at least some of the instructions. Execution of two-mode instructions is attempted in the first mode for successive two-mode instructions while the first execution mode is successful. When an unsuccessful execution of a two-mode instruction under the first mode is detected, following two-mode instructions are executed in the second mode. 
     In general, in a thirty-sixth aspect, the invention features a method and a microprocessor chip for performance of the method. Computer instructions are executed in instruction pipeline circuitry having first and second modes for processing at least some instructions. On expiration of a timer, the instruction pipeline circuitry switches from the first mode to the second, the mode switch persisting for instructions subsequently executed on behalf of a program that was in execution immediately before the timer expiry. 
     In general, in a thirty-seventh aspect, the invention features a method and a microprocessor chip for performance of the method. Events of a computer are assigned into event classes. As part of the basic execution cycle of a computer instruction pipeline, without software intervention, a record of responses to events of the class is maintained. As each classified event comes up for execution in the instruction pipeline circuitry, the record is queried to determine the response to the previous attempt of an event of the same class. The response is attempted if and only if the record indicates that the previous attempt succeeded. 
     Embodiments of the invention may include one or more of the following features. The first and second modes may be alternative cache policies, or alternative modes for performing floating-point arithmetic. Unsuccessful execution may includes correct completion of an instruction at a high cost. The cost metric may be execution time. The cost of an instruction in the first mode may be only ascertainable after completion of the instruction. The instruction pipeline circuitry may be switched back from the second mode to the first, the switch persisting until the next timer expiry. All of the records may be periodically set to indicate that previous attempts of the corresponding events succeeded. 
     In general, in a thirty-eighth aspect, the invention features a method and a microprocessor chip for performance of the method. As part of the basic instruction cycle of executing an instruction of a non-supervisor mode program executing on a computer, a table is consulted, the table being addressed by the address of instructions executed, for attributes of the instructions. An architecturally-visible data manipulation behavior or control transfer behavior of the instruction is controlled based on the contents of a table entry associated with the instruction. 
     Embodiments of the invention may include one or more of the following features. The different instruction may be coded in an instruction set architecture (ISA) different than the ISA of the executed instruction. The control of architecturally-visible data manipulation behavior may include changing an instruction set architecture under which instructions are interpreted by the computer. Each entry of the table may correspond to a page managed by a virtual memory manager, circuitry for locating a table entry being integrated with virtual memory address translation circuitry of the computer. An interrupt may be triggered on execution of an instruction of a process, synchronously based at least in part on a memory state of the computer and the address of the instruction, the architectural definition of the instruction not calling for an interrupt. Interrupt handler software may be provided to service the interrupt and to return control to an instruction flow of the process other than the instruction flow triggering the interrupt, the returned-to instruction flow for carrying on non-error handling normal processing of the process. 
     In general, in a thirty-ninth aspect, the invention features a method and a microprocessor chip for performance of the method. A microprocessor chip has instruction pipeline circuitry, address translation circuitry; and a lookup structure. The lookup structure has an entry associated with each corresponding address range translated by the address translation circuitry, the entry describing a likelihood of the existence of an alternate coding of instructions located in the respective corresponding address range. 
     Embodiments of the invention may include one or more of the following features. The entry may be an entry of a translation look-aside buffer. The alternate coding may be coded in an instruction set architecture (ISA) different than the ISA of the instruction located in the address range. 
     In general, in a fortieth aspect, the invention features a method and a microprocessor chip for performance of the method. A microprocessor chip has instruction pipeline circuitry and interrupt circuitry. The interrupt circuitry is cooperatively designed with the instruction pipeline circuitry to trigger an interrupt on execution of an instruction of a process, synchronously based at least in part on a memory state of the computer and the address of the instruction, the architectural definition of the instruction not calling for an interrupt. 
     Embodiments of the invention may include one or more of the following features. Interrupt handler software may be designed to service the interrupt and to return control to an instruction flow of the process other than the instruction flow triggering the interrupt, the returned-to instruction flow for carrying on non-error handling normal processing of the process. The interrupt handler software may be programmed to change an instruction set architecture under which instructions are interpreted by the computer. The instruction text beginning at the returned-to instruction may be logically equivalent to the instruction text beginning at the interrupted instruction. 
     In general, in a forty-first aspect, the invention features a method and a microprocessor chip for performance of the method. As part of executing a stream of instructions, a series of memory loads is issued from a computer CPU to a bus, some directed to well-behaved memory and some directed to non-well-behaved devices in I/O space. A storage of the computer records addresses of instructions of the stream that issued memory loads to the non-well-behaved memory, the storage form of the recording allowing determination of whether the memory load was to well-behaved memory or not-well-behaved memory without resolution of any memory address stored in the recording. 
     In general, in a forty-second aspect, the invention features a method and a computer for performance of the method. A successful memory reference is issued from a computer CPU to a bus. A storage of the computer records whether a device accessed over the bus by the memory reference is well-behaved memory or not-well-behaved memory. Alternatively, the memory may store a record of a memory read instruction that references a device other than well-behaved memory. 
     Embodiments of the invention may include one or more of the following features. The recording may be a portion of a profile primarily recording program control flow. The recording may be read by a binary translation program, wherein the binary translation program translates the memory load using more conservative assumptions when the recording indicates that the memory load is directed to non-well-behaved memory. References to I/O space may be recorded as being references to non-well-behaved memory. The recording may be slightly in error, the error being induced by a conservative estimate in determining when the memory reference accesses well-behaved memory. The form of the recording may allow determination of whether the memory reference was to well-behaved memory or not-well-behaved memory without resolution of any memory address stored in the recording. The form of the recording may indicates an address of an instruction that issued the memory reference. The memory reference may be a load. The profile monitoring circuitry may be interwoven with the computer CPU. A TLB (translation lookaside buffer) may be designed to hold a determination of whether memory mapped by entries of the TLB is well-behaved or non-well-behaved memory. The profile monitoring circuitry may generate the record into a general purpose register of the computer. The profile monitoring circuitry may be designed to induce a pipeline flush of the computer CPU. 
     In general, in a forty-third aspect, the invention features a method and computer circuitry for performance of the method. DMA (direct memory access) memory write transactions of a computer are monitored, and an indication of a memory location written by a DMA memory write transaction is recorded, by circuitry operating without being informed of the memory write transaction by the CPU beforehand. The indication is read by the CPU. 
     In general, in a forty-fourth aspect, the invention features a method and computer for performance of the method. A first process of a computer generates a second representation in a computer memory of information stored in the memory in a first representation. Overwriting of the first representation by a DMA memory write transaction initiated by a second process is detected by the first process, without the second process informing the first process of the DMA memory write transaction, the detecting guaranteed to occur no later than the next access of the second representation following the DMA memory write transaction. 
     In general, in a forty-fifth aspect, the invention features a method and computer for performance of the method. A computer&#39;s main memory is divided into pages for management by a virtual memory manager. The manager manages the pages using a table stored in the memory. Circuitry records indications of modification to pages of the main memory into a plurality of registers outside the address space of the main memory. The virtual memory management tables do not provide backing store for the modification indications stored in the registers. 
     In general, in a forty-sixth aspect, the invention features a method and computer circuitry for performance of the method. Modifications to the contents of a main memory of a computer are monitored, and on detection of a modification, an approximation of the address of the modification is written into an address tag of one of a plurality of registers, and a fine indication of the address of the modification is written into a memory cell of a plurality of cells of the register. The fine indication of the address of the modification is provided to a CPU of the computer through a read request from the CPU. 
     Embodiments of the invention may include one or more of the following features. The recorded indication may record only the memory location, and not the datum written to the location. Based at least in part by the value read by the CPU, a cached datum may be erased. Two DMA memory writes near each other in address and time may generate only a single record of a write. The recorded indication of a location in the main memory may indicate a physical address in the memory. A value of each bit of a bit vector may indicate whether a corresponding region in the main memory has been recently modified. Matching circuitry may be provided to match an address of a memory modification to an address of a previously-stored indication of a previous nearby memory modification. The recorded indication of a location in the main memory may be initially recorded in an architecturally-visible location outside the main memory and outside a general register file of the computer. The recorded indication of a location in the main memory may be recorded, at least in part, based on a subdivision of the main memory into regions each consisting of a naturally-aligned block of pages of the memory. The DMA monitoring circuitry being designed to monitor transactions on I/O gateway circuitry between the CPU and the DMA devices. The DMA monitoring circuitry may dismiss a content of the DMA monitoring circuitry as a side-effect of being read. The address of the modification stored in the address tag may be a physical memory address. The vector of memory cells may include a bit vector, a value of each bit of the bit vector designed to indicate whether a corresponding region in the main memory has been recently modified. The address tag may include a content-addressable memory. A one of the plurality of registers may be associated with an address range by writing an address into the address tag of the one register. Later, the one register may be associated with a different address range by writing a different address into the address tag of the one register. A value of each bit of a bit vector may indicate whether a corresponding region in the main memory has been recently modified. 
     In general, in a forty-seventh aspect, the invention features a method and computer for performance of the method. As a program is executed in a computer, writes to a protected region of a main memory of the computer are detected, the reporting being performed by monitoring circuitry of the computer. On receiving the report of the detection, a data structure of content corresponding to the content of the protected region to which the write was detected is deleted from the memory. 
     In general, in a forty-eighth aspect, the invention features a method and computer for performance of the method. Memory read references are generated in a CPU of a computer, the memory references referring to logical addresses. Circuitry and/or software evaluates whether main memory pages of the references are in a protected state. Pages that are unprotected are put into a protected state. 
     In general, in a forty-ninth aspect, the invention features a method and computer for performance of the method. Memory references are generated by a CPU of a computer, the memory references referring to logical addresses. The translation of logical addresses into a physical addresses evaluates whether the page of the reference is protected against the access. Pages that are protected have their protection modified, without modifying the contents of the page. 
     Embodiments of the invention may include one or more of the following features. The monitoring and detection circuitry may be responsive to memory writes generated by store operations initiated by instructions executed by pipeline circuitry of the computer. The evaluation circuitry may be incorporated into address translation circuitry designed to translate logical addresses, generated as part of memory read accesses by a CPU of the computer, into physical addresses. The protection of memory regions may be recorded in a table of entries, each entry corresponding to a page of the main memory. The table entries may be organized in correspondence to physical pages of the main memory. The table entries may constitute a table in main memory distinct from a page table used by a virtual memory manager of the computer. The table of entries may be a translation lookaside buffer. A profiling or monitoring function of the computer may be enabled or disabled for regions of the memory of the computer, based on whether the respective regions are protected or unprotected. An arithmetic result or branch destination of an instruction may be controlled based on whether a region containing the instruction is protection or unprotected. The data structure may be formed by translating a computer program stored in the protected region in a first instruction set architecture into a second instruction set architecture. On receiving the report of the detection, an interrupt may be raised to invoke software, the invoked software affecting the contents of the memory without reference to the contents of the protected region. The memory read reference may be an instruction fetch. 
     In general, in a fiftieth aspect, the invention features a method and computer for performance of the method. Memory references generated as part of executing a stream of instructions on a computer are evaluated to determined whether an individual memory reference of an instruction references a device having a valid memory address but that cannot be guaranteed to be well-behaved. 
     In general, in a fifty-first aspect, the invention features a method and computer for performance of the method. While translating at least a segment of a binary representation of a program from a first instruction set architecture to a second representation in a second instruction set architecture, individual memory loads that are believed to be directed to well-behaved memory are distinguished from memory loads that are believed to be directed to non-well-behaved memory device(s). While executing the second representation, a load is identified that was believed at translation time to be directed to well-behaved memory but that at execution is found to be directed to non-well-behaved memory. The identified memory load is aborted. Based at least in part on the identifying, at least a portion of the translated segment of the program is re-executed in the first instruction set. 
     In general, in a fifty-second aspect, the invention features a method and computer for performance of the method. A binary translator translates at least segment of a program from a first representation in a first instruction set architecture to a second representation in a second instruction set architecture, a sequence of side-effects in the second representation differing from a sequence of side-effects in the translated segment of the first representation. Instruction execution circuitry and/or software identifies cases during execution of the second representation in which the difference in sequence of side-effects may have a material effect on the execution of the program. A program state, equivalent to a state that would have occurred in the execution of the first representation, is established. Execution resumes from the established state in an execution mode that reflects the side-effect sequence of the first representation. 
     Embodiments of the invention may include one or more of the following features. If the reference cannot be guaranteed to be well-behaved, the instruction may be re-executed in an alternative execution mode, or program state may be restored to a prior state. The second representation may be annotated with an indication of the distinction between individual memory loads that are believed to be directed to well-behaved memory from memory loads that are believed to be directed to non-well-behaved memory. The device having a valid memory address may have an address in an I/O space of the computer. Code in a preamble of a program unit embracing the memory-reference instruction may establish a state of the instruction execution circuitry, the instruction execution circuitry designed to raise an exception based on an evaluation of both the state and the evaluation of the reference to the device. An annotation embedded in the instruction may be evaluated to determine whether the reference to the non-well-behaved device is to raise an exception. An evaluation of whether the instruction of the individual side-effect is to raise an exception may occur in circuitry embedded in an address translation circuitry of the computer. An exception may be raised, based on an evaluation of both a segment descriptor and the evaluation of the side-effect. An annotation encoded in a segment descriptor may be evaluated to determine whether the reference to the non-well-behaved device is to raise an exception. The segment descriptor may be formed by copying another segment descriptor, and altering the annotation. The formed segment descriptor may copy a variable indicating an assumed sensitivity of the translation to alteration of the sequence of side-effects. The difference of ordering of side-effects may include a reordering of two side-effects relative to each other, an elimination of a side-effect by the translating, or combining two side-effects in the binary translator. The restoring step may be initiated when an exception occurs in the object program. Execution may resume from the restored state, the resumed execution executing a precise side-effect emulation of the reference implementation. A descriptor generated during the translation may be used to restore state to the pre-exception reference state. 
     In general, in a fifty-third aspect, the invention features a method and computer for performance of the method. A first interpreter executes a program coded in an instruction set, the first interpreter being less than fully correct. A second, fully-correct interpreter, primarily in hardware, executes instructions of the instruction set. A monitor detects any deviation from fully-correct interpretation by the first interpreter, before any side-effect of the incorrect interpretation is irreversibly committed. When the monitor detects the deviation, execution is rolled back by at least a full instruction to a safe point in the program, and execution is re-initiated in the second interpreter. 
     In general, in a fifty-fourth aspect, claimed as invention 21B in claim 21.29 and 21.40, the invention features a method and computer for performance of the method. A binary translator translates a source program into an object program, the translated object program having a different execution behavior than the source program. An interrupt handler responds to an interrupt occurring during execution of the object program by establishing a state of the program corresponding to a state that would have occurred during an execution of the source program, and from which execution can continue, and initiates execution of the source program from the established state. 
     Embodiments of the invention may include one or more of the following features. The first interpreter may include a software emulator, and/or a software binary translator. The second interpreter may interpret instructions in an instruction set not native to the computer. The software binary translator may operate concurrently with execution of the program to translate a segment less than the whole of the program. Continuing execution may include rolling back execution of the first interpreter by at least two full instructions. Continuing execution may include rolling back execution of the first interpreter from a state in which a number of distinct suboperations of several instructions have been intermixed by the first interpreter. Continuing execution may include rolling back execution to a checkpoint, or allowing execution to progress forward to a checkpoint in the first interpreter. The detected deviation from fully-correct interpretation may includes detection of the invalidity of a program transformation introduced by the binary translator, or detection of a synchronous execution exception. 
     Embodiments of the invention may offer one or more of the following advantages. 
     A program produced for a computer of an old architecture can be executed on a computer of a new architecture. The old binary can be executed without any modification. Old binaries can be mixed with new—for instance, a program coded for an old architecture can call library routines coded in the new instruction set, or vice-versa. Old libraries and new libraries may be freely mixed. New and old binaries may share the same address space, which improves the ability of new and old binaries to share common data. Alternatively, an old binary can be run in a protected separate address space on a new computer, without sharing any data with any new binary. A caller need not be aware of the ISA in which the callee is coded, avoiding the burden of explicitly saving and restoring context. The invention reduces software complexity: software need not make explicit provision for all possible entries and exits from all possible modes and mixtures of binaries. The pipelines for processing old instructions and new instructions can share pieces of the implementation, reducing the cost of supporting two instruction sets. A new computer can fully model an older computer, with no reliance on any software convention that may be imposed by any particular software product, allowing the new computer to run any program for the old computer, including varying off-the-shelf operating systems. Because translated target code is tracked in association with the physical pages of the source code, even if the physical pages are mapped at different points in the virtual address spaces, a single translation will be reused for all processes. This is particularly advantageous in the case of shared libraries. 
     The profile data may be used in a “hot spot” detector, that identifies portions of the program as frequently executed. Those frequently-executed portions can then be altered, either by a programmer or by software, to run more quickly. The profile data may be used by a binary translator to resolve ambiguities in the binary coding of instructions. The information generated by the profiler is complete enough that the hot spot detector can be driven off the profile, with no need to refer to the instruction text itself. This reduces cache pollution. Ambiguities in the X86 instruction text (the meaning of a given set of instructions that cannot be inferred from the instruction text, for instance the operand size information from the segment descriptors) are resolved by reference to the profile information. The information collected by the profiler compactly represents the information needed by the hot spot detector and the binary translator, with relatively little overhead, thereby reducing cache pollution. The profiler is integrated into the hardware implementation of the computer, allowing it to run fast, with little delay on a program—the overhead of profiling is only a few percent of execution speed. 
     Control may be transferred from an unoptimized instruction stream to an optimized instruction stream, without any change to the unoptimized instruction stream. In these cases, the unoptimized instruction stream remains available as a reference for correct execution. The instruction stream may be annotated with information to control a variety of execution conditions. 
     A profile may be used to determine which program transformation optimizations are safe and correct, and which present a risk of error. Rather than foregoing all opportunities unsafe optimizations or speed-ups, the optimization or speed-up may be attempted, and monitored for actual success or failure. The slower, unoptimized mode of execution can be invoked if the optimization in fact turns out to be unsafe. 
     The above advantages and features are of representative embodiments only, and are presented only to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims. Additional features and advantages of embodiments of the invention will become apparent in the following description, from the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a ,  1   b ,  1   c ,  1   d  and  3   a  are block diagrams of a computer system. 
         FIG. 1   e  is a diagram of a PSW (program status word) of a system as shown in  FIGS. 1   a - 1   d.    
         FIG. 2   a  is a table relating the meaning of several bits of the PSW of  FIG. 1   e.    
         FIGS. 2   b  and  2   c  are tables relating the actions of exception handlers. 
         FIGS. 3   b ,  3   c ,  3   d ,  3   e ,  3   f ,  3   l ,  3   m ,  3   n  and  3   o  are block diagrams showing program flow through memory. 
         FIGS. 3   g ,  3   h ,  3   i ,  3   j ,  6   c ,  7   d ,  8   b , and  8   c  are flow diagrams. 
         FIGS. 3   k ,  4   c ,  4   d , and  7   j  show data declarations or data structures. 
         FIGS. 4   a ,  4   e  and  4   f  are block diagrams showing program flow through memory, and profile information describing that program flow. 
         FIG. 4   b  is a table of profiling event codes and their meanings. 
         FIGS. 4   g ,  4   h ,  4   i ,  7   c ,  7   i  and  8   a  show processor registers of the computer. 
         FIG. 5   a  shows a finite state machine for control of a profiler. 
         FIGS. 5   b ,  6   b ,  7   a ,  7   b ,  7   e ,  7   f ,  7   g , and  7   h  are circuit block diagrams. 
         FIG. 6   a  is a block diagram of PIPM (Physical IP map) and an entry thereof. 
     
    
    
     DESCRIPTION 
     The description is organized as follows. 
     I. Overview of the Tapestry system, and features of general use in several aspects of the invention 
     I.A. System overview 
     I.B. The Tapestry instruction pipeline 
     I.C. Address translation as a control point for system features 
     I.D. Overview of binary translation, TAXi and the converter safety net 
     I.E. System-wide controls 
     I.F. The XP bit and the unprotected exception 
     II. Indicating the instruction set architecture (ISA) for program text 
     III. Saving Tapestry processor context in association with an X86 thread 
     III.A. Overview 
     III.B. Subprogram Prologs 
     III.C. X86-to-Tapestry transition handler 
     III.D. Tapestry-to-X86 transition handler 
     III.E. Handling ISA crossings on interrupts or exceptions in the Tapestry operating system 
     III.F. Resuming Tapestry execution from the X86 operating system 
     III.G. An example 
     III.H. Alternative embodiments 
     IV. An alternative method for managing transitions from one ISA to the other 
     IV.A. Indicating the calling convention (CC) for program text 
     IV.B. Recording Transfer of Control Semantics and Reconciling Calling Conventions 
     V. Profiling to determine hot spots for translation 
     V.A. Overview of profiling 
     V.B. Profileable events and event codes 
     V.C. Storage form for profiled events 
     V.D. Profile information collected for a specific example event—a page straddle 
     V.E. Control registers controlling the profiler 
     V.F. The profiler state machine and operation of the profiler 
     V.G. Determining the five-bit event code from a four-bit stored form 
     V.H. Interaction of the profiler, exceptions, and the XP protected/unprotected page property 
     V.I. Alternative embodiments 
     VI. Probing to find a translation 
     VI.A. Overview of probing 
     VI.B. Overview of statistical probing 
     VI.C. Hardware and software structures for statistical probing 
     VI.D. Operation of statistical probing 
     VI.E. Additional features of probing 
     VI.F. Completing execution of TAXi code and returning to the X86 code 
     VI.G. The interaction of probing and profiling 
     VI.H. Alternative uses of adaptive opportunistic statistical techniques 
     VII. Validating and invalidating translated instructions 
     VII.A. A simplified DMU model 
     VII.B. Overview of a design that uses less memory 
     VII.C. Sector Monitoring Registers 
     VII.D. Interface and Status Register 
     VII.E. Operation 
     VII.F. Circuitry 
     VII.G. DMU_Status register 
     VII.H. DMU_Command register 
     VIII. Managing out-of-order effects 
     VIII.A. Ensuring in-order handling of events reordered by optimized translation 
     VIII.B. Profiling references to non-well-behaved memory 
     VIII.C. Reconstructing canonical machine state to arrive at a precise boundary 
     VIII.D. Safety net execution 
     IX. Interrupt priority 
     I. Overview of the Tapestry System, and Features of General Use in Several Aspects of the Invention 
     I.A. System Overview 
     Referring to  FIGS. 1   a ,  1   b  and  1   c , the invention is embodied in the Tapestry product of Chromatic Research, Inc. of Sunnyvale, Calif. Tapestry is fast RISC processor  100 , with hardware and software features that provide a correct implementation of an Intel X86-family processor. (“X86” refers to the family including the 8086, 80186, . . . 80486, Pentium, and Pentium Pro. The family is described in I NTEL  A RCHITECTURE  S OFTWARE  D EVELOPER&#39;S  M ANUAL, VOL.  1-3, Intel Corp. (1997)) Tapestry fully implements the X86 architecture, in particular, a full Pentium with MMX extensions, including memory management, with no reliance on any software convention imposed, for instance, by a Microsoft or IBM operating system. A Tapestry system will typically be populated by two to four processors (only one of which is shown in  FIGS. 1   a ,  1   b  and  1   c ), interconnected as symmetric shared memory multiprocessors. 
     Tapestry processor  100  fetches (stage  110 ) instructions from instruction cache (I-cache)  112 , or from memory  118 , from a location specified by IP (instruction pointer, generally known as the PC or program counter in other machines)  114 , with virtual-to-physical address translation provided by I-TLB (instruction translation look-aside buffer)  116 . The instructions fetched from I-cache  112  are executed by a RISC execution pipeline  120 . In addition to the services provided by a conventional I-TLB, I-TLB  116  stores several bits  182 ,  186  that choose an instruction environment in which to interpret the fetched instruction bytes. One bit  182  selects an instruction set architecture (ISA) for the instructions on a memory page. Thus, the Tapestry hardware can readily execute either native instructions or the instructions of the Intel X86 ISA. This feature is discussed in more detail in section II, infra. 
     The execution of a program encoded in the X86 ISA is typically slower than execution of the same program that has been compiled into the native Tapestry ISA. Profiler  400  records details of the execution flow of the X86 program. Profiling is discussed in greater detail in section V, infra. Hot spot detector  122  analyzes the profile to find “hot spots,” portions of the program that are frequently executed. When a hot spot is detected, a binary translator  124  translates the X86 instructions of the hot spot into optimized native Tapestry code, called “TAXi code.” During emulation of the X86 program, prober  600  monitors the program flow for execution of X86 instructions that have been translated into native code. When prober  600  detects that translated native Tapestry code exists corresponding to the X86 code about to be executed, and some additional correctness predicates are satisfied, prober  600  redirects the IP to fetch instructions from the translated native code instead of from the X86 code. Probing is discussed in greater detail in section VI, infra. The correspondence between X86 code and translated native Tapestry code is maintained in PIPM (Physical Instruction Pointer Map)  602 . 
     Because the X86 program text may be modified while under execution, the system monitors itself to detect operations that may invalidate a previous translation of X86 program text. Such invalidating operations include self-modifying code, and direct memory access (DMA) transfers. When such an operation is detected, the system invalidates any native Tapestry translation that may exist corresponding to the potentially-modified X86 text. Similarly, any other captured or cached data associated with the modified X86 data is invalidated, for instance profile data. These validity-management mechanisms are discussed in greater detail in sections I.F, VII and VIII, infra. 
     The system does not translate instructions stored in non-DRAM memory, for instance ROM BIOS for I/O devices, memory-mapped control registers, etc. 
     Storage for translated native Tapestry code can also be released and reclaimed under a replacement policy, for instance least-recently-used (LRU) or first-in-first-out (FIFO). 
     A portion of the X86 program may be translated into native Tapestry code multiple times during a single execution of the program. Typically, the translation is performed on one processor of the Tapestry multiprocessor while the execution is in progress on another. 
     For several years, Intel and others have implemented the X86 instruction set using a RISC execution core, though the RISC instruction set has not been exposed for use by programs. The Tapestry computer takes three new approaches. First, the Tapestry machine exposes both the native RISC instruction set and the X86 instruction set, so that a single program can be coded in both, with freedom to call back and forth between the two. This approach is enabled by ISA bit  180 ,  182  control on converter  136 , and context saving in the exception handler (see sections II and III, infra), or in an alternative embodiment, by ISA bit  180 ,  182 , calling convention bit  200 , semantic context record  206 , and the corresponding exception handlers (see section IV, infra). Second, an X86 program may be translated into native RISC code, so that X86 programs can exploit many more of the speed opportunities available in a RISC instruction set. This second approach is enabled by profiler  400 , prober  600 , binary translator, and certain features of the memory manager (see sections V through VIII, infra). Third, these two approaches cooperate to provide an additional level of benefit. 
     Most of the features discussed in this disclosure are under a global control, a single bit in a processor control register named “PP_enable” (page properties enabled). When this bit is zero, ISA bit  180 ,  182  is ignored and instructions are interpreted in Tapestry native mode, profiling is disabled, and probing is disabled. 
     I.B. The Tapestry Instruction Pipeline 
     Referring to  FIG. 1   c , a Tapestry processor  100  implements an 8- or 9-stage pipeline. Stage  1  (stage  110 ) fetches a line from I-cache  112 . Stages  2  (Align stage  130 ) and  3  (Convert stage  134 ,  136 ,  138 ) operate differently in X86 and native Tapestry modes. In native mode, Align stage  130  runs asynchronously from the rest of the pipeline, prefetching data from I-cache  112  into elastic prefetch buffer  132 . In X86 mode, Align stage  130  partially decodes the instruction stream in order to determine boundaries between the variable length X86 instructions, and presents integral X86 instructions to Convert stage  134 . During X86 emulation, stage  3 , Convert stage  134 ,  136  decodes each X86 instruction and converts  136  it into a sequence of native Tapestry instructions. In decomposing an X86 instruction into native instructions, converter  136  can issue one or two Tapestry instructions per cycle. Each Tapestry processor  100  has four parallel pipelined functional units  156 ,  158 ,  160 ,  162  to implement four-way superscalar issue of the last five stages of the pipeline. In native mode, convert stage  134 ,  138  determines up to four independent instructions that can be executed concurrently, and issues them downstream to the four superscalar execution pipelines. (In other machine descriptions, this is sometimes called “slotting,” deciding whether sufficient resources and functional units are available, and which instruction is to be issued to which functional unit.) The Decode  140 , Register-read  142 , Address-Generate  144 , Memory  146 , Execute  148 , and Write-back  150  stages are conventional RISC pipeline stages. 
     Converter  136  decodes each X86 instruction and decomposes it into one or more simple Tapestry instructions. The simple instructions are called the “recipe” for the X86 instruction. 
     Referring to Table 1, when X86 converter  136  is active, there is a fixed mapping between X86 resources and Tapestry resources. For instance, the EAX, EBX, ECX, EDX, ESP and EBP registers of the X86 architecture are mapped by converter hardware  136  to registers R48, R49, R50, R51, R52 and R53, respectively, of the Tapestry physical machine. The eight floating-point registers of the X86, split into a 16-bit sign and exponent, and a 64-bit fraction, are mapped to registers R32-47. The X86 memory is mapped to the Tapestry memory, as discussed in section I.C, infra. 
     The use of the registers, including the mapping to X86 registers, is summarized in Table 1. The “CALL” column describes how the registers are used to pass arguments in the native Tapestry calling convention. (Calling conventions are discussed in detail in sections III.A, III.B, and IV, infra.) The “P/H/D” column describes another aspect of the Tapestry calling convention, what registers are preserved across calls (if the callee subprogram modifies a register, it must save the register on entry and restore it on exit), which are half-preserved (the low-order 32 bits are preserved across calls, but the upper 32 bits may be modified), and which are destroyable. The “X86 p/d” column shows whether the low-order 32 bits of the register, corresponding to a 32-bit X86 register, is preserved or destroyed by a call. The “Converter,” “Emulator” and “TAXi” columns show the mapping between Tapestry registers and X86 registers under three different contexts. For registers r32-r47, “hi” in the X86 columns indicates that the register holds a 16-bit sign and exponent portion of an X86 extended-precision floating-point value, and “lo” indicates the 64-bit fraction. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Tap 
                 Tap 
                   
                 X86 
                 X86 
                 X86 
                   
               
               
                   
                 CALL 
                 P/H/D 
                 Description 
                 p/d 
                 Converter 
                 Emulator 
                 TAXi 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 r63 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r62 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r61 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r60 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r59 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r58 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r57 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r56 
                   
                 P 
                 — 
                   
                 — 
                 — 
                 — 
               
               
                 r55 
                   
                 H 
                 X86 code will preserve only low 32 bits 
                 P 
                 edi 
                 edi 
                 edi 
               
               
                 r54 
                   
                 H 
                 X86 code will preserve only low 32 bits 
                 P 
                 esi 
                 esi 
                 esi 
               
               
                 r53 
                 [FP] 
                 H 
                 must be Frame-Pointer if stack frame has variable size. 
                 P 
                 ebp 
                 ebp 
                 ebp 
               
               
                 r52 
                 SP 
                 H 
                 stack pointer 
                 P 
                 esp 
                 esp 
                 esp 
               
               
                 r51 
                 RV3 
                 D 
                 if (192 bits &lt; size &lt;= 256 bits) fourth 64 bits of 
                 d 
                 ebx 
                 ebx 
                 ebx 
               
               
                   
                   
                   
                 function result 
               
               
                 r50 
                 RV2 
                 D 
                 X86_fastcall 2nd arg; 
                 d 
                 edx 
                 edx 
                 edx 
               
               
                   
                   
                   
                 if (128 bits &lt; size &lt;= 256 bits) third 64 bits of 
               
               
                   
                   
                   
                 function result 
               
               
                 r49 
                 THIS 
                 D 
                 X86_fastcall 1st arg; 
                 d 
                 ecx 
                 ecx 
                 ecx 
               
               
                   
                 RV1 
                   
                 “thiscall” object address (unadorned C++ non-static method); 
               
               
                   
                   
                   
                 if (64 bits &lt; size &lt;= 256 bits) second 64 bits of 
               
               
                   
                   
                   
                 function result 
               
               
                 r48 
                 RV0 
                 D 
                 X86 function result 
                 d 
                 eax 
                 eax 
                 eax 
               
               
                   
                   
                   
                 first 64 bits of function result (unless it is DP floating-point) 
               
               
                 r47 
                 P15 
                 D 
                 parameter register 15 
                   
                 f7-hi 
                 f7-hi 
                 f7-hi 
               
               
                 r46 
                 P14 
                 D 
                 parameter register 14 
                   
                 f7-lo 
                 f7-lo 
                 f7-lo 
               
               
                 r45 
                 P13 
                 D 
                 parameter register 13 
                   
                 f6-hi 
                 f6-hi 
                 f6-hi 
               
               
                 r44 
                 P12 
                 D 
                 parameter register 12 
                   
                 f6-lo 
                 f6-lo 
                 f6-lo 
               
               
                 r43 
                 P11 
                 D 
                 parameter register 11 
                   
                 f5-hi 
                 f5-hi 
                 f5-hi 
               
               
                 r42 
                 P10 
                 D 
                 parameter register 10 
                   
                 f5-lo 
                 f5-lo 
                 f5-lo 
               
               
                 r41 
                 P9 
                 D 
                 parameter register 9 
                   
                 f4-hi 
                 f4-hi 
                 f4-hi 
               
               
                 r40 
                 P8 
                 D 
                 parameter register 8 
                   
                 f4-lo 
                 f4-lo 
                 f4-lo 
               
               
                 r39 
                 P7 
                 D 
                 parameter register 7 
                   
                 f3-hi 
                 f3-hi 
                 f3-hi 
               
               
                 r38 
                 P6 
                 D 
                 parameter register 6 
                   
                 f3-lo 
                 f3-lo 
                 f3-lo 
               
               
                 r37 
                 P5 
                 D 
                 parameter register 5 
                   
                 f2-hi 
                 f2-hi 
                 f2-hi 
               
               
                 r36 
                 P4 
                 D 
                 parameter register 4 
                   
                 f2-lo 
                 f2-lo 
                 f2-lo 
               
               
                 r35 
                 P3 
                 D 
                 parameter register 3 
                   
                 f1-hi 
                 f1-hi 
                 f1-hi 
               
               
                 r34 
                 P2 
                 D 
                 parameter register 2 
                   
                 f1-lo 
                 f1-lo 
                 f1-lo 
               
               
                 r33 
                 P1 
                 D 
                 parameter register 1 
                   
                 f0-hi 
                 f0-hi 
                 f0-hi 
               
               
                 r32 
                 P0 
                 D 
                 parameter register 0 
                   
                 f0-lo 
                 f0-lo 
                 f0-lo 
               
               
                 r31 
                 RVA, 
                 D 
                 address of function result memory temporary (if any); 
                   
                 Prof15 
                 Prof15 
               
               
                   
                 RVDP 
                   
                 DP floating-point function result 
               
               
                 r30 
                   
                 D 
                   
                   
                 Prof14 
                 Prof14 
               
               
                 r29 
                   
                 D 
                   
                   
                 Prof13 
                 Prof13 
               
               
                 r28 
                   
                 D 
                   
                   
                 Prof12 
                 Prof12 
               
               
                 r27 
                   
                 D 
                   
                   
                 Prof11 
                 Prof11 
               
               
                 r26 
                   
                 D 
                   
                   
                 Prof10 
                 Prof10 
               
               
                 r25 
                   
                 D 
                   
                   
                 Prof9 
                 Prof9 
               
               
                 r24 
                   
                 D 
                   
                   
                 Prof8 
                 Prof8 
               
               
                 r23 
                   
                 D 
                   
                   
                 Prof7 
                 Prof7 
               
               
                 r22 
                   
                 D 
                   
                   
                 Prof6 
                 Prof6 
               
               
                 r21 
                   
                 D 
                   
                   
                 Prof5 
                 Prof5 
               
               
                 r20 
                   
                 D 
                   
                   
                 Prof4 
                 Prof4 
               
               
                 r19 
                   
                 D 
                   
                   
                 Prof3 
                 Prof3 
               
               
                 r18 
                   
                 D 
                   
                   
                 Prof2 
                 Prof2 
               
               
                 r17 
                   
                 D 
                   
                   
                 Prof1 
                 Prof1 
               
               
                 r16 
                   
                 D 
                   
                   
                 Prof0 
                 Prof0 
               
               
                 r15 
                 XD 
                 D 
                 Cross-ISA transfer descriptor (both call and return) 
                   
                 RingBuf 
                 RingBuf 
               
               
                 r14 
                   
                 D 
                   
                   
                   
                 CT10 
               
               
                 r13 
                   
                 D 
                   
                   
                   
                 CT9 
               
               
                 r12 
                   
                 D 
                   
                   
                   
                 CT8 
               
               
                 r11 
                   
                 D 
                   
                   
                   
                 CT7 
               
               
                 r10 
                   
                 D 
                   
                   
                   
                 CT6 
               
               
                 r9 
                   
                 D 
                   
                   
                   
                 CT5 
               
               
                 r8 
                   
                 D 
                   
                   
                   
                 CT4 
               
               
                 r7 
                 GP 
                 D 
                 pointer to global static environment (per-image) 
                   
                 CT3 
                 CT3 
               
               
                 r6 
                 LR 
                 D 
                 linkage register 
                   
                 CT2 
                 CT2 
               
               
                 r5 
                 AP 
                 D 
                 argument list pointer (overflow arguments in memory) 
                   
                 CT1 
                 CT1 
               
               
                 r4 
                 AT 
                 D 
                   
                   
                   
                 AT 
               
               
                 r3 
                   
                 vol 
                 volatile, may only be used in exception handlers 
                 vol 
                 vol 
                 vol 
                 vol 
               
               
                 r2 
                   
                 vol 
                 volatile, may only be used in exception handlers 
                 vol 
                 vol 
                 vol 
                 vol 
               
               
                 r1 
                   
                 vol 
                 volatile, may only be used in exception handlers 
                 vol 
                 vol 
                 vol 
                 vol 
               
               
                 r0 
                   
                 n/a 
                 always zero 
                 n/a 
                 n/a 
                 n/a 
                 n/a 
               
               
                   
               
            
           
         
       
     
     Tapestry supersets many features of the X86. For instance, the Tapestry page table format is identical to the X86 page table format; additional information about page frames is stored in a Tapestry-private table, the PFAT (page frame attribute table)  172 , as shown in  FIG. 1   d . As will be shown in  FIG. 1   e , the Tapestry PSW (Program Status Word)  190  embeds the X86 PSW  192 , and adds several bits. 
     The Tapestry hardware does not implement the entire X86 architecture. Some of the more baroque and less-used features are implemented in a software emulator ( 316  of  FIG. 3   a ). The combination of hardware converter  136  and software emulator  316 , however, yields a full and faithful implementation of the X86 architecture. 
     I.C. Address Translation as a Control Point for System Features 
     Referring to  FIG. 1   d , X86 address translation is implemented by Tapestry&#39;s native address translation. During X86 emulation, native virtual address translation  170  is always turned on. Even when the X86 is being emulated in a mode where X86 address translation is turned off, Tapestry address translation is turned on, to implement an identity mapping. By forcing every memory reference through the Tapestry address translation hardware, address translation becomes a convenient place for intercepting much of the activity of X86 converter  136 , and controlling the converter&#39;s execution. Further, control information for many features of the invention is conveniently stored in tables associated with, or tables analogous to those conventionally used for, address translation and virtual memory management. These “hooks” into address translation allow the Tapestry processor and software to intervene to emulate portions of the X86 that have “strange” behavior, like VGA graphics hardware, control registers, memory mapped device controls, and parts of the X86 address space that are given special treatment by traditional Intel chip sets. 
     To avoid changing the meaning of any portion of storage that X86 programs might be using, even if that use is unconventional, the Tapestry processor does not store any of its information in the X86 address translation tables. Tapestry-specific information about pages is stored in structures created specifically for Tapestry emulation of the X86. These structures are not defined in the X86 architecture, and are invisible to the emulated X86 or any program executing on the X86. Among these structures are PFAT (page frame attribute table)  172 . PFAT  172  is a table whose entries correspond to physical page frames and hold data for processing and managing those page frames, somewhat analogous to the PFN (page frame number) database of the VAX/VMS virtual memory manager (see, e.g., L AWRENCE  K ENAH AND  S IMON  B ATE , VAX/VMS I NTERNALS AND  D ATA  S TRUCTURES , Digital Press, 1984, incorporated herein by reference). PFAT  172  has one 1-byte entry  174  corresponding to each physical page frame. 
     As will be discussed in sections II, IV, and V and VI, infra, PFAT entries  174  also include bits that control which ISA is used to decode the instructions of the corresponding page, which calling convention is used on the corresponding page, and to control probing. 
     I.D. Overview of Binary Translation, TAXi and the Converter Safety Net 
     Referring again to  FIGS. 1   a  and  1   b , TAXi (“Tapestry accelerated execution,” pronounced “TAXi”) is a binary translation system. TAXi marries two modes of execution, hardware converter  136  (with software assistance in the run-time system) that faithfully implements a gold standard implementation of the full X86 architecture, and a software binary translator  124  that translates X86 binaries to Tapestry native binaries, but optimizes the translated code by making certain optimistic assumptions that may violate correctness. 
     As a pre-existing X86 binary is executed in converter  136 , hot spots (frequently-executed portions) in the X86 binary are recognized  122 , and translated  124  on-the-fly into native Tapestry instructions. The hardware converter  136  (coupled with a software X86 emulator  316  for especially complex instructions) is necessarily slower than the translated code, because the X86 instructions must be executed in strict sequence. By translating complete hot spots of an X86 binary, as opposed to “translating” single instructions in converter  136 , more optimization opportunities are exposed: X86 instructions can be decomposed into small data-independent Tapestry instructions, which in turn can be executed out of order, pipelined, or executed in parallel in the four superscalar pipelines ( 156 ,  158 ,  160 ,  162  of  FIG. 1   c ). 
     Execution of X86 code is profiled. This profiling information is used to identify  122  the “hot spots” in the X86 program, the most-executed parts of the program, and thus the parts that can most benefit from translation into native Tapestry code. The hot spots in the X86 code are translated by translator  124  into native Tapestry code (TAXi code). As execution of the X86 program proceeds, execution is monitored to determine whether a translated equivalent exists for the X86 code about to be executed. If so, execution is transferred to the translated native Tapestry code. 
     TAXi translator  124  adopts a somewhat simplified view of the machine behavior; for instance, some X86 instructions are not translated. Translator  124  also takes an optimistic view. For instance, translator  124  assumes that there will be no floating-point exceptions or page faults, so that operations can be reordered or speculatively rescheduled without changing program behavior. Translator  124  also assumes that all memory references are to well-behaved memory. (“Well-behaved memory” is a memory from which a load will receive the data last stored at the memory location. Non-well-behaved memory is typified by memory-mapped device controllers, also called “I/O space,” where a read causes the memory to change state, or where a read does not necessarily return the value most-recently written, or two successive reads return distinct data.) For instance, binary translator  124  assumes that memory reads can be reordered. Translated native Tapestry code runs faster than converter  136 , and is used when translation can be guaranteed to be correct, or when any divergence can be caught and corrected. 
     The execution of the TAXi code is monitored to detect violations of the optimistic assumptions, so that any deviation from correct emulation of the X86 can be detected. Either a pre-check can detect that execution is about to enter a region of translated code that can not be trusted to execute correctly, or hardware delivers an exception after the fact when the optimistic assumptions are violated. In either case, when correctness cannot be guaranteed, or for code that translator  124  does not know how to translate, execution of the translated native Tapestry code is aborted or rolled back to a safe check point, and execution is resumed in the hardware converter  136 . The hardware converter  136  adopts the most conservative assumptions, guaranteeing in-order, gold standard correctness, and serves as a safety net for the less risk-averse binary translator  124 . 
     This safety net paradigm allows binary translator  124  to be more aggressive, and makes development easier, because developers can focus on performance issues and leave correctness issues to be caught in the safety net. Additional details of the safety net paradigm are discussed in section VIII. 
     Tapestry and TAXi implement a full X86 architecture. No concession is required from X86 software; indeed, any X86 operating system can run on Tapestry, including off-the-shelf operating systems not specially adapted for Tapestry. Tapestry and TAXi make no assumptions about operating system entities, such as processes, threads, virtual address spaces, address mappings. Thus, Tapestry and TAXi operate in terms of the physical memory of the virtual X86, not the X86 virtual or linear addresses. (The distinction between Intel&#39;s “virtual” addresses and “linear” addresses seldom arises in the context of this disclosure; thus, unless a fine distinction between the two is required, this disclosure uses the term “virtual address” to embrace both concepts.) For instance, library code that is shared between different processes at the operating system level, by using physical addresses, is automatically shared by TAXi processes because the physical memory is shared on the Tapestry implementation. Code shared by the operating system is shared even if it is mapped at different addresses in different processes. If the processes are actually sharing the same physical page, then TAXi will share the same translated code. 
     Buffers of translated code are recycled in a first-in-first-out (FIFO) order. Once a translated code buffer is marked for reclamation, it is not immediately discarded; rather it is marked available for reuse. If execution re-enters an available-for-reuse buffer before the contents are destroyed, the buffer is recycled to the head of the FIFO queue. In an alternative embodiment, whenever the buffer is entered, it is moved to the head of the FIFO queue; this approximates a least-recently-used (LRU) replacement policy. 
     A number of features of the TAXi system are tied to profiling. For instance, a region of code that is not profiled can never be identified as a hot spot, and thus will never be translated. Similarly, probing (see section VI, infra) is disabled for any region that is not profiled, because without a translation, a probe can never succeed. This invariant simplifies a number of design details, as will be discussed at various points infra. 
     I.E. System-wide Controls 
     The PSW  190  has a TAXi_Active bit  198  that enables user-mode access to functionality that is otherwise disallowed in user mode. PSW.TAXi_Active  198  will be set true while a native Tapestry translation of an X86 program is being executed. When PSW.TAXi_Active  198  is true, a user-mode program may access the LDA/STA lock functionality of the X86, it has read and write access to all Tapestry processor registers, and it may access extended TRAP instruction vectors (specifically, to enable calling emulator functions). Further, X86-compatible semantics for extended precision floating-point operations is enabled. 
     A successful probe will set PSW.TAXi_Active  198  before it RFE&#39;s to the TAXi-translated code. When the TAXi-translated code completes execution, the process of returning to untranslated X86 code will clear PSW.TAXi_Active  198  before RFE-ing back to converter  136 . If an exception occurs in the TAXi-translated code, then emulator  316  will be called to surface the exception back to the X86 virtual machine. Emulator  316  will check EPC.TAXi_Active  198  and return control to TAXi to restore the X86 machine context and RFE back to converter  136  to re-execute the X86 instruction. 
     I.F. The XP Bit and the Unprotected Exception 
     Referring again to  FIGS. 1   a ,  1   b  and  2   a , TAXi translator  124  produces a translation of an X86 binary. The TAXi system as a whole represents a very complex cache, where the X86 code represents the slower memory level and the translated TAXi code represents the faster memory level. TAXi begins caching information at the time of profiling, because profiling records knowledge about what events occurred at what addresses, where the instruction boundaries were, etc. Further caching occurs when binary translator  124  translates X86 code into semantically equivalent Tapestry native code. In order not to violate the X86 architectural model, TAXi protects against execution of translated Tapestry native code that corresponds to stale X86 code, X86 code that has either disappeared or been modified. If the underlying primary datum (the X86 instruction text) is modified, whether by a memory write from the CPU, or by a DMA write from a device, the cached data (the profile describing the X86 code and the TAXi code generated from it) is invalidated, so that it will not be executed. Execution will revert to the X86 text, in its modified form. If the modified X86 text becomes a hot spot, it may be recognized  122  and retranslated  124 . 
     Like an ordinary cache, the TAXi cache has a valid bit—the XP bit ( 184  in PIPM entry  640 ,  186  in the I-TLB, see  FIGS. 1   a ,  1   b ). X86 code, and the validity of the “cached” translated native Tapestry code, is protected against modification by CPU writes by XP write-protect bit  184 ,  186 , and exception handlers that manage the protection of pages. Together, the flags and exceptions maintain a coherent translated Tapestry binary as a “cached” copy of the X86 program, while allowing the X86 program (whether encoded in its original X86 form or in translated native Tapestry form) to write to memory, even if that write implements self-modifying code. In either mode, the machine (either X86 converter  136  or the TAXi system) will faithfully execute the program&#39;s semantics. The protected and unprotected exceptions do not terminate processing in the manner of a conventional write-protect exception, but merely signal to the TAXi system that it must intervene to manage the validity of any TAXi code. 
     When a page of X86 code is protected, that is, when its XP protected bit  184 ,  186  is One, there are two classes of events that invalidate the TAXi code associated with the X86 code. First, a Tapestry processor could do a store into one of the X86 pages. This could arise if the program uses self-modifying code, or if the program creates code in writeable storage (stack or heap) on the fly. Second, a DMA device could write onto the page, for instance, when a page of program text is paged in on a page fault following a program load or activation. In either case, Tapestry generates an interrupt, and a handler for the interrupt resets the XP “valid” bit to indicate that any TAXi code corresponding to the X86 page cannot be reached by a probe (recall from section VI.D that probing is only enabled on X86 pages whose XP bit  184 ,  186  is One). 
     The write-protect bit is named “XP,” originally an acronym for “extended property.” Thus, when ISA bit ( 180  in PFAT  172 ,  182  in I-TLB) for a page indicates X86 ISA, the XP bit ( 184  in PFPM entry  640 ,  186  in the I-TLB) is interpreted to encode the modify-protect property for the page. XP bit  184 ,  186  controls the protection mechanism on a page-by-page granularity. The protection system for the machine as a whole is enabled and disabled by the TAXi_Control.unpr bit (bit &lt; 60 &gt; of the TAXi_Control register,  468  of  FIG. 4   g , see section V.E, infra). 
     Physical pages are divided for management between Tapestry operating system ( 312  of  FIG. 3   a ) and X86 operating system  306 , and PFAT.ISA bit  180  for the page (which is cached in the I-TLB.ISA bit  182 ) is set accordingly, Zero for Tapestry, One for X86. For all X86 pages, the XP bit ( 184  in PFAT  172 ,  186  in I-TLB  116 ) is cleared to Zero to indicate “unprotected.” XP bit  184 ,  186  has no effect on Tapestry pages. 
     XP bit  184 ,  186  behaves somewhat analogously to a MESI (Modified, Exclusive, Shared, Invalid) cache protocol. The XP “unprotected” state is roughly equivalent to the MESI “Exclusive” state, and means that no information from this page may be cached while the page remains unprotected. The “protected” XP state is roughly equivalent to the MESI “Shared” state, and means that information from the page may be cached, but cached information must be purged before the page can be written. Four points of the analogy are explained in Table 2. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 MESI 
                 TAXi XP protection 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 fetch for 
                 write 
                   
                   
                 fetch for 
                 write 
               
               
                   
                   
                 sharing 
                   
                   
                   
                 sharing 
               
               
                 Shared 
                 cached 
                   
                 action 1 
                 Protected 
                   
                   
                 action 1 
               
               
                 Exclusive 
                 uncached/ 
                 action 2 
                 3 
                 Unprotected 
                 uncached/ 
                 action 2 
                 3 
               
               
                   
                 exclusive 
                   
                   
                   
                 exclusive 
               
               
                   
               
               
                 action 1: discard all cached copies of the data, transition to the uncached/exclusive state 
               
               
                 action 2: fetch a shared/duplicate copy, and transition to the cached/shared state. 
               
            
           
         
       
     
     A write to a MESI “Shared” cache line forces all other processors to purge the cache line, and the line is set to “Exclusive.” Analogously, a write to an XP-protected  184 ,  186  page causes the page to be set to unprotected. These two analogous actions are designated “action 1” in table 2. If a page&#39;s ISA bit  180 ,  182  is One and XP bit  184 ,  186  is One, then this is an X86 instruction page that is protected. Any store to an X86 ISA page whose XP bit  184 ,  186  is One (protected), whether the current code is X86 native code or TAXi code, is aborted and control is passed to the protected exception handler. The handler marks the page unprotected by setting the page&#39;s XP bit  184 ,  186  to Zero. Any TAXi code associated with the page is discarded, and PIPM database  602  that tracks the TAXi code is cleaned up to reflect that discarding. Then the store is retried—it will now succeed, because the page&#39;s XP bit  184 ,  186  has been cleared to Zero (unprotected). If TAXi code writes onto the X86 page of which this TAXi code is the translation, then the general mechanism still works—the exception handler invalidates the TAXi code that was running, and will return to the converter and original X86 text instead of the TAXi code that executed the store. 
     A write to a “Exclusive” cache line, or to an XP-unprotected  184 ,  186  page, induces no state change. If XP bit  184 ,  186  is Zero (unprotected), then stores are allowed to complete. These two states are labeled “3” in Table 2. 
     A read from a MESI “Shared” cache line proceeds without further delay, because the data in the cache are current. Analogously, converter  136  execution of an instruction from an XP-protected  184 ,  186  page proceeds without delay, because if any translated TAXi code has been generated from the instructions on the page, the TAXi code is current, and the profiling and probing mechanisms ( 400 ,  600 , see sections V and VI, infra) will behave correctly. These analogous responses are labeled “4” in Table 2. 
     A read from a cache line, where that cache line is held in another processor in “Exclusive” state, forces the cache line to be stored to memory from that other processor, and then the line is read into the cache of the reading processor in “Shared” state. Analogously, when converter  136  executes code from XP-unprotected  184 ,  186  page (ISA is One, representing X86 code, and XP bit  184 ,  186  is Zero, indicating unprotected), and is about to write a profile trace-packet entry, with certain additional conditions, the machine takes an “unprotected” exception and vectors to the corresponding handler. The handler makes the page protected and synchronizes that page with other processors. These analogous actions are labeled “action 2” in Table 2. An unprotected exception is raised when an instruction is fetched from an unprotected X86 page (the page&#39;s I-TLB.ISA bit  182  is One, see section II, infra, and I-TLB.XP  186  bit is Zero), and TAXi_Control.unpr  468  is One and either of the following:
         (1) a profile capture instruction is issued to start a new profile packet (TAXi_State.Profile_Active ( 482  of  FIG. 4   h ) is Zero, TAXi_State.Profile_Request  484  is One, and TAXi_State.Event_Code_Latch  486 ,  487  contains an event code for which “initiate packet”  418  is True in  FIG. 4   b ), or   (2) when the first instruction in a converter recipe is issued and TAXi_State.Profile_Active  482  is One.
 
The TAXi_State terms of this equation are explained in sections V.E and V.F and  FIGS. 4   g ,  4   h ,  5   a  and  5   b.  
       

     The unprotected exception handler looks up the physical page address of the fetched instruction from EPC.EIP (the EPC is the native exception word (instruction pointer and PSW) pushed onto the stack by the exception, and EPC.EIP is the instruction pointer value), or from a TLB fault address processor register. The interrupt service routine sets the PFAT.XP bit  184  and I-TLB.XP bit  186  for the page to One, indicating that the page is protected. This information is propagated to the other Tapestry processors and DMU (DMA monitoring unit)  700 , in a manner similar to a “TLB shoot-down” in a shared-memory multiprocessor cache system. The exception handler may either abort the current profile packet (see section V.F, infra), or may put the machine in a context from which the profile packet can be continued. Then the exception handler returns to converter  136  to resume execution. 
     When TAXi_Control.unpr ( 468  of  FIG. 4   g ) is clear, then the value of the XP bit  184 ,  186  is ignored: no exception is generated and TAXi software is responsible for validating the profile packet and setting the “Protected” page attribute. 
     In an alternative embodiment, the unprotected exception handler aborts the current profile packet, and enqueues the identity of the page. Later, a lazy agent, analogous to a page purifier in a virtual memory system, manipulates the PFAT.XP bit  184 , I-TLB.XP bit  186 , and DMU (DMA monitoring unit) to protect the page. When execution next enters the page, the page will be protected, and profiling proceeds in the normal course. 
     Attempts to write to a protected page (for instance, by self-modifying code, or a write to a mixed text-and-data page) will be trapped, and the page will be set unprotected again. 
     Profiling is effectively disabled for unprotected pages, because an attempt to profile on an unprotected page, while TAXi_Control.unpr  468  is One, raises an unprotected exception, and the unprotected exception handler either makes the page protected, or aborts the profile packet. Turning off profiling for unprotected pages ensures that an unprotected page will not be recognized as a hot spot, and thus not translated. Conversely, if a page cannot be protected (for instance, the page is not the well-behaved memory of address space zero, but rather is mapped to an I/O bus), then any profile packet currently being collected is aborted. The implementation of this rule, and some limited exceptions, are discussed in section V.H, infra. 
     Further details of the XP protection mechanism are discussed in VIII, infra. A second protection mechanism, for protecting pages against writes by DMA devices, is described in section VII, infra. 
     II. Indicating the Instruction Set Architecture (ISA) for Program Text 
     Referring to  FIGS. 1   a ,  1   b ,  1   c  and  1   d , a program is divided into regions  176 , and each region has a corresponding flag  180 . Flag  180  asserts  178  an ISA under which instruction decode unit  134 ,  136 ,  140  is to decode instructions from the corresponding region. For instance, the address space is divided into pages  176  (the same pages used for virtual memory paging), and ISA bit  180  in a page table entry (PTE) asserts the ISA to be used for the instructions of the page. When instructions are fetched from a page  176  whose ISA bit  180 ,  182  is a Zero, the instructions are interpreted as Tapestry native instructions and fed  138  by ISA select  178  directly to pipeline  120 . When instructions are fetched from a page  176  whose ISA bit  180 ,  182  is a One, the instructions are fed under control of ISA select  178  to Convert stage  134 ,  136  of the pipeline, which interprets instructions as Intel X86 instructions. The regions need not be contiguous, either in virtual memory or in physical memory—regions of X86 text can be intermingled with regions of native Tapestry text, on a page-by-page basis. 
     A program written for one ISA can call library routines coded in either ISA. For instance, a particular program may use both a database management system and multimedia features. The multimedia services might be provided by libraries in optimized Tapestry native code. The database manager may be an off-the-shelf database system for the X86. The calling program, whether compiled for the X86 or for Tapestry, can readily call both libraries, and the combination will seamlessly cooperate. 
     In one embodiment, ISA bit is instantiated in two places, a master copy  180  and a cached copy  182  for fast access. The master copy is a single bit  180  in each entry  174  in PFAT  172 . There is one PFAT entry  174  corresponding to each physical page of the memory  118 , and the value of the value of ISA bit  180  in a given PFAT entry  174  controls whether Tapestry processor  100  will interpret instructions fetched from the corresponding page under the native instruction set architecture or as X86 instructions. On an I-TLB miss, the PTE from the Intel-format page tables is loaded into the I-TLB, as cached copy  182 . The physical page frame number from the page table entry is used to index into PFAT  172 , to find the corresponding PFAT entry  174 , and information from the PFAT entry  174  is used to supplement the Intel-format I-TLB entry. Thus, by the time the bit is to be queried during an instruction fetch  110 , the ISA bit  180  bit is in its natural location for such a query, I-TLB  116 . Similarly, if the processor uses a unified instruction and data TLB, the page table and PFAT information are loaded into the appropriate entry in the unified TLB. 
     In alternative embodiments, ISA bit  180  may be located in the address translation tables, whether forward-mapped or reverse-mapped. This embodiment may be more desirable in embodiments that are less constrained to implement a pre-existing fixed virtual memory architecture, where the designers of the computer have more control over the multiple architectures to be implemented. In another alternative, ISA bit  180 ,  182  may be copied as a datum in I-cache  112 . 
     When execution flows from a page of one ISA  180 ,  182  to a page of another (e.g., when the source of a control flow transfer is in one ISA and the destination is in the other), Tapestry detects the change, and takes a exception, called a “transition exception.” The exception vectors the processor to one of two exception handlers, a Tapestry-to-X86 handler ( 340  of  FIG. 3   i ) or an X86-to-Tapestry handler ( 320  of  FIG. 3   h ), where certain state housekeeping is performed. In particular, the exception handler changes the ISA bit  194  in the EPC (the copy of the PSW that snapshots the state of the interrupted X86 process), so that the RFE (return from exception instruction) at the end of the transition exception handler  320 ,  340  will load the altered EPC.ISA bit  194  into the PSW. The content of the PSW.ISA bit  194  is the state variable that controls the actual execution of the processor  100 , so that the changed ISA selection  178  takes effect when execution resumes. The PFAT.ISA copy  180  and I-TLB.ISA copy  182  are mere triggers for the exceptions. The exception mechanism allows the instructions in the old ISA to drain from the pipeline, reducing the amount of control circuitry required to effect the change to the new ISA mode of execution. 
     Because the Tapestry and X86 architectures share a common data representation (both little endian, 32-bit addresses, IEEE-754 floating-point, structure member alignment rules, etc.), the process can resume execution in the new ISA with no change required to the data storage state of the machine. 
     In an alternative embodiment, the execution of the machine is controlled by the I-TLB.ISA copy of the bit ISA bit  194 , and the PSW.ISA copy  190  is a history bit rather than a control bit. When execution flows onto a page whose ISA bit  180 ,  182  does not match the ISA  180 ,  182  of the previous page, at the choice of the implementer, the machine may either take a transition exception, or “change gears” without taking a transition exception. 
     There is a “page properties enable” bit in one of the processor control registers. On system power-on, this bit is Zero, disabling the page properties. In this state, the PSW.ISA bit is manipulated by software to turn converter  136  on and off, and transition and probe exceptions are disabled. As system initialization completes, the bit is set to One, and the PFAT and TLB copies of the ISA bit control system behavior as described supra. 
     III. Saving Tapestry Processor Context in Association with an X86 Thread 
     III.A. Overview 
     Referring to  FIGS. 3   a - 3   f , the ability to run programs in either of two instruction sets opens the possibility that a single program might be coded in both instruction sets. As shown in  FIG. 3   b , the Tapestry system provides transparent calls from caller to callee, without either knowing the ISA of the other, without either caller or callee being specially coded to work with the other. As shown in  FIG. 3   c , an X86 caller  304  might make a call to a callee subprogram, without being constrained to work with only callees coded in the X86 instruction set or the native Tapestry RISC instruction set  308 . If the callee is coded in the X86 instruction set, the call will execute as a normal call. If the callee  308  is coded in the native Tapestry instruction set, then Tapestry processor  100  will take a transition exception  384  on entry to the callee  308 , and another transition exception  386  on returning from the Tapestry callee  308  to the X86 caller  304 . These transition exceptions  384 ,  386  and their handlers ( 320  of  FIG. 3   h  and  340  of  FIG. 3   i ) convert the machine state from the context established by the X86 caller to the context expected by the Tapestry callee  308 . 
     Referring to  FIGS. 3   c - 3   f , analogous transition exceptions  384 ,  386  and handlers  320 ,  340  provide the connection between an X86 caller and its callees ( FIG. 3   c ), a native Tapestry caller and its callees ( FIG. 3   d ), between an X86 callee and its callers ( FIG. 3   e ), and between a native Tapestry callee its callers ( FIG. 3   f ), and provides independence between the ISA of each caller-callee pair. 
     Referring to  FIGS. 3   a  and  3   l  and to Table 1, X86 threads (e.g.,  302 ,  304 ) managed by X86 operating system  306 , carry the normal X86 context, including the X86 registers, as represented in the low-order halves of r32-r55, the EFLAGS bits that affect execution of X86 instructions, the current segment registers, etc. In addition, if an X86 thread  302 ,  304  calls native Tapestry libraries  308 , X86 thread  302 ,  304  may embody a good deal of extended context, the portion of the Tapestry processor context beyond the content of the X86 architecture. A thread&#39;s extended context may include the various Tapestry processor registers, general registers r1-r31 and r56-r63, and the high-order halves of r32-r55 (see Table 1), the current value of ISA bit  194  (and in the embodiment of section IV, infra, the current value of XP/calling convention bit  196  and semantic context field  206 ). 
     The Tapestry system manages an entire virtual X86  310 , with all of its processes and threads, e.g.,  302 ,  304 , as a single Tapestry process  311 . Tapestry operating system  312  can use conventional techniques for saving and restoring processor context, including ISA bit  194  of PSW  190 , on context switches between Tapestry processes  311 ,  314 . However, for threads  302 ,  304  managed by an off-the-shelf X86 operating system  306  (such as Microsoft Windows or IBM OS/2) within virtual X86 process  311 , the Tapestry system performs some additional housekeeping on entry and exit to virtual X86  310 , in order to save and restore the extended context, and to maintain the association between extended context information and threads  302 ,  304  managed by X86 operating system  306 . (Recall that Tapestry emulation manager  316  runs beneath X86 operating system  306 , and is therefore unaware of entities managed by X86 operating system  306 , such as processes and threads  302 ,  304 .) 
       FIGS. 3   a - 3   o  describe the mechanism used to save and restore the full context of an X86 thread  304  (that is, a thread that is under management of X86 operating system  306 , and thus invisible to Tapestry operating system  312 ) that is currently using Tapestry extended resources. In overview, this mechanism snapshots the full extended context into a memory location  355  that is architecturally invisible to virtual X86  310 . A correspondence between the stored context memory location  355  and its X86 thread  304  is maintained by Tapestry operating system  312  and X86 emulator  316  in a manner that that does not require cooperation of X86 operating system  306 , so that the extended context will be restored when X86 operating system  306  resumes X86 thread  304 , even if X86 operating system  306  performs several context switches among X86 threads  302  before the interrupted X86 thread  304  resumes. The X86 emulator  316  or Tapestry operating system  312  briefly gains control at each transition from X86 to Tapestry or back, including entries to and returns from X86 operating system  306 , to save the extended context and restore it at the appropriate time. 
     The description of the embodiment of  FIGS. 3   g - 3   k , focuses on crossings from one ISA to the other under defined circumstances (subprogram calls and returns and interrupts), rather than the fully general case of allowing transitions on any arbitrary transfer (conditional jumps and the like). Because there is always a Tapestry source or destination at any cross-ISA transfer, and the number of sites at which such a transfer can occur is relatively limited, the Tapestry side of each transition site can be annotated with information that indicates the steps to take to convert the machine state from that established in the source context to that expected in the destination context. In the alternative embodiment of section IV, the hardware supplements this software annotation, to allow the fully general ISA crossing. 
     The interaction between the native Tapestry and X86 environments is effected by the cooperation of an X86-to-Tapestry transition exception handler ( 320  of  FIG. 3   h ), a Tapestry-to-X86 transition exception handler ( 340  of  FIG. 3   i ), interrupt/exception handler ( 350  of  FIG. 3   j ) of Tapestry operating system  312 , and X86 emulator  316  (the software that emulates the portions of the X86 behavior that are not conveniently executed in converter hardware  136 ). 
     Because all native Tapestry instructions are naturally aligned to a 0 mod 4 boundary, the two low-order bits &lt;1:0&gt; of a Tapestry instruction address are always known to be Zero. Thus, emulator  316 , and exception handlers  320 ,  340 ,  350  of Tapestry operating system  312 , can pass information to each other in bits &lt;1:0&gt; of a Tapestry instruction address. To consider an example, the return address of a call from native Tapestry code, or the resume address for an interrupt of native code, will necessarily have two Zeros in its least significant bits. The component that gains control (either Tapestry-to-X86 transition handler  340  or Tapestry operating system  312 ) stores context information in these two low-order bits by setting them as shown in Table 3: 
                         TABLE 3                  00   default case, where X86 caller set no value of these bits - by           elimination, this means the case of calling a native           Tapestry subprogram       01   resuming an X86 thread suspended in a native Tapestry           subprogram       10   returning from an X86 callee to a native Tapestry caller, result           already in register(s)       11   returning from an X86 callee to a native Tapestry caller, where           the function result is in memory as specified in the X86           calling convention, and is to be copied into registers as           specified by the Tapestry calling convention.                    
Then, when control is to be returned to a Tapestry caller or to interrupted Tapestry native code, X86-to-Tapestry transition handler  320  uses these two bits to determine the context of the caller that is to be restored, and restores these two bits to Zero to return control to the correct address.
 
     A second information store is the XD register (register R15 of Table 1). The Tapestry calling convention (see section III.B, infra) reserves this register to communicate state information, and to provide a description of a mapping from a machine state under the X86 calling convention to a semantically-equivalent machine context under the Tapestry convention, or vice-versa. The Tapestry cross-ISA calling convention specifies that a caller, when about to call a callee subprogram that may be coded in X86 instructions, sets the XD register to a value that describes the caller&#39;s argument list. Similarly, when a Tapestry callee is about to return to what may be an X86 caller, the calling convention requires the callee to set XD to a value that describes the return value returned by the function. From that description, software can determine how that return value should be converted for acceptance by the callee under the X86 calling convention. In each case, the XD value set by the Tapestry code is non-zero. Finally, X86-to-Tapestry transition handler  320  sets XD to zero to indicate to the Tapestry destination that the argument list is passed according to the X86 calling convention. As will be described further infra, each Tapestry subprogram has a prolog that interprets the XD value coming in, to convert an X86 calling convention argument list into a Tapestry calling convention argument list (if the XD value is zero), and Tapestry-to-X86 exception handler  340  is programmed to interpret the XD value returned from a Tapestry function to convert the function return value into X86 form. 
     The Tapestry calling convention requires a callee to preserve the caller&#39;s stack depth. The X86 convention does not enforce such a requirement. X86-to-Tapestry transition handler  320  and Tapestry-to-X86 transition handler  340  cooperate to enforce this discipline on X86 callees. When Tapestry-to-X86 transition handler  340  detects a call to an X86 callee, transition handler  340  records ( 343  of  FIG. 3   i ) the stack depth in register ESI (R54 of Table 1). ESI is half-preserved by the X86 calling convention and fully preserved by the native convention. On return, X86-to-Tapestry transition handler  320  copies ESI back to SP, thereby restoring the original stack depth. This has the desired side-effect of deallocating any 32 byte hidden temporary created ( 344  of  FIG. 3   i ) on the stack by Tapestry-to-X86 transition handler  340 . 
     III.B. Subprogram Prologs 
     A “calling convention” is simply an agreement among software components for how data are to be passed from one component to the next. If all data were stored according to the same conventions in both the native RISC architecture and the emulated CISC architecture, then a transition between two ISA environments would be relatively easy. But they do not. For instance, the X86 calling convention is largely defined by the X86 architecture. Subroutine arguments are passed on a memory stack. A special PUSH instruction pushes arguments onto the stack before a subprogram call, a CALL instruction transfers control and saves the return linkage location on the stack, and a special RET (return) instruction returns control to the caller and pops the callee&#39;s data from the stack. Inside the callee program, the arguments are referenced at known offsets off the stack pointer. On the other hand, the Tapestry calling convention, like most RISC calling conventions, is defined by agreement among software producers (compilers and assembly language programmers). For instance, all Tapestry software producers agree that the first subprogram argument will be passed in register  32 , the second in register  33 , the third in register  34 , and so on. 
     Referring to  FIG. 3   g , any subprogram compiled by the Tapestry compiler that can potentially be called from an X86 caller is provided with both a GENERAL entry point  317  and a specialized NATIVE entry point  318 . GENERAL entry point  317  provides for the full generality of being called by either an X86 or a Tapestry caller, and interprets  319  the value in the XD register (R15 of Table 1) to ensure that its parameter list conforms to the Tapestry calling convention before control reaches the body of the subprogram. GENERAL entry point  317  also stores some information in a return transition argument area (RXA,  326  of  FIG. 3   h ) of the stack that may be useful during return to an X86 caller, including the current value of the stack pointer, and the address of a hidden memory temp in which large function return values might be stored. NATIVE entry point  318  can only be used by Tapestry callers invoking the subprogram by a direct call (without going through a pointer, virtual function, or the like), and provides for a more-efficient linkage; the only complexities addressed by NATIVE entry point  318  are varargs argument lists, or argument lists that do not fit in the sixteen parameter registers P0-P15 (R32-R47 of Table 1). The value of GENERAL entry point  317  is returned by any operation that takes the address of the subprogram. 
     III.C. X86-to-Tapestry Transition Handler 
     Referring to  FIG. 3   h , X86-to-Tapestry transition handler  320  is entered under three conditions: (1) when code in the X86 ISA calls native Tapestry code, (2) when an X86 callee subprogram returns to a native Tapestry caller, and (3) when X86 operating system  306  resumes a thread  304  that was interrupted by an asynchronous external interrupt while executing native Tapestry code. 
     X86-to-Tapestry transition handler  320  dispatches  321  on the two-low order bits of the destination address, as obtained in EPC.EIP, to code to handle each of these conditions. Recall that these two bits were set to values reflected in Table 3, supra. 
     If those two low-order bits EPC&lt;01:00&gt; are “00,” case  322 , this indicates that this transition is a CALL from an X86 caller to a Tapestry callee (typically a Tapestry native replacement for a library routine that that caller expected to be coded in X86 binary code). Transition handler  320  pops  323  the return address from the memory stack into the linkage register LR (register R6 of Table 1). Pop  323  leaves SP (the stack pointer, register R52 of Table 1) pointing at the first argument of the X86 caller&#39;s argument list. This SP value is copied  324  into the AP register (the argument pointer, register R5 of Table 1). SP is decremented  326  by eight, to allocate space for a return transition argument area (the return transition argument area may be used by the GENERAL entry point ( 317  of  FIG. 3   g ) of the callee), and then the SP is rounded down  327  to 32-byte alignment. Finally, XD is set  328  to Zero to inform the callee&#39;s GENERAL entry point  317  that this call is arriving with the machine configured according to the X86 calling convention. 
     If the two low-order bits of the return address EPC&lt;01:00&gt; are “10” or “11,” cases  329  and  332 , this indicates a return from an X86 callee to a Tapestry caller. These values were previously stored into EPC&lt;01:00&gt; by Tapestry-to-X86 transition handler  340  at the time the X86 callee was called, according to the nature of the function return result expected. 
     Low-order bits of “11,” case  329 , indicate that the X86 callee created a large function result (e.g., a 16-byte struct) in memory, as specified by the X86 calling convention. In this case, transition handler  320  loads  330  the function result into registers RV0-RV3 (registers R48-R51—see Table 1) as specified by the Tapestry calling convention. Low-order bits of “10,” case  332 , indicate that the function result is already in registers (either integer or FP). 
     In the register-return-value “10” case  332 , X86-to-Tapestry transition handler  320  performs two register-based conversions to move the function return value from its X86 home to its Tapestry home. First, transition handler  320  converts the X86&#39;s representation of an integer result (least significant 32 bits in EAX, most significant 32 bits in EDX) into the native convention&#39;s representation, 64 bits in RV0 (R48 of Table 1). Second, transition handler  320  converts  334  the X86&#39;s 80-bit value at the top of the floating-point stack into the native convention&#39;s 64-bit representation in RVDP (the register in which double-precision floating-point results are returned, R31 of Table 1). 
     The conversion for 64-bit to 80-bit floating-point is one example of a change in bit representation (as opposed to a copy from one location to another of an identical bit pattern) that may be used to convert the process context from its source mode to a semantically-equivalent form in its destination mode. For instance, other conversions could involve changing strings from an ASCII representation to EBCDIC or vice-versa, changing floating-point from IBM base 16 format to Digital&#39;s proprietary floating-point format or an IEEE format or another floating-point format, from single precision to double, integers from big-endian to little-endian or vice-versa. The type of conversion required will vary depending on the characteristics of the native and non-native architectures implemented. 
     In the “01” case  370  of resuming an X86 thread suspended during a call out to a native Tapestry subprogram, transition handler  320  locates the relevant saved context, confirms that it has not been corrupted, and restores it (including the true native address in the interrupted native Tapestry subprogram). The operation of case  370  will be described in further detail in sections III.F and III.G, infra. 
     After the case-by-case processing  322 ,  329 ,  332 ,  370 , the two low-order bits of return address in EPC&lt;1:0&gt; (the error PC) are reset  336  to “00” to avoid a native misaligned I-fetch fault. At the end of cases  329  and  332 , Register ESI (R54 of Table 1) is copied  337  to SP, in order to return to the stack depth at the time of the original call. An RFE instruction  338  resumes the interrupted program, in this case, at the target of the ISA-crossing control transfer. 
     III.D. Tapestry-to-X86 Transition Handler 
     Referring to  FIG. 3   i , Tapestry-to-X86 handler  340  is entered under two conditions: (1) a native Tapestry caller calls an X86 callee, or (2) a native Tapestry callee returns to an X86 caller. In either case, the four low-order bits XD&lt;3:0&gt; (the transfer descriptor register, R15 of Table 1) were set by the Tapestry code to indicate  341  the steps to take to convert machine context from the Tapestry calling convention to the X86 convention. 
     If the four low-order bits XD&lt;03:00&gt; direct  341  a return from a Tapestry callee to an X86 caller, the selected logic  342  copies any function return value from its Tapestry home to the location specified by the X86 calling convention. For instance, XD may specify that a 64-bit scalar integer result returned in RV0 is to be returned as a scalar in EAX or in the EDX:EAX register pair, that a double-precision floating-point result is to be copied from RV0 to the top of the X86 floating-point stack as an 80-bit extended precision value, or that a large return value being returned in RV0-RV3 (R48-R51 of Table 1) is to be copied to the memory location specified by original X86 caller and saved in the RXA. The stack depth is restored using the stack cutback value previously saved in the RXA by the GENERAL entry point prolog  317 . 
     If a Tapestry caller expects a result in registers but understands under the X86 calling convention that an X86 function with the same prototype would return the result via the RVA mechanism (returning a return value in a memory location pointed to by a hidden first argument in the argument list), the Tapestry caller sets XD&lt;3:0&gt; to request the following mechanism from handler  340 . The caller&#39;s stack pointer is copied  343  to the ESI register (R54 of Table 1) to ensure that the stack depth can be restored on return. A naturally-aligned 32-byte temporary is allocated  344  on the stack and the address of that temporary is used as the RVA (R31 of Table 1) value. Bits LR&lt;1:0&gt; are set  345  to “11” to request that X86-to-Tapestry transition handler  320  load 32 bytes from the allocated buffer into RV0-RV3 (R48-R51 of Table 1) when the X86 callee returns to the Tapestry caller. 
     For calls that will not use the RVA mechanism (for instance, the callee will return a scalar integer or floating-point value, or no value at all), Tapestry-to-X86 transition handler  340  takes the following actions. The caller&#39;s stack pointer is copied  343  to the ESI register (R54 of Table 1) to ensure that the stack depth can be restored on return. Bits LR&lt;1:0&gt; are set  346  to “10” as a flag to X86-to-Tapestry transition handler  320 ,  332  on returning to the native caller. For calls, handler  340  interprets  347  the remainder of XD to copy the argument list from the registers of the Tapestry calling convention to the memory locations of the X86 convention. The return address (LR) is pushed onto the stack. 
     For returns from Tapestry callees to X86 callers, the X86 floating-point stack and control words are established. 
     Tapestry-to-X86 transition handler  340  concludes by establishing  348  other aspects of the X86 execution environment, for instance, setting up context for emulator  316  and profiler  400 . An RFE instruction  349  returns control to the destination of the transfer in the X86 routine. 
     III.E. Handling ISA Crossings on Interrupts or Exceptions in the Tapestry Operating System 
     Referring to  FIG. 3   j  in association with  FIGS. 3   a  and  3   l , most interrupts and exceptions pass through a single handler  350  in Tapestry operating system  312 . At this point, a number of housekeeping functions are performed to coordinate Tapestry operating system  312 , X86 operating system  306 , processes and threads  302 ,  304 ,  311 ,  314  managed by the two operating systems  306 ,  312 , and the data configuration of those processes and threads that may need to be altered to pass from one calling convention to the other. 
     A number of interrupts and exceptions are skimmed off and handled by code not depicted in  FIG. 3   j . This includes all interrupts directed to something outside virtual X86  310 , including all synchronous exceptions raised in other Tapestry processes, the interrupts that drive housekeeping functions of the Tapestry operating system  312  itself (e.g., a timer interrupt), and exceptions raised by a Tapestry native process  314  (a process under the management of Tapestry operating system  312 ). Process-directed interrupts handled outside  FIG. 3   j  include asynchronous interrupts, the interrupts not necessarily raised by the currently-executing process (e.g., cross-processor synchronization interrupts). These interrupts are serviced in the conventional manner in Tapestry operating system  312 : the full Tapestry context of the thread is saved, the interrupt is serviced, and Tapestry operating system  312  selects a thread to resume. 
     Thus, by the time execution reaches the code shown in  FIG. 3   j , the interrupt is guaranteed to be directed to something within virtual X86  310  (for instance, a disk completion interrupt that unblocks an X86 thread  302 ,  304 , or a page fault, floating-point exception, or an INT software interrupt instruction, raised by an X86 thread  302 ,  304 ), and that this interrupt must be reflected from the Tapestry handlers to the virtual X86  310 , probably for handling by X86 operating system  306 . 
     Once X86 operating system  306  gains control, there is a possibility that X86 operating system  306  will context switch among the X86 processes  302 ,  304 . There are two classes of cases to handle. The first class embraces cases  351 ,  353 , and  354 , as discussed further infra. In this class of cases, the interrupted process has only X86 state that is relevant to save. Thus, the task of maintaining the association between context and thread can be handed to the X86 operating system  306 : the context switch mechanism of that operating system  306  will perform in the conventional manner, and maintain the association between context and process. On the other hand, if the process has extended context that must be saved and associated with the current machine context (e.g., extended context in a Tapestry library called on behalf of a process managed by X86 OS), then a more complex management mechanism must be employed, as discussed infra in connection with case  360 . 
     If the interrupted thread was executing in converter  136 , as indicated by ISA bit  194  of the EPC, then the exception is handled by case  351 . Because the interrupted thread is executing X86 code entirely within the virtual X86, the tasks of saving thread context, servicing the interrupt, and selecting and resuming a thread can be left entirely to X86 operating system  306 . Thus, Tapestry operating system  306  calls the “deliver interrupt” routine ( 352  of  FIG. 3   a ) in X86 emulator  316  to reflect the interrupt to virtual X86  310 . The X86 operating system  306  will receive the interrupt and service it in the conventional manner. 
     If an interrupt is directed to something within virtual X86  310 , while TAXi code (a translated native version of a “hot spot” within an X86 program, see section I.D, supra, as indicated by the TAXi_Active bit  198  of the EPC) was running, then the interrupt is handled by case  353 . Execution is rolled back to an X86 instruction boundary. At an X86 instruction boundary, all Tapestry extended context external to the X86  310  is dead, and a relatively simple correspondence between semantically-equivalent Tapestry and X86 machine states can be established. Tapestry execution may be abandoned—after the interrupt is delivered, execution may resume in converter  136 . Then, if the interrupt was an asynchronous external interrupt, TAXi will deliver the appropriate X86 interrupt to the virtual X86 supplying the reconstructed X86 machine state, and the interrupt will be handled by X86 operating system  306  in the conventional manner. Else, the rollback was induced by a synchronous event, so TAXi will resume execution in converter  136 , and the exception will be re-triggered, with EPC.ISA  194  indicating X86, and the exception will be handled by case  351 . 
     If the interrupted thread was executing in X86 emulator  316 , as indicated by the EM86 bit of the EPC, the interrupt is handled by case  354 . This might occur, for instance, when a high-priority X86 interrupt interrupts X86 emulator  316  while emulating a complex instruction (e.g. far call through a gate) or servicing a low-priority interrupt. The interrupt is delivered to emulator  316 , which handles the interrupt. Emulator  316  is written using re-entrant coding to permit re-entrant self-interruption during long-running routines. 
     Case  360  covers the case where the interrupt or exception is directed to something within virtual X86  310 , and the current thread  304 , though an X86 thread managed by X86 operating system  306 , is currently executing Tapestry code  308 . For instance, an X86 program may be calling a native Tapestry library. Here, the interrupt or exception is to be serviced by X86 operating system  306 , but the thread currently depends on Tapestry extended context. In such a case, X86 operating system  306  may perform a context switch of the X86 context, and the full Tapestry context will have to be restored when this thread is eventually resumed. However, X86 operating system  306  has no knowledge of (nor indeed has it addressability to) any Tapestry extended context in order to save it, let alone restore it. Thus, case  360  takes steps to associate the current Tapestry context with the X86 thread  304 , so that the full context will be re-associated (by code  370  of  FIG. 3   h ) with thread  304  when X86 operating system  306  resumes the thread&#39;s execution. 
     Referring briefly to  FIG. 3   k , during system initialization, the Tapestry system reserves a certain amount of nonpageable storage to use as “save slots”  355  for saving Tapestry extended context to handle case  360 . The save slot reserved memory is inaccessible to virtual X86  310 . Each save slot  355  has space  356  to hold a full Tapestry context snapshot. Each save slot  355  is assigned a number  357  for identification, and a timestamp  358  indicating the time at which the contents of the save slot were stored. Full/empty flag  359  indicates whether the save slot contents are currently valid or not. In an alternative embodiment, a timestamp  358  of zero indicates that the slot is unused. 
     Returning to  FIG. 3   j , case  360  is handled as follows. A save slot  355  is allocated  361  from among those currently free, and the slot is marked as in use  359 . If no save slot is free, then the save slot with the oldest time stamp  358  is assumed to have been stranded, and is forcibly reclaimed for recycling. Recall that the save slots  355  are allocated from non-paged storage, so that no page fault can result in the following stores to the save slot. The entire Tapestry context, including the X86 context and the extended context, and the EIP (the exception instruction pointer, the address of the interrupted instruction) is saved  362  into the context space  356  of allocated save slot  355 . The two low-order bits of the EIP (the address at which the X86 IP was interrupted) are overwritten  363  with the value “01,” as a signal to X86-to-Tapestry transition handler  320 ,  370 . The EIP is otherwise left intact, so that execution will resume at the interrupted point. (Recall that case  360  is only entered when the machine was executing native Tapestry code. Thus, the two low-order bits of the EIP will arrive at the beginning of handler  350  with the value “00,” and no information is lost by overwriting them.) The current 64-bit timestamp is loaded  364  into the EBX:ECX register pair (the low order halves of registers R49 and R51, see Table 1) and redundantly into ESI:EDI (the low order halves of registers R54-R55) and the timestamp member ( 358  of  FIG. 3   k ) of save slot  355 . The 32-bit save slot number  357  of the allocated save slot  355  is loaded  365  into the X86 EAX register (the low order half of register R48) and redundantly in EDX (the low order half of register R50). Now that all of the Tapestry extended context is stored in the save slot  355 , interrupt handler  350  of Tapestry operating system  312  now transfers control to the “deliver interrupt” entry point  352  of X86 emulator  316 . X86 operating system  306  is invoked to handle the interrupt. 
     Interrupt delivery raises a pending interrupt for the virtual X86  310 . The interrupt will be accepted by X86 emulator  316  when the X86 interrupt accept priority is sufficiently high. X86 emulator  316  completes delivery of the interrupt or exception to the X86 by emulating the X86 hardware response to an interrupt or exception: pushing an exception frame on the stack (including the interrupted X86 IP, with bits &lt;1:0&gt; as altered at step  363  stored in EPC), and vectoring control to the appropriate X86 interrupt handler. 
     Execution now enters the X86 ISR (interrupt service routine), typically in X86 operating system  306  kernel, at the ISR vectored by the exception. The X86 ISR may be an off-the-shelf routine, completely unmodified and conventional. A typical X86 ISR begins by saving the X86 context (the portion not already in the exception frame—typically the process&#39; registers, the thread ID, and the like) on the stack. The ISR typically diagnoses the interrupting condition, services it, and dismisses the interrupt. The ISR has full access to the X86 context. X86 operating system  306  will not examine or rely on the contents of the X86 processor context; the context will be treated as a “black box” to be saved and resumed as a whole. As part of servicing the interrupt, the interrupted thread is either terminated, put to sleep, or chosen to be resumed. In any case, the ISR chooses a thread to resume, and restores the X86 context of that thread. The ISR typically returns control to the selected thread either via an X86 IRET instruction or an X86 JUMP. In either case, the address at which the thread is to be resumed is the address previously pushed in an X86 exception frame when the to-be-resumed thread was interrupted. The thread resumed by X86 operating system  306  may be either interrupted thread  304  or another X86 thread  302 . 
     III.F. Resuming Tapestry Execution From the X86 Operating System 
     Referring again to  FIG. 3   h , X86 operating system  306  eventually resumes interrupted thread  304 , after a case  360  interrupt, at the point of interruption. X86 operating system  306  assumes that the thread is coded in X86 instructions. The first instruction fetch will be from a Tapestry page (recall that execution enters case  360  only when interrupted thread  304  was executing Tapestry native code). This will cause an X86-to-Tapestry transition exception, which will vector to X86-to-Tapestry transition handler  320 . Because the low-order two bits of the PC were set (step  363  of  FIG. 3   j ) to “01,” control dispatches  321  to case “01”  370 . 
     In step  371 , the save slot numbers in the X86 EAX and EDX registers are cross-checked (recall that the save slot number was stored in these registers by step  365  of  FIG. 3   j ), and the timestamp stored  362  in EBX:ECX is cross-checked with the timestamp stored in ESI:EDI. If either of these cross-checks  371  fails, indicating that the contents of the registers was corrupted, an error recovery routine is invoked  372 . This error routine may simply kill the corrupted thread, or it may bring the whole TAXi system down, at the implementer&#39;s option. If the time stamps pass validation, the timestamp from the EBX:ECX register pair is squirreled away  373  in a 64-bit exception handler temporary register that will not be overwritten during restoration of the full native context. The contents of register EAX is used as a save slot number to locate  374  the save slot  355  in which the Tapestry context is stored  362 . The entire Tapestry native context is restored  375  from the located save slot  355 , including restoration of the values of all X86 registers. Restore  375  also restores the two low-order bits EPC&lt;1:0&gt; to Zero. The save slot&#39;s timestamp  358  is cross-checked  376  against the timestamp squirreled away  373  in the temporary register. If a mismatch of the two timestamps indicates that the save slot was corrupted, then an error recovery routine is invoked  377 . The save slot is now empty, and is marked  378  as free, either by clearing full/empty flag  359  or by setting its timestamp  358  to zero. Execution is resumed at the EPC.EIP value by RFE instruction  338 , in the Tapestry code at the point following the interrupt. 
     Referring again to  FIG. 3   k , in an alternative embodiment, save slots  355  are maintained in a variation of a queue: hopefully-empty save slots to be filled are always allocated from the head  379   a  of the queue, full save slots to be emptied may be unlinked from the middle of the queue, and save slots may be entered into the queue at either the head  379   a  or tail  379   b , as described infra. A double-linked list of queue entries is maintained by links  379   c . At step  361 , a save slot is allocated from the head  379   a  of the allocation queue. After step  365 , the filled save slot  355  is enqueued at tail  379   b  of the save slot queue. At step  377 , the emptied save slot  355  is queued at the head  379   a  of the queue. 
     This alternative head-and-tail queuing protocol  361 ,  379   a ,  379   b ,  379   c ,  375  for save slots  355  has the following effects. The queue remains sorted into two partitions. The portion toward head  379   a  accumulates all save slots  355  known to be free. The portion toward the tail  379   b  holds all slots thought to be busy, in least-recently-used order. Over time, all stale slots (those thought to be busy but whose threads have disappeared) will accumulate at the boundary between the two partitions, because any time a slot with a timestamp older than that of a stale slot is resumed, the emptied slot is removed from the busy tail partition is moved to the free head partition. Normally, allocations will occur by intensively recycling the most recently freed slots at the head of the free partition while truly busy slots will cluster at the tail of the busy partition. When all known-free save slots  355  are exhausted and an apparently-busy save slot  355  is overwritten, the busy save slots  355  will be selected in least recently used to most recently busied. 
     In an alternative embodiment, a native Tapestry process would be allowed to call into an X86 library  308 . Exceptions raised in the X86 code would be serviced by Tapestry operating system  312 , filtered out in handler  350  of  FIG. 3   j  before the decision point reaches the beginning of the code shown in  FIG. 3   j.    
     III.G. AN EXAMPLE 
     Referring to  FIG. 3   m  in conjunction with  FIGS. 3   a ,  3   g ,  3   h ,  3   i ,  3   l  and  3   n , consider an example of a call by an X86 caller thread  304  to a Tapestry callee library  308 , an interrupt  388  in the library that is serviced by X86 operating system  306 , a context switch to another X86 thread  302  and a resumption of Tapestry callee  308 , and a return to the X86 caller  304 . 
     Tapestry library  308  is mapped  382  into a 32-bit flat address space  380 . From the point of view of X86 caller thread  304 , this is the process&#39; address space. From the point of view of the Tapestry machine and operating system  312 , the 32-bit address space is simply an address space that is mapped through page tables ( 170  of  FIGS. 1   a  and  1   d ), and whose contents and meaning are left entirely to the management of X86 operating system  306 . 
     Initially, thread  304  is executing on virtual X86  310 . Thread  304  executes an X86 CALL instruction  383 , seeking a library service. The binary code for thread  304  is conventional X86 code, not specially compiled for use in a Tapestry system. CALL instruction  383  transfers control (arrow ({circle around ( 1 )})) to the entry point of library  308 . This is the GENERAL entry point ( 317  of  FIG. 3   g ) for a Tapestry-binary replacement for the library. Fetching the first instruction from the entry preamble  317 ,  319  for Tapestry native library routine  308 , induces a change from X86 ISA to Tapestry ISA. Processor  100  takes a transition exception  384 , and vectors (arrow ({circle around ( 2 )})) to X86-to-Tapestry transition handler ( 320  of  FIG. 3   h ). Because all Tapestry instructions are aligned to a 0 mod 4 boundary, the two low-order bits of the interrupt address are “00.” Accordingly, transition handler  320  dispatches  321  to the “00” case  322  to establish the preconditions for execution in the Tapestry context (32-byte aligned stack, etc.). At the end of transition handler  320 , execution resumes  338  (arrow ({circle around ( 3 )})) at GENERAL entry point  317 . GENERAL entry point  317  begins by executing the X86 preamble ( 319  of  FIG. 3   g ), which copies the parameter list into the P0-P15 parameter registers, and execution of the body of Tapestry library routine  308  begins. 
     Assume that Tapestry library routine  308  runs to completion without an interrupt or call back to X86 code. 
     When Tapestry library routine  308  completes  385 , routine  308  loads a value describing the form of its return value into XD register (R15 of Table 1). This value will indicate a return value in RV0, RVFP, or a memory location, as appropriate. Routine  308  concludes with a Tapestry JALR instruction to return (arrow ({circle around ( 4 )})). As the first instruction is fetched from X86 caller thread  304 , a transition  386  from Tapestry ISA to X86 ISA is recognized, and control vectors (arrow ({circle around ( 5 )})) to Tapestry-to-X86 transition handler ( 340  of  FIG. 3   i ). Transition handler dispatches  341  on the value of XD&lt;03:00&gt; to one of the return cases  342 , which copies the return value from its Tapestry home to its home under the X86 calling convention. When transition handler  340  completes, it returns control (RFE instruction  349  of  FIG. 3   i , arrow {circle around ( 6 )} of  FIGS. 3   a ,  3   l  and  3   m ) to the instruction in thread  304  following the initial CALL  383 . 
     Referring now to  FIG. 3   n  in conjunction with  FIGS. 3   a ,  3   h ,  3   j  and  3   l , assume that an external asynchronous interrupt  388  occurred midway through the execution of Tapestry library routine  308 . To establish the example, assume that the interrupt is a disk-completion interrupt that unblocks a second, higher-priority X86 thread  302 . The interrupt vectors (arrow ({circle around ( 7 )})) to the interrupt/exception handler ( 350  of  FIG. 3   j ) of Tapestry operating system  312 . After disqualifying cases  351 ,  353 ,  354 , interrupt handler  350  selects case  360 . The full processor context is saved  362  in a save slot  355 , the two low-order bits EIP&lt;01:00&gt; are overwritten  363  with “01,” as described in Table 3, and the save slot number and timestamp information are loaded  364 ,  365  into the X86 registers. The interrupt handler  360  delivers the interrupt ( 369  of  FIG. 3   j ) to the interrupt entry point  352  of X86 emulator  316  (arrow ({circle around ( 8 )})). X86 emulator  316  passes control to X86 operating system  306  (arrow ({circle around ( 9 )})). X86 operating system  306  services the interrupt in the conventional manner. However, the context that X86 operating system  306  saves for thread  304  is the collection of timestamp and save slot number information with the EIP intact except for its two low-order bits, cobbled up by step  363  of Tapestry exception handler  360  to conform to Table 3. As assumed earlier in this paragraph, X86 operating system  306  selects thread  302  to be resumed (arrow ({circle around ( 10 )})). 
     After X86 thread  302  has executed for a time, it eventually cedes control (arrow back to X86 operating system  306 , for instance because its time slice expires, it issues a new disk request, or the like. Assume that the X86 operating system&#39;s scheduler now selects thread  304  to be resumed. The context restored by X86 operating system  306  is the timestamp and save slot number “context” cobbled up by exception handler  360 . The EIP of this restored context points to the instruction following the interrupted  388  instruction, with “01” in the two low-order bits. X86 operating system  306  executes an IRET instruction to resume execution at this restored context (arrow ({circle around ( 12 )})). This instruction fetch will recognize the transition  389  from the X86 ISA of X86 operating system  306  to the Tapestry ISA of Tapestry library  308 , and will vector (arrow ({circle around ( 13 )})) to X86-to-Tapestry transition handler  320  ( FIG. 3   h ). Transition handler  320  dispatches  321  on the two low-order bits of the EIP address to case  370 . The code of case  370  looks in the X86 registers to find the address of the save slot  355  corresponding to the process to be resumed. The content of the X86 registers and found save slot  355  are validated  371 ,  374 ,  376  by comparing the redundantly-stored timestamps and save slot numbers against each other. The content of save slot  355  restores  375  the full Tapestry processor context. Transition handler  320  resumes  378  execution of the Tapestry library routine  308  (arrow ({circle around ( 14 )})) at the point of the original external interrupt  388 . 
     Referring to  FIG. 3   o  in conjunction with  FIGS. 3   a ,  3   h ,  3   j  and  3   l , consider the case of a call from a Tapestry native caller  391  to an X86 callee  392 . (Recall from the discussion of  FIG. 3   b  that neither is specially coded to be tailored to this scenario—the X86 callee was generated by a conventional X86 compiler, and the Tapestry caller  391  is coded to work equally well whether the callee is an X86 callee  392  or a Tapestry callee.) Caller  391  sets  393  the value of the XD register (R15 of Table 1) to a value that describes the layout in the Tapestry registers (R32-R47 of Table 1) of its argument list. Then caller  391  issues a JALR instruction  394  to call to callee  392 . On arrival at the first instruction of callee  392 , processor  100  recognizes a Tapestry-to-X86 transition  395 . Execution vectors (arrow ({circle around ( 15 )})) to Tapestry-to-X86 exception handler ( 340  of  FIG. 3   i ). The four low-order bits XD&lt;3:0&gt; were set by instruction  393  to contain a basic classification of the XD descriptor, and execution is dispatched  341  according to those four bits, typically to code segment  343 - 345  or to segment  343 ,  346 ,  347 . The dispatched-to code segment moves  347  the actual parameters from their Tapestry homes to their X86 homes, as directed by the remainder of the XD register. Handler  340  overwrites  345 ,  346  the two low-order bits of the return PC, LR&lt;1:0&gt; with either “10” or “11” to indicate the location in which caller  391  expects the return result, as described in Table 3. Handler  340  returns (arrow ({circle around ( 16 )})) to the first instruction of X86 callee  392 , which executes in the conventional manner. When callee  392  completes, an X86 RET instruction returns control to caller  391  (arrow ({circle around ( 17 )})). The first instruction fetch from caller  391  will trigger a transition exception  396 . The exception vectors (arrow ({circle around ( 18 )})) control to X86-to-Tapestry handler  320 . Based on the two low-order bits of LR, handler  320  reformats and/or repositions  330 ,  333 ,  334  the function return value. The handler completes  336 ,  338 , and returns control (arrow ({circle around ( 19 )})) to the instruction in caller  391  following the original call  394 . 
     Referring again to  FIGS. 3   a  and  3   l , the complexity is confined to cases of cross-ISA calls. Complexity in handling cross-ISA calls is acceptable because transparent cross-ISA calling is not previously known in the art. In a case where caller, callee, and operating system all share a common ISA, no transition exceptions occur. For instance, when a Tapestry process  314  calls (arrow ({circle around ( 20 )})) the same Tapestry library routine  308 , routine  308  enters through NATIVE entry point  318 , or takes the Tapestry short path through GENERAL entry point  317 . (Note that routine  308  will have to be separately mapped  397  into the address space of Tapestry process  314 —recall that Tapestry process  314  is under the management of Tapestry OS  312 , while the address space  380  of an X86 process is entirely managed by X86 operating system  306 , entirely outside the ken of Tapestry operating system  312 .) If the same external interrupt  388  occurs (arrow ({circle around ( 21 )})), the interrupt can be handled in Tapestry operating system  312  (outside the code of  FIG. 3   j ), and control will directly resume (arrow ({circle around ( 22 )})) at the instruction following the interrupt, without tracing through the succession of handlers. When Tapestry library routine  308  completes, control will return to the caller (arrow ({circle around ( 23 )})) in the conventional manner. The only overhead is a single instruction  393 , setting the value of XD in case the callee is in X86 code. 
     III.H. ALTERNATIVE EMBODIMENTS 
     In an alternative embodiment, a “restore target page” of memory is reserved in the operating system region of the X86 address space. In PFAT  172 , ISA bit  180  for the restore target page is set to indicate that the instructions on the page are to be interpreted under the Tapestry instruction set. This restore target page is made nonpageable. At step  363  of  FIG. 3   j , the EPC.EIP value is replaced with an X86 address pointing into the restore target page, typically with byte offset bits of this replacement EPC.EIP storing the number of the save slot. In an alternative embodiment, the EPC.EIP is set to point to the restore target page, and the save slot number is stored in one of the X86 registers, for instance EAX. In either case, when X86 operating system  306  resumes the thread, the first instruction fetch will trigger an X86-to-Tapestry transition exception, before the first actual instruction from the restore target page is actually executed, because the restore target page has the Tapestry ISA bit set in its PFAT and I-TLB entries. X86-to-Tapestry transition handler  320  begins by testing the address of the fetched instruction. An address on the restore target page signals that there is extended context to restore. The save slot number is extracted from the instruction address (recall that the save slot number was coded into the EPC or EAX on exception entry, both of which will have been restored by X86 operating system  306  in the process of resuming the thread). The processor context is restored from the save slot, including the EPC.EIP value at which the thread was originally interrupted. In an alternative embodiment, only the extended context (not including the X86 context) is restored from the save slot, so that any alterations to the X86 context effected by Tapestry operating system  312  are left intact. X86-to-Tapestry transition handler  320  executes an RFE  338  to resume execution in the interrupted Tapestry code. 
     Note that no instruction from the restore target page is actually executed; the address is simply a flag to X86-to-Tapestry transition handler  320 . All that is required is that the address of the restore target page be representable in the X86 address space, so that the address can pass through X86 operating system  306  and its thread scheduler. In alternative embodiments, a fetch from the restore target page could raise another exception—an unaligned instruction fault, or an access protection fault. It is desirable, however, that the fault raised be one not defined in the X86 architecture, so that no user program can register a handler for the fault. 
     In this alternative embodiment, the “01” case  370  of X86-to-Tapestry transition handler  320  may also save the X86 thread&#39;s privilege mode, and reset the privilege level to user, even if the X86 caller was running in privilege ring zero. The privilege mode is changed to protect system integrity, to disallow a Tapestry Trojan horse from subverting X86 security checks. 
     In an alternative embodiment, the correspondence between save slots and X86 threads is maintained by using thread-ID calls into X86 operating system  306 . Each save slot  355  may be associated with a Windows thread number for the duration of that thread. A garbage collector may be used to recognize save slots that were filled a long time ago and are now apparently abandoned. The garbage collector reclaims save slots after a system-tunable time period, or on a least-recently-filled basis, on the assumption that the thread was terminated by X86 operating system  306 . 
     In another alternative embodiment, when Tapestry takes an exception while in X86 converter mode, the extended context is snapshotted as well. If the operating system uses the X86 TSS (Task-State Segment) to implement multi-tasking, then the PSW portion of the extended context (ISA  194 , XP/calling convention  196 , and semantic class  206 , see section IV, infra) can be snapshotted into unused parts of the X86 TSS. Otherwise the amount of data involved, five bits (ISA bit  194 , XP/calling convention bit  196 , and semantic context  206 ), is small enough that it can be squirreled away within the ten unused bits at the top of EFLAGS. In some embodiments, it may be possible to push the extended context as an additional word pushed onto the exception stack in X86 space. 
     In another alternative embodiment, the extended context can be stored in memory in Tapestry space, where it is inaccessible to the X86. A hash table (or an equivalent associative software structure) links a particular X86 exception frame to its associated snapshot of the extended Tapestry context, so that on exception exit or task rescheduling, when the processor reloads a particular X86 context into the EPC (error PC and program status word), in turn to be reloaded into the PSW by an RFE instruction (or when an X86 POPF instruction is emulated), the extended Tapestry context can be located and placed in the EPC as well. 
     IV. An Alternative Method for Managing Transitions From One ISA to the Other 
     IV.A. Indicating the Calling Convention (CC) for Program Text 
     Sections IV.A and IV.B together describe an alternative mechanism used to determine the conventions under which data are passed to or from a subprogram, and thus the locations in which subprogram arguments or a function return value are stored before a control-transfer event, so that an exception handler can move the data to the locations expected by the code to be executed after the control-flow event. 
     In the alternative Tapestry emulation of the X86 CISC architecture, any particular extent of native code observes one of two different calling conventions (see section III.B, supra): one RISC register-based calling convention for calls from native Tapestry code to native Tapestry code, and another quasi-CISC memory-based convention that parallels the emulated CISC calling convention, for use when it is believed likely that the call will most frequently cross from one ISA to the other. The features described in sections IV.A and IV.B provide sufficient information about the machine context so that a transition from one ISA to the other can be seamlessly effected. 
     Referring again to  FIG. 3   a , programs coded in the native Tapestry instruction set, when calling a subprogram, may use either a register-based RISC calling convention, or a memory-based calling convention that parallels the X86 convention. In X86 converter mode, all subprogram calls use the memory-stack-based calling convention. In either mode, control may be transferred by an internal jump in which the data passes from source to destination simply by its location in certain memory or register locations. 
     Program text regions  176  are annotated with a bit  200  that indicates the calling convention used by the code in the region. When execution flows from a source observing one calling convention to a destination observing another, the difference in calling convention bits  200  will trigger a transition exception. The transition exception handler copies the subprogram arguments from the well-known location established by the source convention to the well-known location expected by the destination. This allows caller and callee subprograms to be compiled with no reliance on the calling convention used by the other, and allows for more seamless system operation in an environment of binaries and libraries of inhomogeneous ISA. 
     Referring to  FIGS. 1   d  and  2   a , calling convention bit  200  is stored in PFAT entries  174  and I-TLB  116  in a manner analogous to ISA bit  180 ,  182  with a record of the calling convention of the previous instruction available in PSW  190  calling convention bit  196 , as discussed in section II, supra; the alternative embodiments discussed there are equally applicable here. (Because the calling convention property  200  is only meaningful for pages of Tapestry code, and the XP write-protect property  184 ,  186  (discussed in section I.F, supra) is only used for pages of X86 code, the two properties for a given page can encoded in a single physical bit, overlaying the XP write-protect bits  184 ,  186 —this single bit has different meanings depending on the PSW.ISA bit  194 .) 
     Referring to  FIGS. 2   b  and  2   c , when execution crosses (column  204 ) from a region of one calling convention  200  to a region of another calling convention  200 , the machine takes an exception. Based on the direction of the transition (Tapestry-to-X86 or X86-to-Tapestry) and a classification (as shown in Table 4 and discussed in IV.B, infra) of the instruction that provoked the transition, the exception is vectored to an exception handler that corresponds to the direction and classification. The eight calling convention transition exception vectors are shown in the eight rows  242 - 256  of  FIG. 2   c . (The eight exception vectors for calling convention transitions are distinct from the two exception vectors for ISA transitions discussed in section II, supra.) The exception vectoring is specific enough that arrival at a specific handler largely determines a mapping from the old machine context to a machine context that will satisfy the preconditions for execution in the new environment. The exception handler implements this mapping by copying data from one location to another. The exception handler operates during an exception interposed between the source instruction and the destination instruction, transforming the machine context from that produced by the last instruction of the source (for instance, the argument passing area established before a CALL) to the context expected by the first instruction of the destination (the expectations of the code that will begin to use the arguments). 
     Further information used to process the transition exception, and the handling of particular exception cases, is described in section IV.B, infra. 
     IV.B. Recording Transfer of Control Semantics and Reconciling Calling Conventions 
     Merely knowing the direction of a transition (from X86 calling convention to Tapestry convention or vice versa) is insufficient to determine the actions that must be taken on a transition exception when the data storage conventions disagree. This section describes a further technique used to interpret the machine context, so that the appropriate action can be taken on a transition exception. In overview, as each control-transfer instruction is executed, the intent or semantic class of the instruction is recorded in the SC (semantic class) field  206  (PSW.SC) of PSW (the Program Status Word)  190 . On a transition exception, this information is used to vector to an exception handler programmed to copy data from one location to another in order to effect the transition from the old state to the new precondition. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 semantic 
                   
                   
               
               
                 ISA of 
                 class 
                   
                 representative 
               
               
                 source 
                 value 
                 Meaning 
                 instructions 
               
               
                   
               
             
            
               
                 Tap 
                 00 
                 Call 
                 JAL, JALR 
               
               
                 Tap 
                 01 
                 Jump 
                 conditional jump, 
               
               
                   
                   
                   
                 J, JALR 
               
               
                 Tap 
                 10 
                 Return with no FP result 
                 JALR 
               
               
                 Tap 
                 11 
                 Return with FP result 
                 JALR 
               
               
                 X86 
                 00 
                 Call 
                 CALL 
               
               
                 X86 
                 01 
                 Jump 
                 JMP, Jcc 
               
               
                 X86 
                 10 
                 Return with no FP result 
                 RET 
               
               
                 X86 
                 11 
                 Return with (possible) FP 
                 RET 
               
               
                   
                   
                 result 
               
               
                   
               
            
           
         
       
     
     Referring to  FIGS. 1   e  and  2   c  and to Table 4, the control-flow instructions of both the Tapestry ISA and the X86 ISA are classified into five semantic classes: JUMP, CALL, RETURN-NO-FP (return from a subprogram that does not return a double-precision floating-point result), RETURN-FP (return from a subprogram that definitely returns a double-precision floating-point result, used only in the context of returning from a Tapestry native callee), and RETURN-MAYBE-FP (return from a subprogram that may return or definitely returns either a 64-bit double-precision or 80-bit extended precision floating-point result, used only in the context of returning from an X86 callee). Because there are four possible transfers for each ISA mode, two bits  206  (combined with PSW.ISA bit  194 ) are sufficient to identify the five states enumerated. 
     Most of this semantic classification is static, by instruction opcode. Some instructions, e.g., the X86 Jump and CALL instructions, are semantically unambiguous. For instance, an X86 RET cannot be mistaken for a CALL or an internal control flow JUMP. Thus, even though the Tapestry system never examines the source code for the X86 binary, the X86 instruction contains sufficient information in its opcode to determine the semantic class of the instruction. 
     Referring to Table 4, some of the semantic classification is encoded into instructions by the compiler. For instance, the Tapestry JALR instruction (ump indirect to the location specified by the instruction&#39;s source register, and store the link IP (instruction pointer) in the destination register), may serve any of several roles, for instance as a return from subprogram (the link IP is stored into the read-zero register), a FORTRAN assigned go-to within a single routine, or a subprogram call. To resolve the ambiguity of a JALR instruction, bits that are unused in the execution of the instruction are filled in by the compiler with one of the semantic class codes, and that code is copied as an immediate from the instruction to PSW.SC  206  when the instruction is executed. In the case of Tapestry native binaries compiled from source code, this immediate field of the JALR instruction is filled in with the aid of semantic information gleaned either from the source code of the program being compiled. In the case of a binary translated from X86 to Tapestry, the semantic class of the X86 instruction is used to determine the semantic class of the corresponding Tapestry instruction. Thus, the Tapestry compiler analyzes the program to distinguish a JALR for a branch to a varying address (for instance a FORTRAN assigned or computed go-to, or a CASE branch through a jump table) from a JALR for a function return (further distinguishing the floating-point from no-floating-point case) from a JALR for a subprogram call, and explicitly fills in the two-bit semantic class code in the JALR instruction. 
     Some of the semantic classification is performed by execution time analysis of the machine context. X86 RET (return from subprogram) instructions are classified into two semantic context classes, RETURN-NO-FP (return from subprogram, definitely not returning a floating-point function result) and RETURN-MAYBE-FP (return, possibly or definitely returning a floating-point function result). The X86 calling convention specifies that a floating-point function result is returned at the top of the floating-point stack, and integer function results are returned in register EAX. The instruction opcode is the same in either case; converter  136  classifies RET instructions on-the-fly based on the X86 floating-point top-of-stack. If the top-of-stack points to a floating-point register marked empty, then the X86 calling convention unambiguously assures that the RET cannot be returning a floating-point value, and the semantic class is set to RETURN-NO-FP. If the top-of-stack register points to a full location, there may nonetheless be an integer return value; the semantic context is set to RETURN-MAYBE-FP to indicate this ambiguity. 
     On an exception, PSW  190  (including ISA bit  194 , calling convention bit  196 , and SC field  206 ) is snapshotted into the exception PSW, a control register of the machine. The PSW bits in the exception PSW are available for examination and modification by the exception handler. When the exception handler completes, the RFE (return from exception) instruction restores the snapshotted exception PSW into the machine PSW  190 , and machine execution resumes. Thus, PSW.SC  206  is preserved across exceptions, even though it is updated during execution of the exception handler (unless the exception handler deliberately modifies it by modifying the exception PSW). 
       FIGS. 2   b  and  2   c  show how calling convention transitions are vectored to the exception handlers. On a calling convention transition exception, five data are used to vector to the appropriate handler and determine the action to be taken by the handler: the old ISA  180 ,  182 , the new ISA  180 ,  182 , the old calling convention  196 , the new calling convention  196 , and PSW.SC  206 . In  FIG. 2   b , the first column  204  shows the nature of the transition based on the first four, the transition of the ISA and CC bits. For instance, the third line  216  discusses a transition from native Tapestry ISA using native Tapestry register-based calling conventions (represented as the value “00” of the ISA and CC bits) to X86 code, which necessarily uses the X86 calling convention (represented as the value “1x,” “1” for the X86 ISA, and “x” for “don&#39;t care” value of the CC bit). Table 4 shows that several different situations may vector to the same exception handler. Note, for instance, that lines  214  and  216  vector to the same group of four handlers, and lines  218  and  224  vector to the same group of handlers. These correspondences arise because the memory manipulation required to convert from native Tapestry calling convention to X86 calling convention, or vice versa, is largely the same, whether the X86 convention is observed by native Tapestry instructions or X86 instructions. 
       FIG. 2   c  shows how the machine vectors to the proper exception handler based on semantic class. For instance, lines  242 ,  244 ,  246 , and  248  break out the four possible handlers for the 00=&gt;01 and 00=&gt;1x (native Tapestry code using native calling conventions, to X86 code using X86 conventions) ISA and CC transitions, based on the four possible semantic classes of control-flow instruction. Lines  250 ,  252 ,  254 , and  256  break out the four possible handlers for the 01=&gt;00 and 1x=&gt;00 transitions, based on the four semantic classes of instruction that can cause this transition. 
     Referring to  FIG. 2   b , when crossing from one subprogram to another, if the source and destination agree on the convention used for passing arguments, either because they agree on ISA and calling convention (rows  212 ,  220 ,  228 ,  230 ), or agree on calling convention even though disagreeing on ISA (rows  222 ,  226 ), or because the data pass simply by virtue of being stored in the same storage location in the source and destination execution environments (rows  244 ,  252 ), then no intervention is required. For instance, when crossing from the X86 ISA using the X86 calling convention to the Tapestry native ISA using the X86 convention, or vice-versa, data passes from one environment to the other, without actually moving from one hardware element to another, using the fixed mapping between X86 virtual resources and Tapestry physical resources using the fixed mapping shown in Table 1 and discussed in section I.B, supra. 
     For instance, as shown in row  222 , if a caller in Tapestry native code, using the memory based quasi-X86 calling convention, calls a routine in X86 code (or vice-versa, row  226 ), no arguments need be moved; only the instruction decode mode need be changed. 
     On the other hand, if the calling conventions  200  disagree, and the arguments are being passed under one calling convention and received under another, the calling convention exception handler intervenes to move the argument data from the well-known locations used by the source convention to the well-known locations expected by the destination convention. For instance, a subprogram CALL from an X86 caller to a callee in native Tapestry code that uses the native Tapestry calling convention (rows  224 ,  250 ), or equivalently, from Tapestry code using X86 conventions to native Tapestry using the native convention (rows  218 ,  250 ), must have its arguments moved from the locations specified by the memory stack-based caller convention to the locations specified by the register-based callee convention. 
     Rows  214 ,  242  of  FIGS. 2   b  and  2   c  show the case of a subprogram call where the caller is in Tapestry native code using the register-based native calling convention, and the callee is in Tapestry native code but uses the quasi-X86 calling convention. (Similarly, as shown in rows  216 , if the caller is in Tapestry native code using the register-based native calling convention, and the callee is coded in the X86 ISA, then the same exception handler  242  is invoked, and it does the same work.) The exception handler will push the subprogram arguments from their register positions in which arguments are passed under the native convention, into their memory stack positions as expected under the X86 calling convention. If the arguments are of varying size, the X86 stack layout of the argument buffer may be rather complex, and the mapping from the arguments in Tapestry registers to that argument buffer will be correspondingly complex. The argument copying is specified by a descriptor, an argument generated by the compiler for annotation of the caller site. This is particularly important for “varargs” routines. Because the native caller was generated by the Tapestry compiler, the compiler is able to produce a descriptor that fully describes the data copying to be performed by the transition exception. The descriptor is analogous to the argument descriptors generated by compilers for use by debuggers. The data will then be in the locations expected by the callee, and execution can resume in the destination ISA mode. 
     When an X86 caller (or a Tapestry caller using the quasi-X86 calling convention), the data of the argument block established by the caller are copied into the locations expected by the Tapestry callee. For instance, the linkage return address is copied from the top of stack to r6 (the Tapestry linkage register, given the alias name of LR for this purpose). The next few bytes of the stack are copied into Tapestry registers, for quick access. A call descriptor (a datum that describes the format of the call arguments) is manufactured in register r51 (alias CD), set to indicate that the arguments are passed under the X86 convention. A null return value descriptor is manufactured on the stack; the return descriptor will be modified to specify the format of the return value, once that information is known. 
     When returning from a callee function, the calling convention  200  of the caller and callee and the semantic class  206  of the return instruction determine the actions needed to put the function return value in the correct location expected by the callee. As shown in Table 1, the X86 calling convention returns double-precision floating-point function return values in the floating-point register indicated by the top-of-floating-point-stack. The X86 calling convention returns other scalars of 32 bits or less in register EAX, results of 33 to 64 bits in the EAX:EDX register pair, and function return values of 65 bits or greater are returned in a memory location pointed to by an argument prepended to the caller&#39;s argument list. The native Tapestry calling convention returns double-precision floating-point values in r31 (for this purpose, given the alias name of RVDP), other return values of 256 bits or less in registers r48, r49, r50, and r51 (given the alias names of RV0, RV1, RV2, and RV3), and larger return values in a memory location pointed to by r31 (for this purpose, given the alias name of RVA). 
     The Tapestry calling convention, and the mapping between Tapestry and X86 resources, are co-designed, at least in part, to maximize common uses, thereby to reduce the amount of data copying required on a calling convention transition. Thus, the two registers used to return scalar function return values—r48 (RV0) in Tapestry, EAX in X86—are mapped to each other. 
     When returning from a native-convention callee to an X86 or a Tapestry-using-X86-convention caller, the semantic class of the return is unambiguously known (because whether the function returns a floating-point value or not was encoded in the semantic class bits of the JALR instruction by the compiler), and the semantic class distinguishes the two actions to take in the two cases that may arise, as discussed in the next two paragraphs. 
     When a native-convention function returns a double-precision (64-bit) floating-point value to an X86-convention caller (the RETURN-FP case of row  248 ), the function return value is inflated from an IEEE-754 64-bit representation in r31 (RVDP, the register in which Tapestry returns double-precision function results) to an 80-bit extended precision representation in the register pair to which the X86 FP top-of-stack currently points (usually r32-r33, the Tapestry register pair mapped to F0 of the X86). The top-of-floating-point stack register is marked full, and all other floating-point registers are marked empty. (Tapestry has a floating-point status register that subsumes the function of the X86 FPCW (floating-point control word), FPSW (floating-point status word), and FPTW (floating-point tag word), and the registers are marked full or empty in the tag bits of this status register.) 
     On a return from a non-floating-point Tapestry native callee function to an X86-convention caller (the RETURN-NO-FP case of row  248 ) to an X86-convention caller, the function return value is left alone in r48, because this single register is both the register in which the Tapestry function computed its result, and the register to which the X86 register EAX (the function-result return register) is mapped. The entire floating-point stack is marked empty. 
     If the native callee is returning a value larger than 64 bits to an X86-convention caller, a return descriptor stored on the stack indicates where the return value is stored (typically in registers r48 (RV0), r49 (RV1), r50 (RV2), and r51 (RV3), or in a memory location pointed to by r31 (RVA)); the return value is copied to the location specified under the X86 convention (typically a memory location whose address is stored in the argument block on the stack). 
     When returning from an X86 callee to a Tapestry-using-X86-convention caller, no action is required, because the register mapping of Table 1 implements the convention transformation. 
     When returning from an X86 callee to a native Tapestry caller, two cases are distinguished by the two semantic classes RETURN-MAYBE-FP and RETURN-NO-FP. For the RETURN-NO-FP case of rows  224  and  254 , no action is required, because the return value was computed into X86 register EAX, which is mapped to r48, the Tapestry scalar return value register. For the RETURN-MAYBE-FP case, the exception handler conservatively ensures that any scalar result is left in r48, and also ensures that the value from the top of the floating-point stack is deflated from an 80-bit extended-precision representation to a 64-bit double-precision representation in r31 (RVDP). 
     When executing translated native code, Tapestry will not execute a JALR subprogram return unless the destination is also in native code. Because the semantic class codes on the present implementation only ambiguously resolve whether an X86 instruction does or does not return a floating-point result (RETURN-FP vs. RETURN-MAYBE-FP), and the native semantic class codes are unambiguous (RETURN-FP vs. RETURN-NO-FP), binary translator  124  does not translate a final X86 RET unless its destination is also translated. 
     An alternative embodiment may provide a third calling convention value, a “transition” value. The machine will not take an exception when crossing to or from a transition page—the transition calling convention “matches” both the X86 calling convention and the Tapestry calling convention. Typically, pages of transition calling convention will have a Tapestry ISA value. These transition pages hold “glue” code that explicitly performs the transition work. For instance, an X86 caller that wants to call a Tapestry callee might first call a glue routine on a transition calling convention page. The glue routine copies arguments from their X86 calling convention homes to their Tapestry homes, and may perform other housekeeping. The glue routine then calls the Tapestry callee. The Tapestry callee returns to the glue routine, where the glue routine performs the return value copying and performs other housekeeping, and returns to its caller, the X86 caller. 
     One of ordinary skill will understand the argument copying that implements each of the cases of transition exception shown in  FIGS. 2   b  and  2   c . One embodiment is shown in full detail in the microfiche appendix of the &#39;394, &#39;443 and &#39;194 parent applications. 
     In an embodiment alternative to any of the broad designs laid out in sections II, III, or IV, the computer may provide three or more instruction set architectures, and/or three or more calling conventions. Each architecture or convention is assigned a code number, represented in two or more bits. Whenever the architecture crosses from a region or page with one code to a region or page with another, an appropriate adjustment is made to the hardware control, or an appropriate exception handler is invoked, to adjust the data content of the computer, and/or to explicitly control the hardware execution mode. 
     V. Profiling to Determine Hot Spots for Translation 
     V.A. Overview of Profiling 
     Referring to  FIGS. 1   a ,  1   b  and  4   a , profiler  400  monitors the execution of programs executing in X86 mode, and stores a stream of data representing the profile of the execution. Because the X86 instruction text is typically an off-the-shelf commercial binary, profiler  400  operates without modifying the X86 binary, or recompiling source code into special-purpose profileable X86 instruction text. The execution rules for profiler  400  are tailored so that the right information will be captured at the right time. Hot spot detector  122  identifies hot spots in the programs based on the profile data. The data collected by profiler  400  are sufficiently descriptive to allow the application of effective heuristics to determine the hot spots from the profile data alone, without further reference to the instruction text. In particular, the profile information indicates every byte of X86 object code that was fetched and executed, without leaving any non-sequential flow to inference. Further, the profile data are detailed enough, in combination with the X86 instruction text, to enable binary translation of any profiled range of X86 instruction text. The profile information annotates the X86 instruction text sufficiently well to resolve all ambiguity in the X86 object text, including ambiguity induced by data- or machine-context dependencies of the X86 instructions. Profiler  400  operates without modifying the X86 binary, or recompiling source code into a special-purpose profileable X86 binary. 
     In its most-common mode of operation, profiler  400  awaits a two-part trigger signal ( 516 ,  522  of  FIG. 5   a ) to start sampling events, and then records every profileable event  416  in a dense sequence, including every profileable event that occurs, until it stops (for instance, on exhaustion of the buffer into which profile information is being collected), as opposed to a conventional profiler that records every n th  event, or records a single event every n microseconds. The profile information records both the source and destination addresses of most control flow transfers. Entries describing individual events are collected into the machine&#39;s general register file, and then stored in a block as a profile packet. This blocking of events reduces memory access traffic and exception overhead. 
     Referring again to  FIGS. 1   a  and  1   b , profiler  400  tracks events by physical address, rather than by virtual address. Thus, a profileable event  416  may be induced by “straight line” flow in virtual address space, when two successive instructions are separated by a physical page boundary, or when a single instruction straddles a virtual page boundary. (As is known in the art, two pages that are sequential in a virtual address space may be stored far from each other in physical memory.) By managing the X86 pages in the physical address space, Tapestry operates at the level of the X86 hardware being emulated. Thus, the interfaces between Tapestry and X86 operating system  306  may be as well-defined and stable as the X86 architecture itself. This obviates any need to emulate or account for any policies or features managed by X86 operating system  306 . For instance, Tapestry can run any X86 operating system (any version of Microsoft Windows, Microsoft NT, or IBM OS/2, or any other operating system) without the need to account for different virtual memory policies, process or thread management, or mappings between logical and physical resources, and without any need to modify X86 operating system  306 . Second, if X86 operating system  306  shares the same physical page among multiple X86 processes, even if at different virtual pages, the page will be automatically shared. There will be a single page. Third, this has the advantage that pages freed deleted from an address space, and then remapped before being reclaimed and allocated to another use. 
     Referring to  FIG. 4   b , events are classified into a fairly fine taxonomy of about thirty classes. Events that may be recorded include jumps, subprogram CALL&#39;s and returns, interrupts, exceptions, traps into the kernel, changes to processor state that alters instruction interpretation, and sequential flow that crosses a page boundary. Forward and backward jumps, conditional and unconditional jumps, and near and far jumps are distinguished. 
     Referring to  FIGS. 4   g  and  4   h , profiler  400  has a number of features that allow profiling to be precisely controlled, so that the overhead of profiling can be limited to only those execution modes for which profile analysis is desired. 
     Referring to  FIGS. 5   a  and  5   b , as each X86 instruction is decoded by the converter ( 136  of  FIG. 1   c ), a profile entry is built up in a 64-bit processor register  594 . During execution of the instruction, register  594  may be modified and overwritten, particularly if the instruction traps into Tapestry operating system  312 . At the completion of the instruction, profiler  400  may choose to capture the contents of the profile entry processor register into a general register. 
     Hot spot detector  122  recognizes addresses that frequently recur in a set of profile packets. Once a hot spot is recognized, the surrounding entries in the profile may indicate (by physical address) a region of code that is frequently executed in correlation with the recurring address, and the path through the physical pages. Hot spot detector  122  conveys this information to TAXi translator  124 , which in turn translates the binary. 
     V.B. Profileable Events and Event Codes 
     Referring to  FIG. 4   b , profiler  400  recognizes and records about thirty classes of events, listed in the table. Each class of event has a code  402 , which is a number between 0 and 31, represented as a five-bit number. The class of events is chosen to provide both the minimum information required to support the design, and additional information that is not strictly necessary but may provide additional hints that allow hot spot detector  122  to achieve better results. 
     The upper half  410  of the table lists events that are (in one embodiment) raised by software, and lower half  404  contains events raised by hardware. The lower half will be discussed first. 
     The lower half  404  of the table, the sixteen entries whose high-order bit is One, are events induced by converter  136 . As each X86 instruction is decoded and executed, the events enumerated in lower half  404  are recognized. If profiler  400  is active when one of these events  404  occurs, a profile entry is recorded in a general register. The events in the lower half of the table fall into two classes: near transfers of control that are executed in converter  136 , and sequential flows of execution across a physical page frame boundary. 
     Profiler  400  captures transfers of control, including IP-relative transfers, subroutine calls and returns, jumps through pointers, and many interrupt-induced transfers. Even though profiler  400  views the machine in its physical address space, the distinction between forward and backwards jumps can be determined for PC-relative jumps by looking at the sign bit of the PC-relative displacement in the X86 instruction. Once the branch is classified, the classification is encoded in event code  402  stored in the profile entry for the branch. There are event codes  402  to separately classify forward conditional branches, backward conditional branches, three separate classes of conditional jump predicates, etc., as shown by event codes 1.0000, 1.0001, 1.0010, 1.0011, 1.0100, 1.0101, and 1.0111. 
     Event code 1.1100 is discussed in section VIII.B. 
     Event code 1.1110  406  indicates a simple sequential instruction with nothing of note. Event code 1.1111  408  denotes an instruction that either ends in the very last byte of a physical page or straddles a page boundary in virtual address space (and is likely separated into two distant portions in the physical address space). 
     The upper half  410  of the table, the top sixteen entries whose high-order bit is Zero, are events that are handled in software emulator  316 , and recorded during execution of a Tapestry RFE (return from exception) instruction at the completion of the emulation handler. RFE is the Tapestry instruction that returns from Tapestry operating system  312  to a user program after a synchronous exception, (for instance a page fault or NaN-producing floating-point exception), an asynchronous external interrupt, or a trap into emulator  3 16 for simulation of a particularly complex X86 instruction that is not implemented in the hardware converter  136 . Generally, the events in the upper half of the table fall into four classes: (1) far control transfer instructions executed in emulator  316 , (2) instructions that update the x86 execution context (e.g. FRSTOR) executed in emulator  316 , (3) delivery of x86 internal, synchronous interrupts, and (4) delivery of x86 external, asynchronous interrupts. In general the upper-half event codes are known only to software. 
     Each RFE instruction includes a 4-bit immediate field ( 588  of  FIG. 5   b ) in which is stored the low-order four bits of the event code  402  associated with the event that invokes the returned-from handler. The fifth bit in an RFE event class is reconstructed (see section V.G, infra) as a Zero, even though the Zero is not explicitly stored. When the RFE is executed, the event code from the RFE is copied into TAXi_State.Event_Code-Latch ( 486 ,  487  of  FIGS. 4   h  and  5   b ) and the temporary processor register ( 594  of  FIG. 5   b ) that collects profile information (see section V.F, infra), overwriting the event code supplied by converter  136 . From register  594 , the event code will be copied into a general register if a profile entry is to be collected. This mechanism allows software to signal profiler  400  hardware  510  that a profileable instruction has been executed in emulator  316 , or that an otherwise non-profileable instruction executed in emulator  316  caused a page crossing and should be profiled for that reason. (RFE&#39;s without X86 significance will set this field to zero, which will prevent the hardware from storing a profile entry—see the discussion of code 0.0000, infra). 
     The “profileable event” column ( 416  of  FIG. 4   b ) specifies whether an the event code is to be included in a profile packet. Events that are not profileable simply occur with no action being taken by profiler  400 . The “initiate packet” column  418  specifies whether an event of this event code ( 402  of  FIG. 4   b ) is allowed to initiate collection of a new profile packet, or whether this event class may only be recorded in entries after the first. “Initiate packet”  418  is discussed at length in sections V.F and V.G, infra, in connection with Context_At_Point profile entries,  FIG. 4   c , and the profiler state machine  510 ,  FIG. 5   a . The “probeable event” column  610  and “probe event bit” column  612  will be discussed in connection with Probing, section VI, infra. The “initiate packet”  418 , “profileable event”  416 , and “probeable event”  610  properties are computed by PLA (programmable logic array)  650 , which is discussed in sections VI.C and VI.D, infra. 
     Discussion of event codes 0.0000, 0.0001, 0.0010 and 0.0011 is deferred for a few paragraphs. 
     An event code of 0.0100 is simply stored over the current value of TAXi_State.Event_Code_Latch ( 486 ,  487  of  FIG. 4   h  and  5   b ), without further effect of the current state of the machine. The effect of this overwrite is to clear the previously-stored event code, ensuring that converter  136  can restart without any effects that might be triggered by the current content of TAXi_State.Event_Code_Latch  486 ,  487 . For instance, if converter  136  takes a probe exception (see section VI, infra), and the first instruction of the translated TAXi code generates an exception (e.g., a floating-point overflow) that should be handled by returning control to converter  136  (rather than allowing execution to resume in the translated TAXi code), the exception handler will return with an RFE whose event code immediate field is 0.0100. This ensures that converter  136  will not restart with the event code pending in TAXi_State.Event_Code_Latch  486 ,  487  that triggered the probe exception in the first place. 
     Event code 0.0101 indicates an emulator completion of an instruction that changes the execution context, for instance, the full/empty state of the floating-point registers or floating-point top-of-stack. This will force the recording of Context_At_Point profile entry (see  430  of  FIG. 4   c  and section V.C, infra) to capture the state change. 
     Events of event code 0.0110, 0.0111, 0.1000, 0.1001 are control-transfer instructions that are conveniently implemented in emulation software instead of hardware converter  134 ,  136  such as far call, far jump, far return, and X86 interrupt return. The event code taxonomy for these far transfers does not differentiate forward and backward jumps, in contrast to the taxonomy of IP-relative near jumps (event codes 1.0000-1.0101). 
     An RFE with an event code of 0.1010 causes TAXi_Control.special_opcode  474  (bits &lt;50:44&gt;) to be captured in the special_opcode  434  field (bits &lt;50:43&gt; of  FIG. 4   c ) of a Context_At_Point profile entry ( 430  of  FIG. 4   c ). This opens up a new seven-bit space of event codes that can be managed completely by software. 
     Event code 0.1011 is used to RFE from an exception handler, to force the current profile packet to be aborted. The Tapestry hardware recognizes the event code in the RFE immediate field and aborts the profile packet by clearing TAXi-State.Profile_Active ( 482  of  FIGS. 4   h  and  5   a ). For instance, this event code might be used after a successful probe RFE&#39;s to TAXi code and aborts any packet in progress. This is because the TAXi code represent a break in the sequential interval of a profile packet, and an attempt to continue the packet would render it ill-formed. Similarly, when X86 single-step mode is enabled, the RFE from emulator  316  uses event code 0.1011 to abort a packet in progress. Profiling will resume at the next profile timer expiry. 
     Event codes 0.1100, 0.1101, 0.1110, and 0.1111 provide two pairs of RFE event codes associated with delivery of X86 exceptions from X86 emulator  316 . This allows software to group exceptions into different categories for TAXi usage. By classifying interrupts into two groups, and further into probeable and non-probeable events (see section VI, infra), these four event codes provide a control framework for software to build upon. This classification exploits the fact that the X86 funnels all exceptions, external interrupts, and traps through a single unified “interrupt” mechanism. 
     Event codes 0.0000, 0.0001, 0.0010, and 0.0011  412  operate somewhat differently from the other events in upper half  410 , as shown by the “reuse event code” column  414 . Events of these classes (that is, RFE instructions with these four-bit codes in their event code immediate field) do not update TAXi_State.Event_Code_Latch ( 486 ,  487  of  FIG. 4   h ) and related signals; the previously-latched event code is simply allowed to persist for the next X86 instruction. For example, event code 0.0000 is for “transparent” exceptions, exceptions that do not get recorded in the profile. As a specific example, the RFE&#39;s at the end of the handlers for TLB miss exceptions, interrupt service routines for purely-Tapestry interrupts, and other exceptions unrelated to the progress of an X86 program have event code 0.0000 (four explicit Zeros in the immediate field, and an assumed high-order Zero), which causes the hardware to resume execution at the interrupted location without storing a profile entry. These events are kept architecturally invisible to the currently-executing process and are not correlated to any hot spot in that process, and thus recording an event would be specious. 
     Event code 0.0001 is used in the software X86 emulator  316 . Very complex X86 CISC instructions that are not implemented in hardware converter  136  are instead implemented as a trap into software, where the instruction is emulated. When X86 emulator  316  completes the instruction, it returns using an RFE with an event code of 0.0001 to indicate that “nothing special happened here,” and so no profile entry is collected (unless the emulated instruction straddled a page). 
     Another use of the “reuse event code” feature of column  414  is illustrated by considering the case of a complex instruction, an instruction that is emulated in software, that does not affect any control flow, for instance a compare string instruction. When such a complex instruction is encountered, converter  136 , non-event circuit  578 , and MUX  580  of  FIG. 5   b  in section V.F, infra, will have made a preliminary decode of the instruction, and supplied a preliminary event code ( 582 ,  592  of  FIG. 5   b ): either the default event code 1.1110  406  or a new page event code 1.1111  408 , depending on whether the instruction straddles a page break. (In some embodiments, converter  136  may in addition supply the event codes for far control transfers, far CALL, code 0.1000; far JMP, code 0.1001; far RET, code 0.0110; IRET, code 0.0111). This preliminary event code  582 ,  592  is latched into TAXi_State.Event_Code_Latch  486 ,  487  as part of trapping into X86 emulator  316 . When X86 emulator  316  completes the complex instruction and RFE&#39;s back to converter  136 , the RFE will have as its event code immediate field ( 588  of  FIG. 5   b ) the simple X86 instruction-complete event code 0.0001. Because event code 0.0001 has “reuse event code” property  414 , the event code from the RFE immediate field will simply be discarded, leaving intact the preliminary event code  582 ,  592  in TAXi_State.Event_Code_Latch  486 ,  487 . On return from the exception, an event with the preliminary event code is then added to the profile packet. 
     Event codes 0.0010 and 0.0011 are used in the RFE from the probe exception handler (see section VI, infra). If a probe fails, that class of probe is disabled. Because probing and profiling are mutually exclusive (see section VI.G, infra), when there is a probe exception, profiling is not active. Thus, these event codes are never stored in a profile packet, but exist to control prober  600 , as described in section VI.D, infra. 
     V.C. Storage Form for Profiled Events 
     Referring to  FIGS. 4   a ,  4   c , and  4   d , profile events are collected and stored in groups called packets  420 . Each profile packet  420  holds a programmable number of entries, initially collected into registers R16-R31, and then stored to memory. In a typical use, there will be sixteen entries per packet, beginning with a 64-bit time stamp, then fourteen event entries  430 ,  440 , and an ending time stamp. Each event is described as a 64-bit entry, of one of two forms: a Context_At_Point entry  430 , or a Near_Edge entry  440 . The first entry in the packet is always a Context_At_Point entry  430 , which gives a relatively complete snapshot of the processor context at the point that profiling begins, a point conceptually between two X86 instructions. Subsequent entries may be of either Context_At_Point or Near_Edge form. A Near_Edge entry  440  describes an intra-segment (i.e., “near”) control transfer, giving the source and destination of the transfer. At a Near_Edge entry  440 , the remainder of the X86 processor context can be determined by starting at the most-recent Context_At_Point entry  430  and inferring the processor context by interpreting the instructions that intervened between that Context_At_Point and the Near_Edge transfer. Sufficient information is present in the profile so that the context can be inferred by binary translator  124  by reference only to the opcodes of those intervening instructions, without requiring any knowledge of the actual data consumed or manipulated by those instructions. The rules for emitting a Context_At_Point entry  430  preserve this invariant: processor context is inferable from the combination of the profile and the opcodes of the intervening instructions, without reference to any data consumed or manipulated by the instructions. If execution of an X86 instruction depends on memory data or the processor context bits in a manner not representable in a Near_Edge entry  440 , then profiler  400  emits a Context_At_Point entry  430 . Thus, Context_At_Point entries ensure that the TAXi binary translator  124  has sufficient information to resolve ambiguity in the X86 instruction stream, in order to generate native Tapestry code. 
     Referring to  FIG. 4   c , a Context_At_Point entry  430  describes an X86 instruction boundary context snapshot, a context in effect as execution of an X86 instruction is about to begin. 
     Bits &lt;63:60&gt;  431  of a Context_At_Point entry  430  are all Zero, to distinguish a Context_At_Point entry  430  from a Near_Edge entry  440 . (As noted in the discussion of done_length  441 , bits &lt;63:60&gt; of  FIG. 4   d , infra, in a Near_Edge  440  the first four bits record the length of an instruction, and there are no zero-length instructions. Thus, a zero value in field  431  unambiguously indicates a Context_At_Point  430 .) 
     Bits &lt;59:51&gt;  432 ,  433  and &lt;42:32&gt;  435  capture the processor mode context of the X86 at the instruction boundary (before the start of the instruction described in next_frame  438  and next_byte  439 , bits &lt;27:00&gt;). The bits of an X86 instruction do not completely specify the action of the instruction; the X86 architecture defines a number of state bits that define the processor context and the operation of instructions. These bits determine operand size (whether a given wide form instruction acts on 16 bits or 32), stack size (whether a PUSH or POP instruction updates 16 bits or 32 of the stack pointer), address size (whether addresses are 16 or 32 bits), whether the processor is in V86 mode, whether addressing is physical or virtual, the floating-point stack pointer, and the full/empty state of floating-point registers. The X86 scatters these bits around code and stack segment descriptors, the EFLAGS register, the floating-point status word, the floating-point tag word, and other places. The Tapestry machine stores these bits in analogs of the X86 structures to actually control the machine; when a Context_At_Point entry  430  is captured, a snapshot of these bits are captured into bits &lt;59:51&gt;  432 ,  433  and &lt;42:32&gt;  435  of the Context_At_Point entry  430 . 
     Bits &lt;59:56&gt;  432  indicate the current state of the operand-size/address-size mode (encoded in the D bit of the X86 code segment descriptor), and the stack address size (encoded in the B bit of the stack segment descriptor). Bit &lt;59&gt;, “c1s1,” indicates that the X86 is in 32-bit-code/32-bit-stack mode. Bit &lt;58&gt;, “c1s0,” indicates that the X86 is in 32-bit-code/16-bit-stack mode. Bit &lt;57&gt;, “c0s1,” indicates that the X86 is in 16-bit-code/32-bit-stack mode. Bit &lt;56&gt;, “c0s0,” indicates that the X86 is in 16-bit-code/16-bit-stack mode. (The D and B bits render the X86 instruction set ambiguous. For instance, a given nine-byte sequence of the instruction stream might be interpreted as a single instruction on one execution, and three entirely different instructions on the next, depending on the values of the D and B bits. Very few architectures share this ambiguity.) Thus, whether or not to profile any particular combination of the four possible combinations of D and B modes can be individually controlled. 
     In field  433 , bit &lt;55&gt;, “pnz,” indicates that the X86 is in non-ring-zero (unprivileged) mode. Bit &lt;54&gt;, “pez,” indicates that the X86 is in X86 ring-zero (privileged) mode. Bits &lt;53&gt;, &lt;52&gt;, and &lt;51&gt;, “v86,” “real,” and “smm,” indicate respectively, that the X86 is in virtual-8086, real, and system management execution modes, as indicated by X86 system flag bits. 
     Bits &lt;50:43&gt;, special_opcode  434 , are filled from TAXi_Control.special_opcode  474  whenever a Context_At_Point entry is generated. These bits are especially relevant to event code 0.1010. 
     In field  435 , bits &lt;42:40&gt; are the floating-point top-of-stack pointer. Bits &lt;39:32&gt; are the floating-point register full/empty bits. 
     Field event_code  436 , bits &lt;31:28&gt;, contains an event code  402 , the four least significant bits from the most recently executed RFE or converter event code (from  FIG. 4   b ). The four bits of the Context_At_Point event_code  436  are the four low order bits of the event code  402  of  FIG. 4   b . The high-order bit is derived from these four by a method that will be described in section V.G, infra. As will be described more fully there, a Context_At_Point entry  430  can describe any of the sixteen events from the upper half  410  of the table, or an event with the “initiate packet” property  418  from anywhere in the table of  FIG. 4   b.    
     Bits &lt;27:00&gt; describe the next X86 instruction, the instruction about to be executed at the time that the Context_At_Point context was snapshotted. Field next_frame  438 , bits &lt;27:12&gt;, give a physical page frame number, and field next_byte  439 , bits &lt;11:00&gt;, give a 12-bit offset into the page. 
     Referring to  FIG. 4   d , a Near_Edge entry  440  describes a completed X86 intra-segment “near” control transfer instruction. Bits &lt;63:60&gt;  441  of a Near_Edge entry  440  describe the length of the transfer instruction. The length  441  value is between one and fifteen (the shortest X86 instruction is one byte, and the longest is fifteen bytes). Because a zero length cannot occur, these four bits  431  distinguish a Near_Edge entry  440  from a Context_At_Point entry  430 . 
     The instruction at the source end of the Near_Edge transfer is described by a page frame number in which the instruction begins, a page frame number in which the instruction ends, a byte offset into the page where the instruction begins, and an instruction length. The page frame number for the beginning of the instruction is not explicitly represented in the Near_Edge entry  440 , but rather is inherited as the next_frame value  438 ,  448  from the immediately-preceding entry in the profile packet (recall that profile packet always start with a Context_At_Point entry  430 , and that a Near_Edge entry  440  is never the first entry). The page frame in which the last byte of the instruction lies is represented in field done_frame  444 , bits &lt;59:44&gt;. These two page frame numbers will differ if the instruction straddles a page boundary. The byte offset into the page where the instruction begins is represented in field done_byte  445 , bits &lt;43:32&gt;. The length is recorded in field done_length  441 , bits &lt;63:60&gt;. Thus, the source instruction ends at the byte found by summing (((done_byte  445 +done_length  441 )−1) mod 4096) (4096 because that is the size of an X86 page). 
     The destination of the Near_Edge transfer is described by next_frame  448  and next_byte  449  fields in bits &lt;27:00&gt;, in the manner of the next_frame  438  and next_byte  439  fields, bits &lt;27:00&gt;, described supra for a Context_At_Point entry  430 . 
     Field event_code  446 , bits &lt;31:28&gt;, contains an event code, parallel to the event code  436  of a Context_At_Point entry  430 . The four bits of the Near_Edge event_code  446  are the four low order bits of the bottom half of  FIG. 4   b ; a leading One is assumed. (Thus a Near_Edge entry  440  can only describe one of the sixteen events in the lower half  404  of  FIG. 4   b .) 
     Thus, all physical pages are mentioned in successive profile entries in their execution order. When execution crosses from one physical page to another because of an explicit branch, the branch is indicated by a Near_Edge entry  440 . When execution crosses from one physical page to another because of sequential execution in virtual address space across a page boundary, a Near_Edge entry  440  will be generated either between the instruction that ended at the end of the page and the instruction that begins the next, or between the instruction that straddles the page break and the first full instruction of the next page. Alternatively, if control enters a page without a Near_Edge event, a Context_At_Point profile entry  430  will describe the arrival at the page. Together, these rules ensure that sufficient information exists in the profile entries that the flow of execution can be retraced, and a hot spot detected, without reference to the binary text. Allowing the hot spot detector to operate without examining the instruction text allows it to run without polluting the cache. Further, the guarantee that all physical pages are mentioned allows for profiling of the program as it exists in the physical memory, even though the X86 executes the instructions from the virtual address space. The guarantee ensures that control flow can be traced through the physical memory, without the need to examine the program text to infer adjacency relationships. 
     For a Near_Edge entry  440 , the X86 processor context on arrival at the destination instruction is inferable from fields  432 ,  433  (bits &lt;59:51&gt;) and  435  (bits 42:32&gt;) of the nearest-preceding Context_At_Point entry  430 , by starting with the context  432 ,  433 ,  435  encoded in that Context_At_Point  430 , and tracing forward through the opcodes of the intervening instructions to capture any updates. 
     V.D. Profile Information Collected for a Specific Example Event—a Page Straddle 
     Referring to  FIGS. 4   e  and  4   f , consider two instances of instructions that straddle a page boundary.  FIGS. 4   e  and  4   f  are drawn in virtual address space, though profiler  400  operates in physical address space. 
     In  FIG. 4   e , consider instruction  450  that straddles a page boundary  451  between pages  452  and  453 , and is not a transfer-of-control instruction. The page-crossing is described by a Near_Edge entry  440 ,  454  with a sequential event code, code 1.1110 ( 406  of  FIG. 4   b ). The instruction begins in the page  452  identified in the next_frame bits (bits &lt;27:12&gt;)  438 ,  448 ,  452   a  of the immediately previous profile entry  455 , whether that previous entry is a Context_At_Point  430  or a Near_Edge  440 . The instruction begins at a byte offset indicated by done_byte  445  (bits &lt;43:32&gt;) of current Near_Edge  454 . The length of the instruction is indicated in done_length  441  (bits &lt;63:60&gt;) of current Near_Edge  454 . The last byte of the instruction is in page  453 , indicated by done_frame (bits &lt;27:12&gt;)  444 ,  453   a  of current Near_Edge  454 . The last byte of the instruction will fall at byte (((done_byte  445  (bits &lt;43:32&gt;)+done_length  441  (bits &lt;63:60&gt;)−1) mod 4096)), which will necessarily equal ((next_byte  449  (bits &lt;11:00&gt;)−1) mod 4096). The first byte of the next sequential instruction  456  falls in page  453 , as indicated in next_frame  448 ,  456   a  (bits &lt;27:12&gt;) of current Near_Edge  440 ,  454 , at byte next_byte  449  (bits &lt;11:00&gt;). Because the maximum length  441  of an instruction (fifteen bytes) is less than the length of a page, done_frame  453   a  of previous profile entry  455  will necessarily equal Next_Frame  456   a  of current Near_Edge  454  in the page-straddling-instruction case shown in  FIG. 4   e.    
     If instruction  450  is entirely within page  452  and ends exactly at the page boundary  451 , and is not a control transfer (or is a control transfer that falls through sequentially), then a Near_Edge  440 ,  454  will be generated whose done_frame  453   a  will point to page  452 , and whose next_frame  456   a  will point to the following page. 
     Referring to  FIG. 4   f , consider another example, a page-straddle control transfer instruction  450  that touches three pages, the two pages  452 ,  453  on which the source instruction itself is coded, and page  458  on which the destination instruction  457  begins. Event code  446  of current Near_Edge entry  454  records the nature of the control transfer, codes 1.0000 through 1.1100 ( FIG. 4   b ). As in the sequential flow case of  FIG. 4   e , transfer instruction  450  begins in page  452 , as indicated identified in next_frame field  438 ,  448 ,  452   a  of immediately previous profile entry  455 , at a byte offset indicated by next_byte  439  (bits &lt;43:32&gt;) of current Near_Edge  455 . The length of instruction  450  is indicated in done_length  441  of current Near_Edge  454 . Instruction  450  ends in page  453 , as indicated by done_frame  444 ,  453   a  (bits &lt;59:44&gt;) of current Near_Edge  440 ,  454 , at byte ((done_byte  445  (bits &lt;43:32&gt;)+done_length  441  (bits &lt;63:60&gt;)−1) mod 4096), each taken from the current Near_Edge  440 ,  454 . Destination instruction  457  begins in page  458 , as indicated by next_frame  448 ,  458   a  (bits &lt;27:12&gt;) of the current Near_Edge  454 , at byte offset next_byte  449  (bits &lt;11:00&gt;). For a page-straddling branch  450 , done_frame  444 ,  453   a  (bits &lt;59:44&gt;) of current Near_Edge  454  now disagrees with the next_frame  438 ,  448  of the previous entry, because of the page straddle. 
     If a profile packet is initiated on a control transfer instruction, the first entry will be a Context_At_Point entry  430  pointing to the target of the transfer instruction. 
     Referring to  FIG. 4   a , the Near_Edge  440  and Context_At_Point  430  entries together provide a compact, efficient description of even the most complex control flow, giving enough information to allow hot spot detector  122  and TAXi binary translator  124  to work, without overwhelming them with an overabundance of information that is not useful for these two tasks. Note that the requirements of hot spot detector  122  and TAXi binary translator  124  are somewhat different, so the information in the profile is designed to superset the requirements of the two. 
     In some embodiments, it may be desirable to record a range as the first byte of the first instruction to the first byte of the last instruction. Recording ranges in this manner is particularly attractive if the architecture has fixed-length instructions. 
     V.E. Control Registers Controlling the Profiler 
     Referring to  FIG. 4   g , the TAXi hardware system is controlled by a 64-bit register called TAXi_Control  460 . TAXi_Control  460  allows fine control over profiling. Because much of the system is driven by the profile, fine control over profiling gives fine control over the entire TAXi system. The various bits allow for enabling and disabling separate pieces of the TAXi mechanism, enabling and disabling profiling for code that meets or does not meet certain criteria, and timer controls that control rates of certain events. In any code region for which profiling is disabled, the TAXi resources will be quiescent, and impose no overhead. 
     In a typical embodiment, the contents of TAXi_Control register  460  will be written once during system initialization, to values determined by system tuning before shipment. In other embodiments, the values may be manipulated on the fly, to adapt to particular systems&#39; usage patterns. The one exception is the special_opcode field  434 , discussed infra. 
     Bit &lt;63&gt;, probe  676  is use to enable or disable the probe exception, and will be discussed in more detail in connection with probing, section VI, infra. Bit &lt;62&gt;, Profile_Enable  464 , “prof,” enables and disables profile trace packet collection and delivery of the profile trace-packet complete exception. The probe  676  and Profile_Enable  4 64 bits will typically be manipulated to disable TAXi operation any time the hardware debugging resources are active. 
     Bit &lt;61&gt;, tio  820 , indirectly controls the TAXi I/O exception, to provide one of the guards that implement the safety net introduced at section I.D, supra, and described in further detail in section VIII.A, infra. 
     Bit &lt;60&gt;, unpr  468 , enables and disables the unprotected exception, discussed in section I.F, supra. Unprotected exceptions are only raised when profiling on unprotected pages. 
     Field  470 , bits &lt;59:56&gt; control the code segment/stack segment size combinations that will be profiled. Bit &lt;59&gt;, “c1s1,” enables profiling for portions of the program whose X86 code segment has its 32-bit default operand-size/address-size bit set, and uses a stack in a segment whose 32-bit stack bit is set. Bit &lt;58&gt;, “c1s0,” enables profiling for 32-bit operand/address, 16-bit stack segments. Bit &lt;57&gt;, “c0s1,” enables profiling for 16-bit operand/address, 32-bit stack segments. Bit &lt;56&gt;, “c0s0,” enables profiling for 16-bit operand/address, 16-bit stack segments. 
     Bit &lt;55&gt;, “pnz,” enables profiling for code in privilege rings one, two, and three (Not Equal to Zero). 
     Bit &lt;54&gt;, “pez,” enables profiling for code in privilege ring zero (Equal to Zero). 
     Bits &lt;53&gt;, &lt;52&gt;, and &lt;51&gt;, “v86,” “real,” and “smm” (with the size and mode controls of bits &lt;59:54&gt;, collectively known as the Global_TAXi_Enables bits  470 ,  472 ), enable and disable profiling for code in the virtual-8086, real, and system management execution modes of the X86 (these execution modes indicated by system flags and the IOPL field in the X86 EFLAGS register). If a given X86 execution mode is not supported by TAXi (in the sense that TAXi will not attempt to produce translated native Tapestry binaries for code of that X86 mode), the system is designed to impose no overhead on code in that mode. Thus, when the Global_TAXi_Enables  470 ,  472  bit for a mode is Zero and virtual X86  310  is executing in that mode, then execution is not profiled, the profile timer ( 492  of  FIG. 4   i ) does not run, and the profile, unprotected, and probe exceptions are all inhibited. 
     Bits &lt;50:44&gt;, special_opcode  474  are used to set the contents of Context_At_Point profile entries  430 . X86 emulator  316  sets special_opcode  474  to a desired value. When an RFE with event code 0.1010 ( FIG. 4   b ) is subsequently executed, the contents of TAXi_Control.special_opcode  474  are copied unmodified into the special_opcode field  434  (bits &lt;50:44&gt;) of a Context_At_Point event  430 . 
     Bits &lt;43:38&gt;, Packet_Reg_First  476 , and &lt;37:32&gt;, Packet_Reg_Last  478 , specify a range of the general registers to be used to accumulate profile trace packets. The first Context_At_Point entry  430  of a packet will be stored in the register pointed to by Packet_Reg_First  476 , then the next entry in register Packet_Reg_First+1, and so on, until the last entry is stored in Packet_Reg_Last  478 . Then a “profile full” exception will be raised ( 536 ,  548  of  FIG. 5   a ), so that the profile registers can be spilled to memory. As shown in Table 1, typically Packet_Reg_First  476  will be set to 17, and Packet_Reg_Last  478  to 31. 
     Bits &lt;31:16&gt;, Profile_Timer_Reload_Constant  494 , and &lt;15:00&gt; Probe_Timer_Reload_Constant  632  (bits &lt;15:00&gt;) are used to control the rate of profile trace-packet collection and probing respectively. This is further discussed in connection with the TAXi_Timers register ( 490 ,  630  of  FIG. 4   i ; see the discussion of  FIG. 4   i  infra, and the discussion of probing in sections VI.C and VI.D, infra). 
     Referring to  FIG. 4   h , the internal state of the TAXi system is available by looking at a register called TAXi_State  480 . In the normal running of the system, the TAXi_State register  480  is read-only, though it is read-write during context switching or design verification. 
     Bit &lt;15&gt;, “preq” or “Profile_Request”  484 , indicates that profile timer  492  has expired and posted the request to collect another packet, but either no event has yet been encountered to initiate the packet, or profile timer  492  expired while a packet was actively being collected. 
     Bit &lt;31&gt;, “pact” or “Profile_Active”  482 , indicates that preq “Profile-Request”  484  was set and that an Initiate Packet event ( 418  of  FIG. 4   b ) was encountered and a profile packet has been initiated and is in progress, but the profile registers are not yet filled. 
     The unused bits of the register are labeled “mbz” for “must be zero.” 
     The “Decoded_Probe_Event”  680  and “Probe_Mask”  620  fields will be discussed in section VI, infra. 
     The “Event_Code_Latch” field  486 ,  487 , bits &lt;12:08&gt;, records a 5-bit event code (the event codes of  FIG. 4   b , or the four-bit events of a Context_At_Point entry  430  of  FIG. 4   c  or Near_Edge profile entry  440  of  FIG. 4   d ), as a retrospective view of the last event that was generated in converter  136  or encoded as the immediate field in an RFE instruction ( 588  of  FIG. 5   b ). Event_Code_Latch  486 ,  487  serves as an architecturally visible place to log the event code until the next logical cycle of this process. The four low order bits  486  are supplied by the RFE immediate field  588  or four bits from converter  136  ( 582  of  FIG. 5   b ). The high-order bit  487  is supplied by context, either One for events from converter  136 , or Zero for events from an RFE. 
     The “Packet_Reg” field  489 , bits &lt;05:00&gt;, gives the number of the register into which the next profile entry will be written, as a post-increment direct address into the register file. When TAXi_State.Packet_Reg  489  exceeds TAXi_Control.Packet_Reg_Last  478 , profile collection is terminated, a Profile Packet Complete exception is raised, and the value of TAXi_State.Packet_Reg is reset to TAXi_Control.Packet_Reg_First  476 . 
     Referring to  FIG. 4   i , TAXi_Timers register  490  has two sixteen-bit countdown timers  492 ,  630 . 
     TAXi_Timers.Profile_Timer  492  (bits &lt;31:16&gt;) counts down at the CPU clock frequency when profile collection is enabled as described in the following paragraph. Profile_Timer  492  is an unsigned value that counts down to zero. On expiry, hardware reloads profile timer  492  with the value TAXi_Control.Profile_Timer_Reload_Constant ( 494  of  FIG. 4   g ). Profile_Timer  492  continually counts down and reloads. The transition to zero is decoded as timer expiration as defined in the profile exception state diagram ( FIG. 5   a ). 
     Profile collection is enabled, and profile timer  492  runs, when these five conditions are met: (1) TAXi_Control.Profile_Enable  464  is One, (2) converter  136  is active (PSW.ISA bit  194  indicates X86, see section II, supra), (3) all bytes of the current instruction have 4K page I-TLB entries, (4) all bytes of the current instruction have I-TLB page attributes in well-behaved memory (Address space zero, with D-TLB.ASI=Zero, is well-behaved, and the other address spaces are assumed to reference non-well-behaved memory) and (5) the machine is currently executing in a mode enabled in the TAXi_Control.Global_TAXi_Enables bits  470 ,  472  (bits &lt;59:51&gt;). When X86 debugging or single-step operation is requested, software clears TAXi_Control.Profile_Enable  464  to disable profile collection. 
     TAXi_Timers.Probe_Timer  630  (bits &lt;15:00&gt;) is discussed in sections VI.C and VI.D, infra. 
     V.F. The Profiler State Machine and Operation of the Profiler 
     Referring to  FIG. 5   a , profiler  400  operates according to state machine  510 . The four states  512 ,  518 ,  530 ,  542  of state machine  510  are identified by the values of the TAXi_State.Profile_Active  482  and TAXi_State.Profile_Request  484  bits. The transitions of TAXi_State.Profile_Active  482  and TAXi_State.Profile-Request  484  bits, and thus of state machine  510 , are triggered by timer expiry, profileable events, and packet aborts. Event “pe” indicates completion of a profileable event in the execution of the X86 program, one of the events enumerated as “profileable”  416  in table of  FIG. 4   b . Timer expiry is the countdown-to-zero-and-reset of timer TAXi_Timers.Profile_Timer  492 , as described in connection with  FIG. 4   i , supra. Aborts are described further infra. 
     State  512  is the initial state, with Profile_Active  482  (PA) and Profile_Request  484  (PR) both equal to Zero. In state  512 , profileable events  416  and abort events are ignored, as indicated by the loop transition  514  labeled “pe, ap.” When the profile timer  492  expires, TAXi_State.Profile_Request  484  is set to One, which transitions  516  state machine  510  to state  518 . 
     In state  518 , Profile_Request  484  is One and Profile_Active  482  is Zero, indicating that the Profile_Timer  492  has expired, priming profiler  400  to begin collecting a profile packet. But that first profileable event  416 ,  418  has not yet occurred, so profiling is not yet in active progress. In state  518 , further timer expirations are ignored (loop transition  520 ), rather than queued. Aborts are also ignored (loop transition  520 ), as there is no profile packet content to abort. 
     The first entry in a profile packet is always an event with the “Initiate Packet” property ( 418  of  FIG. 4   b ). State  518  waits until the first “initiate packet” pe init  event  418  occurs, initiating transition  522 . Profileable events ( 416  of  FIG. 4   b ) that are not “Initiate Packet” events ( 418  of  FIG. 4   b ) are ignored, indicated by the “pe init ” label on loop transition  520 . On transition  522 , several actions  524  are initiated. TAXi_State.Packet_Reg  489  is initialized from TAXi_Control.Packet_Reg_First  476 . The hardware captures a timestamp from the Global_Timestamp processor register into the Packet_Timestamp control register (or, in an alternative embodiment, into the general register preceding the first profile event capture register). A Context_At_Point profile entry  430  is captured into the general register indicated by TAXi_State.Packet_Reg  489 . At decision box  526 , TAXi_State.Packet_Reg  489  is incremented, and compared against TAXi_Control.Packet_Reg_Last  478 . For the first profile entry, the packet registers will never be full, so control follows path  528 . TAXi_State.Profile_Active  482  is set to One, and TAXi_State.Profile_Request  484  is cleared to Zero, putting state machine  510  in state  530 . 
     This first entry in a packet is the only circumstance in which converter  136  can generate a Context_At_Point entry  430 . For second-and-following entries in a profile packet, converter  136  only generates Near_Edge entries  440 . Any subsequent Context_At_Point entry  430  in the packet is generated by the RFE mechanism. 
     In state  530 , Profile_Request  484  is Zero and Profile_Active  482  is One. At least one profileable event ( 416  of  FIG. 4   b ) has been recognized and recorded, a profile packet  420  is in progress, and profiler  400  is awaiting the next profileable event  416 . When the next profileable event  416  occurs  532 , the profileable event is recorded  534  in the general register indicated by TAXi_State.Packet_Reg  489 . After the event is captured by a TAXi instruction (see discussion of  FIG. 5   b , infra), control reaches decision box  526 . If the range of profile registers is not full (TAXi_State.Packet_Reg  489  ++&lt;TAXi_Control.Packet_Reg_Last  478 —the old value of TAXi_State.Packet_Reg  489  is tested and then TAXi_State.Packet_Reg  489  is incremented), then control returns  528  to state  530  to collect more profileable events  416 . If the profile registers are full (TAXi_State.Packet_Reg  489  equals TAXi_Control.Packet_Reg_Last  478 ), then the machine takes a profile exception  536 . TAXi_State.Packet_Reg  489  is incremented after the comparison. The profile exception handler stores the collected profile into a ring buffer in memory, along with the timestamp captured by action  524 . The ring buffer write pointer, pointing to the next location in the ring buffer, is maintained in R15 (“RingBuf” of Table 1). After the collected profile packet is stored at the location indicated by R15, R15 is postincremented by the size of a profile packet. TAXi_State.Profile_Active  482  and TAXi_State.Profile_Request  484  are both cleared to Zero, and control returns  538  to start state  512 . 
     If TAXi_Timers.Profile_Timer  492  expires while state machine  510  is in state  530 , that is, while a profile packet was in progress, state machine  510  sets TAXi_State.Profile_Active  482  and TAXi_State.Profile_Request  484  both to One, and transitions  540  to state  542 . 
     The behavior of state  542  is largely similar to state  530 , in that a partially-complete packet is in progress, and new profileable events  416  are logged  544  as they occur. The difference between states  530  and  542  arises when the packet is complete. A profile-registers-full exception  548  from state  542  spills the profile registers to memory, just as profile exception  536 , but then as part of transition  546 , TAXi_State.Profile_Request  484  is set to One, to transition to state  518 , instead of to Zero as in transition  538 , which transitions into start state  512  to await the next timer expiry  516 . From state  518 , collection of the next packet can begin immediately on the next “initiate packet” event  418 , rather than awaiting another timer expiry  516 . This effects one level of queuing of pending timer expiries. 
     Collection of a profile packet may be aborted  550 ,  552  mid-packet by a number of events. For instance, an abort packet event code is provided (row 0.1011 of  FIG. 4   b )—an RFE with this event code clears TAXi_State.Profile_Active  482 , which in turn discards the current profile packet and aborts profile collection until at least the next profile timer expiry. If the predicate for enabling profiling (from the discussion of TAXi_Control  460  in section V.E, supra) becomes unsatisfied, then the packet is aborted. For instance, a packet will be aborted if control passes to a page that is not well-behaved memory (for instance, a page on the I/O bus), or a byte of instruction lies on a page that does not have a 4K page I-TLB entry, or the X86 execution mode transitions to a mode for which profiling is not enabled in the TAXi_Control.Global_TAXi_Enables bits  470 ,  472 . This abort protocol  550 ,  552  assures hot spot detector  122  that each packet describes an actual execution path of the X86 machine, without omission. 
     A transition from X86 code to Tapestry code (for instance, a successful probe exception, see section VI, infra) may be an abort  550 ,  552  event. Profiler  400  is configured to allow the choice between entirely discarding the aborted packet or padding out and then spilling the partial packet to the ring buffer before abort  550 ,  552  occurs. This choice is implemented in the code of the X86-to-Tapestry transition handler  320 . 
       FIG. 5   b  is a block diagram of a portion of profiler  400 , the logic  554  to collect and format a profile entry  430 ,  440  into a processor register. The inputs to logic  554  include TAXi_State register  480 , and a number of lines produced by X86 instruction decode logic  556  within converter  136 . The output of logic  554  is a profile entry in register  594 . Logic  554  as a whole is analogous to a processor pipeline, with pipeline stages in horizontal bands of  FIG. 5   b , progressing from the top of the diagram to the bottom. The stages are clocked at X86 instruction boundaries  566 . Recall from the discussion of  FIG. 1   c  that Align stage  130  parsed the X86 instruction stream, to identify full X86 instructions, and the spatial boundaries in the stored form. Convert stage  134 ,  136  further decodes X86 instructions and decomposes the complex X86 CISC instructions into simple RISC instructions for execution by Tapestry pipeline  120 . The temporal division between X86 instructions is marked by a tag  566  on the last instruction of the recipe of constituent Tapestry instructions emitted by converter  136 . The temporal boundaries between X86 instructions are flagged in a bit of the Tapestry PSW, PSW.X86_Completed  566 . The first native instruction in the converter recipe (which may be a TAXi instruction), resets PSW.X86_Completed  566  to Zero. The last native instruction in the converter recipe sets PSW.X86_Completed to One. If a converter recipe contains only one native instruction, then PSW.X86_Completed  566  is set to One. Since an emulator trap is guaranteed to be the last instruction in a converter recipe, upon normal completion of an emulated instruction recipe, PSW.X86_Completed will be One. 
     The Tapestry processor provides a special instruction for capturing a profile entry from processor register  594  into a general register. This special instruction is called the “TAXi instruction.” The TAXi instruction is injected into the Tapestry pipeline when a profile entry is to be captured. Recall from the discussion of  FIG. 1   c , supra, that converter  136  decomposes each X86 instruction into one or more Tapestry instructions according to a “recipe” for the X86 instruction. The TAXi instruction is simply one more Tapestry instruction injected into the pipeline under the cooperation of profiler  400  and converter  136 . Thus, profile generation is an integral part of the basic Tapestry instruction execution cycle. The TAXi instruction is typically injected into the pipeline at the beginning of the recipe for the instruction at the destination of a control transfer. At the choice of the hardware implementer, the TAXi instruction may be either a special move instruction not encodeable in the Tapestry instruction set, or it may be a move from a processor register. Depending on implementation choice, the instruction can take the form of a “move from register  594  to general register TAXi_State.Packet_Reg  489 ” or converter  136  can extract the contents of register  594  and inject a move-immediate of this 64-bit datum into the profile collection general register specified by TAXi_State.Packet_Reg  489 . 
     Instruction decode logic  556  of the Align and Convert pipeline stages ( 130 ,  134 ,  136  of  FIG. 1   c ) produces signals  558 - 562  describing the current instruction and certain other profileable properties of each instruction, and this description is latched. The information generated includes the instruction length  558  (which, if the instruction generates a profileable Near_Edge event  416 , will end up as done_length  441  (bits &lt;64:61&gt;) of a Near_Edge entry  440 ), the page frame for the last byte of the instruction  559  (done_byte  445  (bits &lt;59:44&gt;) of a Near_Edge entry  440 ), and the page frame  560  and byte offset  561  of the first byte of the next instruction (bits &lt;27:00&gt;, the next_frame  438 ,  448  and next_byte  439 ,  449  of a Near_Edge  440  or Context_At_Point  430 ). Also generated by decode logic  556  is a raw event code  562  associated with the X86 instruction when that instruction is executed by converter  136 , an indication of whether the instruction ends on or straddles a page boundary  563 , whether the instruction is a control transfer (conditional or unconditional)  584 , whether a PC-relative branch is forward or backward, and whether converter  136  is currently active (which in turn is copied from the PSW)  590 . 
     At the next X86 instruction boundary  566 , the information from the just-completed instruction is clocked from signals  558 ,  559 ,  561  to registers  568 ,  569 ,  570 . Registers  568 ,  569 ,  570  are simply a buffer for time-shifting information about an X86 instruction to make it available during the next instruction, in case a profile event is to be captured. Because the native control transfer instruction is always the last instruction of the recipe for an X86 transfer instruction, the virtual-to-physical translation of the address of the destination of the transfer (especially in the case of a TLB miss) is not available until the transfer instruction itself is complete. If an event is to be captured, the TAXi profile capture instruction is injected into the pipeline as the first instruction in the recipe of the destination instruction. Thus, the time shifting defers the capture of the profile event until the address translation of the destination is resolved. Registers  569 ,  570  together drive a 28-bit bus  572  with the “done” part (bits &lt;59:32&gt;) of a Near_Edge profile entry  430 . 
     Simultaneously, the X86 processor context for the current X86 instruction is made available on a 28-bit bus  574 , in a form that parallels bits &lt;59:32&gt; of a Context_At_Point entry  440 . 
     Event codes are generated by circuits  576 ,  591 , and used to control capture of profile entries, as follows. 
     X86 instruction decode logic  556  generates a new raw event code  562  for each X86 instruction. This event code designates a control transfer instruction (event codes 1.0000-1.1011 of  FIG. 4   b ), an instruction that straddles or ends on the last byte of a page frame (code 1.1111,  408  of  FIG. 4   b ), or the default converter event code (1.1110,  406  of  FIG. 4   b ) for all other cases. (For instructions executed in emulator  316 , as converter  136  parses the instruction, logic  576 ,  578  generates the default event code 1.1110  406  or page-straddle event code 1.1111  408 , and then this raw event code  562  is overwritten or selected by the event code immediate field  588  of the RFE instruction at the end of the X86 instruction&#39;s emulation routine.) 
     If the instruction is not a control transfer instruction, the two special “non-event” event codes 1.1110  406  and 1.1111  408  (sequential flow or page straddle) are manufactured by circuit  578 , using the “straddles a page boundary” signal  563  to set the low-order bit. 
     MUX  580  generates final converter event code  582 , selecting between the raw event code  562  generated by instruction decode logic  556  and the 1.111x non-event event code  406 ,  408  from circuit  578  by the following mechanism. If the current instruction is a “control transfer” (either an unconditional or a conditional transfer) as indicated by line  584 , or the branch predictor predicts  586  that the branch is taken, then MUX  580  selects the raw event code  562  generated by decode logic  556 , else MUX  580  selects the non-event event code from 1.111x circuit  578 . 
     When the branch is predicted  586  taken, MUX  580  selects the raw conditional branch event code  562  associated with the instruction. When the branch is predicted  586  not taken, MUX  580  selects the 1.111x non-event event code (either the page boundary event code 1.1111  408  or the default event code 1.1110  406 ) from circuit  578 . Recall that the native control transfer instruction is always the last instruction of the recipe for an X86 transfer instruction, and that the TAXi profile capture instruction is injected into the pipeline as the first instruction in the recipe of the destination instruction of a profileable transfer. Thus, if it turns out that the branch prediction  586  was incorrect, the entire pipeline ( 120  of  FIG. 1   c ) downstream of converter  136  is flushed, including the TAXi instruction that would capture the contents of register  594  into the next general register pointed to by TAXi_State.Packet_Reg  489 . (This is because the TAXi instruction is injected into the pipeline following the native branch instruction that ends the X86 recipe.) The instruction stream is rerun from the mispredicted branch. The branch prediction line  586 , on rerun, will be asserted to the correct prediction value, and MUX  580  will thus select the correct event code, and the TAXi instruction will correctly be injected or not injected. This event code resolution allows the profile packet to correctly record taken branches or taken conditional branches that straddle (or end on) a page boundary, and to correctly omit capture of not-taken branches that do not cross a page boundary. 
     For emulated instructions, converter  136  always supplies an event code  582  that is either the default or new page event code  578 . Since converter  136  completely decodes all instructions, it could supply the event code corresponding to far control transfer instructions (far CALL, far JMP, far RET or IRET) instead of the default or new page event code  578 . This event code is latched as part of the emulator trap recipe. When emulator  316  completes an instruction that straddles a page frame and RFE&#39;s back to converter  136  with the simple X86 instruction complete event code 0.0001, the new page event 1.1111  408  in Event_Code_Latch ( 486 ,  487 , bits &lt;44:40&gt; of  FIG. 4   i ) will be used. Since the high-order bit is set, a reuse event code  414  RFE will result in a Near_Edge profile entry being captured; this is correct, because the RFE implies no data-dependent alteration of context that would require a Context_At_Point. If emulator  316  supplies an RFE event code that doesn&#39;t reuse  414  the Event_Code_Latch, then the RFE event code  588  will be latched. This convention allows the profile packet to record either interesting emulated instructions or simple emulated instructions that straddle a page frame. 
     Similarly, if an X86 instruction fails and must be restarted, the profile information  558 ,  559 ,  560 ,  561 ,  562 ,  563 ,  584  for the instruction is regenerated and runs down the profile pipeline  554  in parallel with the instruction. For instance, if an instruction fetch misses in the TLB, the TLB miss routine will run to update the TLB, and the instruction will be restarted with regenerated profile information in the profile pipeline. 
     When an event code comes from the immediate field  588  of an RFE instruction ( 410  of  FIG. 4   b ), Converter_Active line  590  is used both as the select line  590   a  into MUX  591  to select between the converter event code  582  and the RFE-immediate event code  588  for the four low-order bits, and also supplies the high-order bit  590   b  of the event code  402 , to form a five-bit event code  592 . This event code  592  is latched into TAXi_State.Event_Code_Latch ( 486 ,  487 , bits &lt;44:40&gt; of  FIG. 4   i ). (The reader may think of TAXi_State.Event_Code_Latch  486 , 487  as being part of the pipeline stage defined by registers  568 ,  569 ,  570 .) Not shown in  FIG. 5   b  is the effect of “reuse event code”  414  of  FIG. 4   b : when an RFE instruction completes with a “reuse event code” event code immediate (0.0000 through 0.0011), update of TAXi_State.Event_Code_Latch  486 ,  487  is suppressed, and the old event code is left intact. 
     Each X86 instruction materializes either a Context_At_Point entry  430  or a Near_Edge entry  440  into 64-bit register  594 . The two possible sets of bits  568 ,  572 ,  574  are presented to MUxes  596   a ,  596   b , and bit TAXi_State.Event_Code_Latch&lt;4&gt;  487  selects between them. Note, for instance, that TAXi_State.Profile_Active  482  must be True (states  530  and  542  of  FIG. 5   a ) in order to generate a One from AND gate  598  to generate a Near_Edge entry  440 ; this enforces the rule that a Near_Edge entry  440  must always be preceded by a Context_At_Point entry  430 . Thus, a Context_At_Point entry is always forced out if TAXi_State.Profile_Active  482  is Zero (states  512  and  518  of  FIG. 5   a ) when a TAXi instruction is issued. 
     If profiler  400  decides that the entry in register  594  ought to actually be captured into a profile, converter  136  injects a TAXi profile capture instruction into the Tapestry pipeline  120  at the boundary  566  between the profiled X86 instruction and the next X86 instruction, in order to capture the profile information from register  594 . 
     In some embodiments, it may be desirable to inject multiple TAXi instructions to capture different kinds of profile information. For instance, multiple TAXi instructions could capture a timestamp, a context (analogous to a Context_At_Point entry  430 ), a control flow event (analogous to a Near_Edge entry  440 ), or one injected instruction could compute the desired information, and the next instruction store that information to memory. It may be desirable to temporarily collect the profile information into a register that is not addressable in the architecture, to reduce contention for the storage resource. While register conflict scheduling hardware would have to be used to schedule access to this temporary register, the addition of this register would isolate the operation of profiler  400  from other portions of the processor. 
     The TAXi instruction is injected (and a “pe” event  416  triggers a transition in state machine  510  of  FIG. 5   a ) when all of the following conditions are met: (1) the machine is currently executing in a mode enabled in the TAXi_Control bits &lt;53:51&gt; (that is, the AND of the current X86 instruction context and TAXi_Control.Global_TAXi_Enables  470 ,  472  is non-zero), (2) the machine is at an X86 instruction boundary, (3) all bytes of the current instruction have 4K page I-TLB entries, (4) all bytes of the current instruction have well-behaved (address space zero) memory I-TLB entries, and (5) at least one of these is true: (a) profile collection is enabled (TAXi_State.Profile_Active  482  is One) and TAXi_State.Profile_Request  484  is One and TAXi_State.Profile_Active  482  is Zero and the event code currently latched in TAXi_State.Event_Code_Latch  486 ,  487  has the “initiate packet” property ( 418  of  FIG. 4   b ), or (b) TAXi_State.Profile_Active  482  is One and the event code of TAXi_State.Event_Code_Latch  486 ,  487  is “profileable” ( 416  in  FIG. 4   b ), or (c) a TAXi probe exception will be generated (this is ancillary to profiling, but rather is a convenient mechanism to control probing, see sections VI.C and VI.D, infra). 
     During an interrupt of the orderly execution of X86 instructions, for instance during a TLB miss, page fault, disk interrupt, or other asynchronous interrupt, the machine queries X86 converter  136  and switches to native execution. During native execution, X86 instruction-boundary clock  566  is halted. Because X86 clock  566  is halted, the Near_Edge state of the previous X86 instruction is held in registers  568 ,  569 ,  570  until X86 execution resumes. 
     Note that in the embodiment of  FIG. 5   b , profiling is only active during X86 execution. In an alternative embodiment, profiler  400  is active during execution of native Tapestry instructions translated from X86 by TAXi translator  124 , so information generated by profiler  400  can be fed back to the next translation to improve optimization the next time the portion is translated. The register usage of the Tapestry program is confined by the compiler, so that the profile entries can be stored in the remaining registers. 
     TAXi_Control.Profile_Timer_Reload_Constant ( 494  of  FIG. 4   g ) can be tuned by this method. If hot spot detector  122  finds a that the working set of the program is changing slowly (that is, if a high proportion of hot spots detected overlap with previously-detected hot spots), then profiler  400  is running too often. In this case, Profile_Timer_Reload_Constant  494  can be increased, reducing the frequency of profiling. Similarly, if hot spot detector  122  is finding a large change in the working set between hot spot detector runs, then Profile_Timer_Reload_Constant  494  can be reduced. 
     An alternative tuning method for TAXi_Control.Profile_Timer_Reload_Constant  494  considers buffer overruns. When the range of profile collection registers is full, the profile registers are spilled ( 536  and  548  of  FIG. 5   a ) to a ring buffer in memory. The hot spot detector  122  consumes the profile information from this ring buffer. If profiler  400  overruns hot spot detector  122  and the ring buffer overflows, then the value in TAXi_Control.Profile_Timer_Reload_Constant  494  is increased, to reduce the frequency at which profiling information is collected. Alternatively, on a buffer overrun, the frequency at which hot spot detector  122  runs can be increased. 
     V.G. Determining the Five-bit Event Code From a Four-bit Stored Form 
     Referring again to  FIGS. 4   b ,  4   c , and  4   d , the event code field  436 ,  446  in a profile entry (either a Context_At_Point entry  430  or a Near_Edge entry  440 ) is four bits. Because the four bits can only encode sixteen distinct values, and thirty-two classes of events are classified in  FIG. 4   b , the high order bit is recovered as follows. 
     A Near_Edge entry  440  can never be the first entry in a packet. The elided high-order bit is always a One, and thus a Near_Edge entry  440  always records an event from the lower half  404  of the table of  FIG. 4   b . The event was always generated by converter  136  (or 1.111x non-event circuit  578 ), and was materialized at line  582  of  FIG. 5   b.    
     When a Context_At_Point  430  is not the first entry in a packet, the elided high-order bit is always a Zero, reflecting an event from the upper half  410  of the table of  FIG. 4   b . These non-initial Context_At_Point entries  430  were always generated by RFE events. 
     Every packet begins with a Context_At_Point entry  430 , and that Context_At_Point is an event with the “initiate packet” property ( 418  of  FIG. 4   b ). The event codes  402  are carefully assigned so that only one RFE event code (lower half  404  of  FIG. 4   b ) and converter event code (upper half  410  of  FIG. 4   b ) both share identical low-order four bits and are also have the “initiate packet” property  418 . These two are event codes 0.0110 and 1.0110, near RET and far RET. Thus, the high-order fifth bit can be recovered from the four bit event code  436 ,  446  of the first event in a packet by a lookup: 
                                                0000 −&gt; 1   1000 −&gt; 0           0001 −&gt; 1   1001 −&gt; 0           0010 −&gt; 1   1010 −&gt; 1           0011 −&gt; 1   1011 −&gt; 1           0100 −&gt; 1   1100 −&gt; 0           0101 −&gt; 1   1101 −&gt; 0           0110 −&gt; *   1110 −&gt; 0           0111 −&gt; 1   1111 −&gt; 0                        
Near and far returns (0.0110 and 1.0110) share the same four low-order bits, and either may appear at the beginning of a packet. An implementation may choose to recover either a 0 or 1. The ambiguity is an acceptable loss of precision.
 
     V.H. Interaction of the Profiler, Exceptions, and the XP Protected/unprotected Page Property 
     Exceptions interact with profile collection in several ways. 
     A first class of exceptions are handled completely by the Tapestry Operating System ( 312  of  FIG. 3   a ). These include TLB, PTE, and PDE exceptions and all native-only exceptions. After handling the exception, sequential execution resumes, with no profile entry collected. The RFE instruction at the end of these exception handlers uses the sequential 0.0000 unchanged event code. 
     A second class includes TAXi profiling exceptions, including the profile-register-full exception and unprotected exception (see section I.F, supra). Exceptions in this second class have special side effects defined by the TAXi environment. These exceptions resume instruction execution and use special RFE event codes to control the profiling environment. 
     A third class includes all emulator traps from converter  136  for x86 instruction emulation. Exceptions in the third category provide additional profile information. Emulator  316  always uses a non-zero RFE event code to resume converter operation. 
     A fourth class includes asynchronous x86 transfers of control from hardware interrupts, page faults, breakpoints, single step, or any other x86 exception detected in converter  136  or emulator  316  that must be manifest to the x86 virtual machine. Exceptions in the fourth class have special capabilities. When emulator  316  is about to cause a change of control flow through the x86 IDT, it uses one of four software defined event codes in the RFE. These event codes are divided into two categories. One category is used just for profiling and the other is used to allow emulator  316  to force a check for translated code on any x86 code page. Emulator  316  maintains a private data structure to test that a probe check should be generated for a particular ISR address. 
     The “unprotected” exception (see section I.F, supra) and profiler  400  interact as follows. One of the effects of an unprotected exception is to issue a TAXi instruction to start a new profile packet. Recall that the unprotected exception is triggered when an X86 instruction is fetched from an unprotected, profileable page: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 TAXi_State.Profile_Active 482 = = 1 
                 // profiling 
               
               
                 TAXi_Control.unpr 468 = = 1 
                 // exception enabled 
               
               
                 Page&#39;s I-TLB.ISA 182 = = 1 and XP 186 = = 0 
                 // unprotected 
               
               
                 Fetch page is 4KB 
                 // no abort... 
               
               
                 Fetch page is ASI = = 0 
                 // no abort... 
               
               
                   
               
            
           
         
       
     
     TAXi_State.Profile_Active  482  is set to prime the collection of a packet in the cycle when an “initiate packet” ( 418  in  FIG. 4   b ) event is recognized. A TAXi instruction is sent flowing down the pipe to update TAXi_State.Profile_Active  482  in the following cycle, after the translated fetch address is known and the next instruction has been successfully fetched. A TAXi instruction is issued when TAXi_State.Profile_Active  482  is clear, TAXi_State.Profile_Request  484  is set and TAXi_State.Event_Code_Latch  486 ,  487  contains an event_code for which Initiate_Packet  418  is true or the first instruction in a converter recipe is issued and TAXi_State.Profile_Active  482  is set. The unprotected exception handler may choose whether to preserve or discard the current profile packet, keeping in mind that profile collection on any page that is not protected is unsafe, since undetected writes to such a page could lead to an incorrect profile database. When TAXi_Control.unpr  468  is clear, no exception is generated and TAXi software is responsible for validating the profile packet and setting the “Protected” page attribute. 
     There are two narrow exceptions to the rule that all pages referenced in a profile packet must be protected—the boundary cases at the beginning and end of the packet. If a profile packet (e.g.,  420  of  FIG. 4   a ) ends with a control transfer instruction, the last byte of the transfer instruction, and thus the source of the transfer (the done_frame member  444 ), must be on a protected page, but the destination of the transfer (the next_frame member  438 ,  448  of the entry) need not be. Similarly, if a packet begins with a control transfer instruction (one having the “initiate packet” property,  418  of  FIG. 4   b ), the destination of the transfer (next_frame  438 ,  448 ) must be on a protected page, but the source need not be. In the latter case, the source will escape mention in the profile packet as a matter of course, because a packet must begin with a Context_At_Point entry ( 430  of  FIG. 4   c ), which does not mention the source of the event. 
     V.I. Alternative Embodiments 
     To provide a good heuristic for when to generate optimistic out-of-order code and when to generate conservative in-order code, profile entries may record references to non-well-behaved I/O space. One mechanism is described in section VII.B, infra, converter event code  1 . 1100  that records accesses to I/O space. In an alternative embodiment, a “profile I/O reference” exception traps into Tapestry operating system  312  on a reference to I/O space, when executing from an X86 code page (PSW.ISA  194  equals One, indicating X86 ISA), and TAXi_State.Profile_Active ( 482  of  FIG. 4   h ) is One. At the completion of the exception handler, the RFE immediate field ( 588  of  FIG. 5   b ) will supply a profile event with event code 1.1100 to indicate an I/O space reference. 
     A profile control register may be used to control profiling at a finer grain level. For instance, a register may have 32 bits, where each bit enables or disables a corresponding one of the event classes of  FIG. 4   b . Another control for profiling is discussed infra, in connection with PLA  650 . 
     VI. Probing to Find a Translation 
     VI.A. Overview of Probing 
     Profiler  400  generates a profile of an X86 program. Hot spot detector  122  analyzes the profile to identify often-executed sections of code. TAXi binary translator  124  translates the hot spot from X86 code to TAXi code (the Tapestry native code generated by TAXi binary translator  124 , functionally equivalent to the X86 binary). Because the X86 binary is left unaltered, it contains no explicit control flow instruction to transfer control to the TAXi code. “Probing” is the process of recognizing when execution has reached a point in an X86 binary that has a corresponding valid entry point into TAXi code, seizing control away from the X86 binary, and transferring control to the TAXi code. 
     In one embodiment, each instruction fetch cycle queries of a table. Each entry of the table maps an X86 physical IP value to an address of a TAXi code entry point. For instance, a large associative memory may map X86 physical IP values to entry points into TAXi code segments. The number of segments of TAXi code will typically be, at most, on the order of a few hundred, and execution can only enter a TAXi code segment at the top, never in the middle. Thus, only a few hundred entries in the mapping will be live at any point in time. Such a sparse mapping can be implemented in an associative memory roughly the size of one of the caches. Again, the hit rate in this table will be extremely low. Conceptually, the other embodiments discussed infra seek to emulate such an associative memory, using less chip real estate. 
     In another embodiment, the mapping from X86 physical IP value to Tapestry entry point is stored in memory in a table, and the most-accessed portions of this mapping table are kept in a cache, analogous to a TLB. Each entry in this mapping table has a valid bit that tells whether the accompanying entry is or is not valid. The cached copy of this table is queried during each instruction fetch cycle. Again, the hit rate in this table will be extremely low. 
     In another embodiment, a bit vector has a bit corresponding to each byte (or each possible instruction beginning, or each basic block) that indicates whether there is an entry point to TAXi code corresponding to that byte of X86 instruction space. Each entry in a mapping table includes a machine state predicate, indicating the X86 machine state assumptions that are coded into the TAXi code associated with the entry, and the address for the TAXi entry point. In this embodiment, probing is implemented as a three step process: query the bit vector to see if a mapping translation exists, and if so, look in the mapping table, and if that succeeds, verify that the X86 machine state currently satisfies the preconditions listed in the table entry. The bit vector is quite large, potentially taking 1/9 of the entire memory. Further, the bit vector and table queries tend to pollute the cache. In this embodiment, an exception is raised after the bit vector query succeeds, and the table query is performed by the exception handler software; thus, an exception is only raised for addresses that have their corresponding bits in the bit vector set, addresses that have valid TAXi code entry points. 
     In another embodiment, each bit in the bit vector corresponds to a page of X86 code. If there is an X86 instruction somewhere on the page with a corresponding translation, then the corresponding bit in the bit vector is set. Then, at each event that may be followed by entry to a TAXi code segment, the mapping table is probed to see if such a translation exists. Thus, this implementation takes less memory to hold the bit vector than the embodiment of the previous paragraph, but generates an exception for every instruction fetch from the pages to query the table, not just the instructions that have corresponding TAXi entry points. This embodiment works especially well if translation is confined to a relatively small number of infrequent events, for instance, subroutine entries, or loop tops. 
     A bit associated with a page can be cached in the TLB, like the other page properties  180 ,  186 . 
     In the embodiment discussed at length in the following sections, TAXi divides the possible event space by space (pages), time (using the Probe timer), and event code (the same event code  402  used in profiling). 
     VI.B. Overview of Statistical Probing 
     TAXi prober  600  uses a set of statistical heuristics to help make a profitable set of choices about when a TAXi translation is highly likely to exist in the TAXi code buffer. Rather than probe for a translation on every occurrence of an event, for instance at every routine call, TAXi prober  600  probes on a larger class of events, including simple control transfers, conditional jumps, near call, far call and delivery of an X86 interrupt, and uses a statistical mechanism to throttle the number of probes on the expanded number of classes down to a number likely to succeed. The statistical probe mechanism is designed to have a high correlation between probe exceptions and actual opportunities to execute TAXi code. 
     TAXi divides the space of possible program events spatially, logically, and temporally, and then forms a statistical association between the X86 code space/logic/time that is not always correct, but that is well correlated with the existence of TAXi code. As in the embodiments described in section VI.A, a table maps X86 physical IP values to entry points in TAXi code segments. This table is called the PIPM (Physical IP Map)  602 . Each physical page has associated properties. The properties are associated with several logical event classes (a subset  612  of the event classes laid out in  FIG. 4   b  and discussed in section V.B, supra). Binary translator  124  maintains five bits  624  of properties per page in PFAT (page frame attribute table)  172 —when a binary translation is created, the bit  624  corresponding to the entry event is set in the X86 page&#39;s PFAT entry  174  to indicate the existence of the translation, and an entry in PIPM  602  is created that maps the X86 physical IP address to the address of the TAXi code segment. The five PFAT bits are loaded into the TLB  116  with the page translation from the page tables. Enablement of the feature that queries these bits is gated by a time-varying probe mask, whose bits correspond to the five PFAT/TLB bits. 
     A probe occurs in several stages, as will be described in detail in connection with  FIG. 6   c . When a stage fails, the rest of the probe is abandoned. The first stage is triggered when an X86 instruction is executed, and that instruction generates an event code that is one of the probeable event codes, and the corresponding probe property for the page is enabled, and the corresponding bit in the current probe mask is enabled. The first stage is essentially an implementation of the associative memory search described for the previous embodiments, but on a memory page granularity. This first stage gives a reasonable-but-imperfect evaluation of whether it is likely to be profitable to generate an exception, so that software can actually probe PIPM  602 . If this first stage test succeeds, then the processor generates a probe exception. A software exception handler probes PIPM  602  to discover whether there is a current translation of the current IP value, and to find the address of that translation. 
     This implementation uses no large hardware structures on the Tapestry microprocessor chip; for instance, it avoids a large associative memory. The implementation reduces the overhead associated with unsuccessful probes of PIPM  602 , while providing a high likelihood that execution will be transferred to the TAXi code that is translated to replace a hot spot of the X86 program. 
     Recall also that probing is an optimization, not a condition for minimum correctness. If prober  600  generates too many probe exceptions, the excess probes of PIPM  602  will fail because there is no translation to which to transfer control, and correct execution will resume in converter ( 136  of  FIGS. 1   a  and  1   c ). The cost of an error is one execution of the probe exception handler. If the mechanism generates too few probes, then control will not be transferred to the TAXi code, and execution will simply continue in converter  136 . The cost of the error is the opportunity foregone (less the cost of the omitted exception). Because errors do not induce any alteration in the result computed, a heuristic, not-always-correct approach does not violate any architectural correctness criteria. This goal is sought by finding fine-grained ways of slicing up time, space, and classes of events, and associating a well-correlated indicator bit with each slice. 
     VI.C. Hardware and Software Structures for Statistical Probing 
     A number of the structures discussed in section V, supra, in connection with profiling are also used in probing. 
     Referring again to  FIG. 4   b , the event code taxonomy  402  for profiling is also used for probing. Column  610  designates a number of events as “probeable.” The events designated probeable  610  are all transfers of control by an X86 instruction or interrupt. The code at the destination of the transfer is a candidate for a probe. Hot spot detector  122  is designed with knowledge of the probeable event classes, and will only translate a detected X86 hot spot when the control transfer that reaches the hot spot is one of the probeable events  610 . Thus, when an X86 program executes a transfer of control, and the transfer is one of the probeable  610  transfers, there is at least the theoretical possibility of the existence of TAXi code, and the rest of the probe circuitry is activated. 
     The probeable events  610  are further classified into six classes, in column  612 . The six classes are “far call,” “emulator probe,” “jnz,” “conditional jump,” “near jump,” and “near call.” 
     Referring again to  FIG. 4   h , probe mask  620  is a collection of six bits, one bit corresponding to each of the six probeable classes  612  of  FIG. 4   b . When a probe mask bit is One, probes for the corresponding class  612  are enabled—when an event of that class occurs (and certain other conditions are satisfied, see the discussion of  FIGS. 6   a - 6   c , infra), the hardware will trigger a probe exception and a probe of PIPM  602 . When a probe mask  620  bit is Zero, probes for the corresponding class  612  are disabled—even if a translation exists for the destination of the event, the hardware will not initiate a probe of PIPM  602  to find the translation. 
     Referring again to  FIG. 1   d , a PFAT entry  174  has five bits  624  of properties for each physical page. These five bits  624  correspond to the “far call,” “jnz,” “conditional jump,” “near jump,” and “near call” probeable properties ( 612  of  FIG. 4   b ,  620  of  FIGS. 4   h  and  6   b , and  660 ,  661 ,  662 ,  663 ,  664  of  FIG. 6   b —the “emulator probe” probe is raised by software, rather than being maintained on a per page basis). The corresponding bit of PFAT probe properties  624  is set to One when hot spot detector  122  has detected a hot spot and binary translator  124  has generated a native Tapestry translation, and the profile for the translation indicates the class of events that lead to entry of the X86 hot spot that is detected and translated. The five bits  624  of a given page&#39;s PFAT entry are AND&#39;ed together with the five corresponding bits of probe mask  620  to determine whether to probe, as described infra in connection with  FIGS. 6   b - 6   c.    
     Referring again to  FIGS. 4   g ,  4   h  and  4   i , TAXi_Timers.Probe_Timer  630  is an unsigned integer countdown timer that counts down at the CPU clock frequency, used to control the average rate of failed probe exceptions on a per-event-class basis. When Probe_Timer  630  counts down to zero, TAXi_State.Probe_Mask  620  is reset to all One&#39;s, and Probe_Timer  630  is reset to the value of TAXi_Control.Probe_Timer_Reload_Constant  632 . An RFE with event code 0.0011 forces an early reset of Probe_Timer  630  from Probe_Timer_Reload_Constant  632 . 
     Together, Probe_Mask  620  and Probe_Timer  630  synthesize the following behavior. As long as probes of a class  612  are successful, the machine continues to probe the class. When a probe fails, the class  612  of the failed probe is disabled for all pages, by setting the class&#39; bit in Probe_Mask  620  to Zero. At the next expiry of Probe_Timer  630 , all classes are re-enabled. 
     Recall that TAXi code segments are created asynchronously to the execution of the X86 binary, after a hot spot is detected by hot spot detector  122 . Translated code segments are retired when they fall into disuse. On a round-robin basis, TAXi native code segments are marked as being in a transition state, and queued as available for reclamation. The code segment, while in transition state, is removed from all address spaces. If the TAXi code segment is invoked while in transition state, it is dequeued from the transition queue, mapped into the invoking address space, and re-set into active state. If the TAXi code segment is not invoked while in transition state, the storage is reclaimed when the segment reaches the tail of the queue. This reclamation policy is analogous to the page replacement policy used in Digital&#39;s VAX/VMS virtual memory system. Thus, because the reclamation policy is somewhat lazy, PFAT  172  may be somewhat out of date. 
     Referring to  FIG. 6   a  in conjunction with  FIGS. 1   c ,  1   d ,  3   a  and  4   b , PIPM  602  is a table of PIPM entries  640 . Each PIPM entry  640  has three classes of information: the X86 physical address  642  that serves as an entry point into a translated hot spot, X86 machine context information  646 ,  648  that was in effect at the time of previous executions and which now serves as a precondition to entry of a translated TAXi code segment, and the address  644  of the translated TAXi code segment. The integer size and mode portion  646  of the context information is stored in a form that parallels the form captured in a Context_At_Point profile entry ( 430  of  FIG. 4   c ), and the form used to control profiling in the TAXi_Control.Global_TAXi_Enables bits ( 470 ,  472  of  FIG. 4   g ). If the current size and mode of virtual X86  310  does not match the state saved in the size and mode portion  646  of PIPM entry  640 , the probe fails. The floating-point portion  648  of PIPM entry  640  parallels the floating-point state  435  captured in a Context_At_Point profile entry  430 . If, at the conclusion of an otherwise successful probe, the floating-point state of virtual X86  310  does not match the state saved in the floating-point portion  648  of PIPM entry  640 , then either the floating-point state is massaged to match the state saved in PIPM entry  640 ,  648 , or the probe fails. 
     Referring to  FIG. 6   a  in combination with  FIG. 1   b , PIPM  602  is kept up-to-date, reflecting the current catalog of translations available, and tracking TAXi code translations as they are created, marked for reclamation, and actually reclaimed and invalidated. The probe bits in PFAT  172  may lag slightly, and the probe bits in TLB  116  are allowed to lag slightly further. Further, the probe bits in TLB  116  only convey information to page granularity. Thus, the probe bits in TLB  116  indicate that at some recent time there has been TAXi code with that entry point class on this page. A Zero bit in TLB  116  suggests that there is no such entry point, and that a probe of the PIPM  602  on this event class would very likely fail, and thus should not be attempted. A One suggests a high likelihood of success. The One may be somewhat stale, still indicting the presence of a TAXi code translation that has since been invalidated and reclaimed. After a hit in TLB  116 , a probe of PIPM  602  will find that the PIPM entry  640  for the reclaimed TAXi code segment will indicate the invalidity of the TAXi segment, for instance, by a Zero in address  644 . 
     Recall from section V.G, supra, that a fifth high-order bit is needed to disambiguate the four-bit event code stored in TAXi_State.Event_Code_Latch  486 ,  487  and Context_At_Point profile entries  430 . The event codes  402  of  FIG. 4   b  are carefully assigned so that no probeable  610  RFE event code (top half  410 ) shares four low-order bits with a probeable  610  converter event code (bottom half  404 ). Probeable  610  RFE events  410 ,  610  are always even, and probeable  610  converter events  404  are always odd. Thus, the least significant four bits of the current event code uniquely identify the probe event, the probe exception handler can always determine whether the probe event came from a RFE instruction or converter execution. (This non-overlap of probeable events  610  is an additional constraint, on top of the non-overlap of “initiate packet” event codes  418  discussed in section V.G, supra.) 
     Referring again to  FIG. 6   b , probing is controlled by a PLA (programmable logic array)  650  and several AND gates. PLA  650  generates several logic functions of event code  592  from event code latch  486 ,  487 . PLA  650  computes the “initiate packet”  418 , “profileable event”  416 , and “probeable event”  610  properties as described in  FIG. 4   b . In addition, the probeable event codes are decoded into single signals as described in column  612  of  FIG. 4   b . For instance, “jnz” bit  660 , corresponding to bit &lt;0&gt; of the probe properties  624  of  FIG. 1   d , is asserted for event code 1.0001. “Conditional jump” bit  661 , corresponding to bit &lt;1&gt; of probe properties  624 , is asserted for event code 1.0011. “Near jump” bit  662 , corresponding to bit &lt;2&gt; of probe properties  624 , is asserted for event code 1.0101. “Near call” bit  663 , corresponding to bit &lt;3&gt; of probe properties  624 , is asserted for event codes 1.0111 and 1.1011. “Far call” bit  664 , corresponding to bit &lt;4&gt; of probe properties  624 , is asserted for event code 0.1000. “Emulator probe” bit  665  is asserted for event codes 0.1100 and 0.1110. 
     VI.D. Operation of Statistical Probing 
     Referring to  FIGS. 6   b  and  6   c , for an X86 transfer of control instruction (either a simple instruction executed in converter  136  or a complex instruction executed in emulator  316 ), the instruction fetch of the transfer target ensures that TLB  116  is updated from PFAT  172  with the current probe page properties  624  for the page of the target instruction—either the information was already current in TLB  116 , or it is refilled as part of the I-TLB miss induced by the instruction fetch. Thus, as part of the instruction fetch, the TLB provides both an address translation and the probe page properties  624  for the target instruction (though, as discussed in section VI.C, supra, the probe properties in TLB  116  may be slightly stale). 
     Further, these control transfer instructions generate an event code  402 , as described in section V.F, supra. At the conclusion of the instruction, either converter  136  or an RFE instruction generates a 5-bit event code  592 . The event code is stored in latch  486 ,  487 . As the target instruction is fetched or begins execution, event code latch  486 ,  487  is fed to PLA  650 . 
     Six 3-input AND gates  670  AND together the probeable event signals  660 ,  661 ,  662 ,  663 ,  664 ,  665  with the corresponding page properties from the TLB ( 624  of  FIG. 1   d ) and the current value of Probe_Mask  620 . The six AND terms are OR&#39;ed together in OR gate  672 . Thus, the output of OR gate  672  is One if and only if the current instruction generated an event  592  whose current Probe_Mask  620  is One and whose probe property bit  624  for the current page is One. The “emulator probe” signal  665  is generated by PLA  650  when RFE event code equals 0.1100 or 0.1110, as indicated by “Emulator Probe” in column  612  of  FIG. 4   b . This class of probe is raised when emulator  316  believes that probe success is likely and the Emulator Probe bit (bit &lt;5&gt;) of Probe Mask  620  is One. 
     The sum of OR gate  672  is AND&#39;ed  674  with several more terms. Probing as a whole is controlled by TAXi_Control.probe  676  (see also  FIG. 4   g ); if this bit is Zero, probing is disabled. To ensure that control is only transferred to TAXi code whose underlying X86 code is unmodified since the translation was generated, probing is only allowed on protected pages of X86 instruction text, as controlled by XP bit  184 ,  186  for the page (see also  FIG. 1   d , and sections I.F, supra, and section VII, infra); if XP bit  184 ,  186  is Zero, no probes are taken on the page. Probing is controlled for X86 contexts by TAXi_Control.Global_TAXi_Enables.sizes  470  and .modes  472  bits, which are set by TAXi system control software. Probing is only enabled for current X86 modes whose TAXi_Control.Global_TAXi_Enables  470 ,  472  are set to One. Probing and profiling are mutually exclusive (see section VI.G, infra); thus probing is disabled when TAXi_State.Profile_Active ( 482  of  FIGS. 4   h , states  530  and  542  of  FIG. 5   a , see section V.E and V.F, supra) is One. If the output  678  of AND gate  674  is One, then the processor continues to the next step of determining whether to probe PIPM  602 , as discussed further infra. 
     TAXi_Control.probe  676  was Zeroed by software when the X86 processor entered a mode that TAXi is not prepared to handle, e.g., X86 debugging, single-step or floating-point error conditions. When operating in “page property processing disabled” mode (with PROC_CTRL.PP_Enable deasserted, see section I.A, supra), TAXi_Control.probe  676  is deasserted. 
     The output  678  of AND gate  674  latches the single bit of the probe event class into Decoded_Probe_Event latch  680 . 
     An intermediate step  690  to be performed in hardware, discussed in detail in section VI.E, infra, may optionally be performed here. 
     If all of the hardware checks described supra pass, then the processor takes a probe exception before completing execution of the instruction at the target of the control transfer. The probe exception transfers control to software that continues to further test whether control should be transferred to the TAXi code. 
     As part of generating a probe exception, converter  136  writes (step  682 ) a Context_At_Point profile entry ( 430  of  FIG. 4   c ) to the register indicated by TAXi_Control.Packet_Reg_First ( 476  of  FIG. 4   g ) defined for profile collection. (as will be explained further in section VI.G, infra, profiling and probing are mutually exclusive, and the X86 does not use the profile collection registers, so the three uses cannot conflict.) The event code ( 436  of  FIG. 4   c ) of the profile entry  430  is set to the least significant 4 bits of the current event code ( 592  of  FIG. 5   b ). 
     On entry to the probe exception handler the following information is available from the converter:
         A Context_At_Point profile entry  430 , containing the X86 physical IP (page frame number and page offset) in low half  438 ,  439     X86 execution context, from high half  432 ,  433 ,  435  of Context_At_Point  430     probe event code in the event code field  436  of Context_At_Point  430     X86 virtual IP (offset into the CS segment) from EPC.EIP       

     The exception handler consults PIPM  602 . PIPM  602  is a table that maps X86 instruction addresses (their physical addresses, after address translation) to addresses of TAXi code segments. The table entry in the PIPM is indexed by X86 physical address, typically using a conventional hashing technique or other table lookup technique. The probe exception handler looks up the physical address of the target instruction in the Physical IP to TAXi code entry point Map (PIPM)  602 . 
     If no PIPM entry  640  with a matching X86 address is found, then the probe has failed, with consequences discussed infra. 
     Once a table entry with an address match is located, the translation must be further qualified by the current X86 mode. Recall that the full execution semantics of an X86 instruction is not fully specified by the bits of the instruction itself; execution semantics depend on whether the processor is in V86 mode, whether addressing is physical or virtual, the floating-point stack pointer, and the full/empty state of floating-point registers, and operand sizes are encoded in segment descriptors, the EFLAGS register, the floating-point status word, the floating-point tag word, etc. The translation into Tapestry native code embeds assumptions about these state bits. These state bits were initially captured in bits &lt;59:51&gt; of a Context_At_Point profile entry  430  (see section V.C, supra) and then hot spot detector  122  and binary translator  124  generated the translation based on the profiled values of the mode bits. The corresponding PIPM entry  640  for the translation records the mode bit assumptions under which the TAXi code segment was created. Thus, once PIPM entry  640  is found, the current X86 mode is compared against the X86 mode stored in PIPM entry  640 . 
     The exception handler makes three general classes of checks of the mode information in PIPM  602 . 
     First, the current execution mode and the value of the CS.D (code and operand size) and SS.D (stack segment size) bits assumed by TAXi translator  124  must be compatible. This is determined by comparing the decoded “sizes” information  432  from the Context_At_Point argument with the mask of acceptable contexts provided in PIPM entry  640 ,  646 . 
     If the current floating-point state does not match the floating-point state  648  in PIPM entry  640 , then the probe fails. In some cases, disagreements can be resolved: the floating-point unit can be unloaded and reloaded to conform to the floating-point state in PIPM entry  640 , for instance, to get the floating-point registers into the canonical locations specified by the current X86 floating-point map. If the height of the floating-point register stack mismatches the stack height in PIPM entry  640 , or the pseudo floating-point tag words mismatch, or the floating-point control words (precision and rounding modes) mismatch, then the probe fails. If the only mismatch is the mapping of the floating-point tag map (the map from the X86 stack-based register model to the register address Tapestry model), then software can reconfigure the floating-point state to allow the probe to succeed. 
     Execution control is tendered to the TAXi code. If the modes mismatch, the probe fails. 
     Second, the current virtual IP value must be such that (a conservative approximation of) the transitive closure of the TAXi code points reachable by invoking this TAXi fragment would not trigger a CS limit exception. This is determined from the virtual IP at the time of the exception and normalized CS limit, and comparing them to values stored in PIPM entry  640 . 
     Third, because the TLB copy of the XP bit  186  may be slightly stale relative to the PFAT copy  184 , the master copy of the XP bit  184  in PFAT  172  is checked to ensure that all cached information (the profile and TAXi code) associated with the X86 page is still valid. 
     Fourth, DMU  700  (see section VII, infra) may be queried to ensure that the X86 page has not been invalidated by a DMA write. 
     If the current X86 mode satisfies the mode checks, then the probe has succeeded. PIPM entry  640  contains the address of the TAXi code corresponding to the address of X86 code at which the probe exception occurred. If the modes mismatch, the probe fails. 
     When a probe exception succeeds, the handler modifies the EPC by setting EPC.TAXi_Active, Zeroing EPC.ISA (native Tapestry mode), setting EPC.EIP to the address of the TAXi code, and setting EPC.ESEG to the special TAXi code segment. The RFE instruction completes the transfer of execution to the TAXi code by loading the EPC into the actual processor PSW. A successful probe leaves the Probe_Mask  620  unaltered. Thus, classes of probeable events remain enabled as long as each probe in the class is successful. 
     By resetting EPC.EIP to point to TAXi translated code, the RFE instruction at the end of the probe exception handler effects a transition to the TAXi code. Because the TAXi code was transliterated from X86 code, it follows the X86 convention, and thus the argument copying that would have been performed by the transition exception handler (see sections II, III, and IV, supra) is not required. Further, because both the probe exception handler and the TAXi code are in Tapestry ISA, no probe exception occurs on this final transition. 
     When a probe exception is triggered, and the software probe fails to find a translation, several steps are taken. The bit in Probe_Mask  620  that corresponds to the event that triggered the probe is cleared to Zero, to disable probes on this class of event until the next expiry of Probe_Timer  630 . This is accomplished by the Probe_Failed RFE signal and the remembered Decoded_Probe_Event latch  680 . The interrupt service routine returns using an RFE with one of two special “probe failed” event codes of  FIG. 4   b . Event code 0.0011 forces a reload of TAXi_Timers.Probe_Timer  630  with the Probe_Timer_Reload_Constant  632 . Event code 0.0010 has no side-effect on Probe_Timer  630 . It is anticipated that when a probe on a backwards branch fails, Probe_Timer  630  should be reset, by returning from the probe exception with an RFE of event code 0.0011, in order to allow the loop to execute for the full timer value, with no further probe exceptions. On the other hand, it is anticipated that when a probe on a “near call” fails, testing other near calls from the same page should be allowed as soon as Probe_Timer  630  expires, and thus this probe exception will return with an event code of 0.0010. The RFE returns to the point of the probe exception, and execution resumes in converter  136 . 
     If an RFE instruction that modifies Probe_Mask  620  is executed at the same time that the probe timer expiry attempts to reset Probe_Mask  620 , then the RFE action has higher priority and the reset request is discarded. 
     VI.E. Additional Features of Probing 
     In the intermediate step  690  mentioned briefly supra, a bit vector of bits indicates whether a translation exists for code ranges somewhat finer than the page level encoded in the PFAT probe bits. After a probeable event occurs, and the class of that event is screened against the PFAT probe bits and the probe mask, the hardware tests the bit vector (in an operation somewhat reminiscent of a page translation table walk) before actually raising the probe exception and transferring control to the software interrupt handler. 
     Only the slices of the bit vector that correspond to pages with non-zero PFAT probe bits are actually instantiated by software, again similar to the way only the relevant portions of a full page table tree are instantiated by a virtual memory system. The bit vector itself is hidden from the X86 address space, in an address space reserved for the probe bit vector and other structures for managing the X86 virtual machine. The bit vector may be cached in the d-cache—because of the filtering provided by the earlier steps, the number of unsuccessful queries of the probe bit vector will be relatively small. 
     The density of the bit vector can be tailored to the operation of the system. In some embodiments, there may be a bit for every byte in the physical memory system. In other embodiments, the effectiveness of the bit vector would most likely be only marginally reduced by having one bit for a small power of two bits, for instance, one bit for every 2, 4, 8, 16, or 32 bytes of physical memory. The block size guarded by each bit of the bit vector may be software configurable. 
     Thus, where the probe properties  624  in PFAT  172  give a fine-grained filter by event code (the five probeable event classes), but are spatially coarse (on a page basis), the bit vector gives a coarse map on event code (all events grouped in a single bit), but is finely grained (a few bytes) by space. 
     A One bit in the bit vector is not a guarantee that translated code exists and should be activated. As with the PFAT probe bits, the bit vector is somewhat over-optimistically heuristic, and may on occasion lag the actual population of translated code segments. Even after testing the bit vector, the mode predicates in PIPM  602  are still to be verified. 
     The quasi-microcoded hardware used for table walking is readily modified to issue the loads to memory to fetch the appropriate slices of the bit vector. 
     The logic of PLA  650  is programmable, at least during initial manufacture. Reprogramming would alter the contents of columns  414 ,  416 ,  418 ,  610 ,  612  of table at  FIG. 4   b . Though the five-bit event codes generated by converter  136  are relatively fixed, the interpretation given to those bits, and whether to profile or probe on those events, is reconfigurable within PLA  650 . In alternative embodiments, PLA  650  may be made programmable at run time, to control operation of profiling and probing by altering the contents of the columns of  FIG. 4   b . The five bits of input (event code latch  486 ,  487 ) to PLA  650  give 2 5 =32 possible inputs. There are nine bits of output (probeable event signals  660 ,  661 ,  662 ,  663 ,  664 ,  665 , profileable event  416 , initiate packet  418 , and probeable event  610 ). Thus, PLA  650  could be replaced by a 32%9 RAM, and the outputs of PLA  650  would then be completely software configurable. With that programmability, both profiling (section V, above) and probing (this section VI) become completely configurable. In a programmable embodiment, the overhead of profiling and probing can be controlled, and strategies can be adapted to experience. 
     Most of the attributes required for a probe are associated with pages (stored in the PFAT and TLB), or with individual translated code segments (stored in PIPM  602 ), a structure queried by converter  136  as simple X86 instructions are executed in hardware. For complex instructions that are executed in the emulator ( 316  of  FIG. 3   a ) the decision to probe or not to probe is made in software. A side table annotates the X86 IVT (interrupt vector table) with probe attributes, much as the PFAT is a side annotation table to the address translation page tables. After emulating an X86 instruction, emulator  316  queries the IVT side table, and analyzes these bits in conjunction with the machine state determined during the course of the emulation. On the basis of this query, emulator  316  decides whether to return to converter  136  using an RFE with an event code that induces a probe, or an RFE with an event code that does not. Event codes 0.1100 and 0.1110 induce a probe (see column  610  of  FIG. 4   b ), and event codes 0.1101 and 0.1111 do not. 
     VI.F. Completing Execution of TAXi Code and Returning to the X86 Code 
     Once a probe exception activates some translated TAXi code within an X86 process, there are only three ways to leave that TAXi code, either a normal exit at the bottom of the translated segment, a transfer of control out of the code segment, or an asynchronous exit via an exception. 
     The fall-out-the-bottom case is handled by epilog code generated by the TAXi translator  124 . The TAXi code will home all X86 machine state and return control to the converter by issuing a trap instruction. A trap instruction transfers control to an exception handler for a TAXi_EXIT exception. The trap handler for exiting TAXi code sets the ISA to X86 and returns control to the point in the X86 code following the translated hot spot. In the alternative embodiment of section IV, epilog code returns data to their X86 homes, and sets the IP to point to the point following the end of the portion of the X86 code that was translated. 
     The transfer of control case may be handled by the state saving mechanism described in section III, supra, or may be handled by code essentially similar to the epilog code discussed supra. In any case, the Tapestry system takes explicit actions to reconstruct the x86 machine state. 
     Asynchronous exits are handled by exception handlers, using the safety net mechanism introduced in section I.D, supra, and discussed in more detail in section VIII, infra. When an exception occurs in TAXi code and the exception handler determines that it must materialize the exception in the X86 virtual machine, it jumps to a common entry in emulator  316  that is responsible for setting the X86 state—establishing the interrupt stack frame, accessing the IDT and performing the control transfer. When this function is invoked, it must first determine if TAXi code was being executed by examining PSW.TAXi_Active  198 , and if so, jump to a TAXi function that reconstructs the X86 machine state and then re-executes the X86 instruction in the converter to provoke the same exception again. Re-executing the X86 instruction is required to establish the correct X86 exception state. Anytime the converter is started to re-execute an x86 instruction, the exception handler uses the RFE with probe failed, reload probe timer event code to prevent a recursive probe exception from occurring. 
     The only exceptions that may not be materialized in the x86 world are those that can be completely executed by native Tapestry code, e.g. TLB miss that is satisfied without a page fault, FP incomplete with no unmasked X86 floating-point exceptions, etc. 
     VI.G. The Interaction of Probing and Profiling 
     Probing and profiling are mutually exclusive. Probing only occurs when there is a probeable event (column  610  of  FIG. 4   b ) while TAXi_State.Profile_Active ( 482  of  FIG. 4   h  and  5   a ) is Zero. These constraints are enforced by AND gate  674  of  FIG. 6   b . On the other hand, profiling is only enabled while TAXi_State.Profile_Active  482  is One. Thus, when the processor takes a probe exception, the mutual exclusion guarantees that the resources used by profiling are quiescent. In particular, the general registers in which profile packets accumulate are guaranteed to be available for use to service the exception. 
     Every probeable event  610  is also an “initiate packet” event  418 . This reflects a practical design consideration: the class of probeable events  610  are the most important events in the flow of a program, and “initiate packet” events  418  are a somewhat broader set of important events. If a probeable event  610  occurs in a class for which probing is enabled, and TAXi_State.Profile_Active ( 482  of  FIG. 4   h  and  5   a ) is Zero, then the event is also an “initiate packet” event  418 . If, further, TAXi_State.Profile_Request  484  is One, then profiler  400  would naturally trigger a transition of TAXi_State.Profile_Active ( 482  of  FIG. 4   h  and  5   a ) and TAXi_State.Profile_Request  484 , transition  522  of  FIG. 5   a . This would violate mutual exclusion. However, the probe exception is higher priority than any activity of profiler  400 . Thus, on a successful probe, control is transferred to the TAXi code, and any profiler action is suppressed. If the probe fails, the probe class is disabled, and profiler  400  is allowed to take its normal course, as described in  FIGS. 5   a  and  5   b  and section V.F, supra. 
     The content of a profile packet, and in particular, a Context_At_Point profile entry ( 430  of  FIG. 4   c ), is tailored to efficiently represent the information required by hot spot detector  122  (to precisely identify the ranges of addresses at which frequently-executed instructions are stored), and efficiently tailored for the binary translator  124  (to capture the X86 semantic mode information that is not represented in the code text itself), and efficiently tailored for prober  600  (the information required to qualify a probe, to ensure that the semantic mode assumptions under which the binary was translated are met by the current X86 semantic mode, before transferring control to the TAXi code). Though the representation is not optimal for any one of the three, it is very good for all three. In other embodiments, the representation may be tailored to promote efficiency of one of the three over the others, or solely for the benefit of one. 
     The fact that probeable events  610  are a subset of “initiate packet” events  418  has a further desirable side effect: the hardware to capture information for the first profile entry  430  in a packet can be reused to capture the information needed by the probe exception handler. When a decision is made in hardware to deliver a probe exception, the exception handler is provided with information about the physical address to which control was being passed and the context of the machine. The information for a probe exception is gathered in register  594  of  FIG. 5   b , in a form that mirrors the form captured in a Context_At_Point profile entry  430 . In the process of either generating a probe exception in hardware, or servicing it in software, the content of register  594  is captured into a general register. This capture (when supplemented with the CS limit (code segment length), as stored in an X86 segment descriptor register) supplies the information needed by the probe exception handler: the physical address of the next instruction, used to index PIPM  602  and find a possible candidate entry, and the X86 mode information needed to qualify that entry. The address captured in the Context_At_Point  430  has the physical page number, ready for use to index into PIPM  602 . Since all probeable events are “initiate packet” events, the mode information is readily available in the Context_At_Point profile entry  430  that initiates the packet identifying the hot spot. The various snapshots can be compared to each other for compatibility by AND&#39;ing the appropriate bits together. 
     Unlike profile collection, which operates by periodic sampling, probing is always enabled when the converter is active, the TAXi_Control.probe flag is One, and the probe mask has at least one surviving One bit. 
     VI.H. Alternative Uses of Adaptive Opportunistic Statistical Techniques 
     The adaptive opportunistic execution policy described in section VI.A through VI.E can be used in a number of settings in a CPU design. 
     In one example embodiment, a CPU might have a fast path and a slow path through the floating-point unit, where the fast path omits full implementation of the IEEE-754 floating-point infinities, denormalized numbers (“denorms”) and NaNs, and the slow path provides a full hardware implementation. Because infinities, denorms and NaNs tend to arise infrequently, but once generated tend to propagate through more and more of the computation, it is advantageous to start with the optimistic assumption that no denorms or NaNs will arise, and to configure the CPU to use the fast path. Once an infinity, denorm or NaN is detected, then the CPU may revert to the slow path. A timer may be set to run, and when the timer expires, the CPU will resume attempting the fast path. 
     In another example embodiment, a cache system might use an analogous adaptive opportunistic technique. For instance, a multi-processor cache might switch between a write-through policy when inter-processor bus snooping indicates that many data in the cache are shared, write-in when it is noted that shared data are being used intensively as a message board, and write-back when the bus snooping indicates that few data are shared. A cache line flush or invalidate is the “failure” that signals that execution must revert to a higher-cost policy, while a successful write in a lower-cost policy is a “success” that allows continued use of the lower-cost policy. The adaptation might be managed on the basis of address ranges, with a record of success and failure maintained for the distinct address ranges. The switch between mode can be managed by a number of techniques. For instance, a counter might count the number of successive memory accesses that would have been more-efficiently handled if the cache were in another mode. When that counter reaches a threshold value, the cache would be switched into the other mode. Or, a timer might set the cache into a more-optimistic mode, and an access that violates the assumption of optimism would set the cache into a less-optimistic mode. 
     The opportunistic policy might be used in branch prediction, cache prefetch or cache enabling. For instance, cache prefetching might be operative for as long as prefetching is successful. Or, a particular LOAD instruction in a loop may be identified as a candidate for cache prefetching, for as long as the prefetch continues successfully. When the prefetch fails, prefetching is disabled. 
     A multiprocessor cache might cache certain data, on optimistic assumptions, and then mark the data non-cacheable when inter-processor cache trashing shows that caching of these data is unprofitable. 
     Opportunistic policies might be useful in memory disambiguation in object-oriented memory systems. For instance, a compiler might generate two alternate codings for a source construct, one assuming that two objects are disjoint, one assuming overlap. The optimistic disjoint code would be used for as long as the optimistic assumption held, then control would revert to the pessimistic code. 
     VII. Validating and Invalidating Translated Instructions 
     The TAXi system is analogous to a complex cache—the profile data and TAXi code are kept current with the pages of X86 instruction text, and must be invalidated when the X86 instruction text is modified. There are two possible sources for modifications to the X86 instruction text: memory writes by the CPU, and writes from DMA devices. Writes from the CPU are protected by the XP protected bit  184 ,  186 , discussed at section I.F, supra, and validity checks in PIPM  602 , as discussed in sections VI.C and VI.D, supra. This section VII discusses protection of the cached information against modification of the X86 instruction text by DMA writes. 
     Referring to  FIG. 7   a , DMU  700  (the DMA Monitoring Unit) monitors DMA writes to ASI Zero (address space zero, “well-behaved” non-I/O space) in order to provide a condensed trace of modification of page frames. DMU  700  performs this monitoring without imposing excessive overhead. DMU  700  is implemented as an I/O device in the I/O gateway, instead of directly on the main processor bus (the G-bus). This gives DMU  700  visibility to detect all non-processor updates of X86 code pages in physical memory (except for those initiated by the processor itself, which are masked by the behavior of a write-back cache). 
     VII.A. A Simplified DMU Model 
     A simple DMU provides modified page frame (MPF) bit for each physical page frame in the system. An MPF bit of Zero indicates that no modification has occurred, and if a DMA transfer were to write into the corresponding page frame then a modification event would need to be reported against that page frame. An MPF bit of One indicates that DMA writes to the corresponding page frame should pass unreported. 
     This simple DMU is initialized by Zeroing all MPF bits. Then, for every DMA write, the relevant MPF bit is checked. If that MPF bit was already One, no further processing occurs. If the MPF bit is still Zero, then it is set to One, and the identity of the modified page frame is reported, for instance by creating an entry in a FIFO. Once a page frame&#39;s MPF bit becomes One, and the modification is reported, no amount of additional DMA writing to that page frame will produce another modification report. 
     This simple DMU provides tremendous condensation in the reporting of page modifications; in fact, it generates a provably minimal number of modification reports. The proof follows from the fact that DMU  700  itself never Zeros any MPF bits—it only sets them to One. The number of modification reports possible is bounded by the number of MPF bits, or equivalently, the number of page frames. Because most DMA writes are to the buffer pages for “data” I/O, and the important writes to be monitored are to pages of X86 instruction text, which are written less often, this behavior reduces overhead while preserving correct behavior. 
     So long as a page frame&#39;s MPF bit remains Zero, the TAXi system is assured that no DMA modification has occurred since that MPF bit was last cleared to Zero. Thus, whenever profiler  400  is about to profile an X86 page, generate a TAXi translation, execute a TAXi translation (the operations that cache information about the page or use cached information), that page&#39;s MPF bit is Zeroed, and any queues or FIFO&#39;s that might contain pending modification reports are flushed. Now profile or translation information from the page may be encached. Whenever a modification of the page frame is reported, any encached information about the page is discarded. Once the cached information is purged, then the MPF bit for the page can be reset to Zero, and information about the page may again be cached. 
     VII.B. Overview of a Design That Uses Less Memory 
     While the simple design described in section VII.A, supra, would execute correctly and would impose little interrupt overhead, it might consume too much memory. On a system with 28 bits of physical address space and 4 KB page frames there are 65K page frames. This translates into 8 KB (or 256 256-bit cache lines) worth of storage just to hold the MPF bits. Those bits could be stored in memory but then, since a DMA read of such a memory based structure in response to every DMA write cycle would be unacceptable, DMU  700  would have to include some kind of caching mechanism. 
     The design described in this section is very similar to the simple model of section VII.A. In the embodiment discussed below, small, regular, naturally-aligned slices of the full MPF array are instantiated as needed, to monitor corresponding ranges of the physical address space. This design monitors only a subset of the entire physical address space at any given moment. When idle monitoring resources are reclaimed to monitor different physical addresses, this design for DMU  700  makes the conservative assumption that no page frame within the range that is about to be monitored has had a modification reported against it. This conservative assumption induces redundant reports of modification to page frames for which modifications had already been reported at some point in the past. 
     VII.C. Sector Monitoring Registers 
     Referring to  FIG. 7   a , DMU  700  has several Sector Monitoring Registers (SMR)  707 , typically four to eight. In the example embodiment discussed here, it is assumed that there are four SMR&#39;s  707  in the SMR file. Each SMR  707  monitors a sector, a naturally-aligned region of a power of 2 number of page frames. In the embodiment of  FIG. 7   a , a sector is a naturally-aligned 128 KB range of the G-bus physical memory address space, or equivalently, a naturally-aligned group of thirty-two 4 KB page frames. Each SMR  707  consists of a content addressable sector CAM (content-addressable memory, analogous to a TLB address tag)  708 , an array of MPF (Modified Page Frame) bits  710 , an Active bit  711 , and a small amount of logic. Sector CAM address tag  708  is eleven bits for a 28-bit physical address space (28, less 12 bits of byte addresses within a page, less 5 bits for the 32 pages per sector—see  FIG. 7   a ). MPF array  710  has 32 bits, one bit for each page frame in the sector. Each MPF array is essentially a 32-bit slice of the large MPF bit array described in section VII.A. (In order to maximize the opportunity to use large DMA transfers, modern operating systems tend to keep sequential virtual pages in sequential clusters in physical memory, so clustering of pages in an MPF array  710  offers much of the advantage of distinct MPF bits at lower address-tag matching overhead.) SMR.Active bit  711  is set to One if there was at least one Zero-to-One transition of an MPF bit  710  since the last time the SMR  707  was read. Thus, an SMR  707  is Active  711  when it contains at least one MPF bit  710  that has transitioned from Zero to One since the last time the SMR  707  was read out via DMU_Status register  720  (see section VII.G, infra.) DMU  700  will never reassign an active SMR  707  to monitor a different sector. 
     A DMU interrupt is asserted when one or more page frames have been modified, that is, when an MPF bit transitions from a Zero to a One. The handler for the DMU interrupt identifies the modified page frame(s). If the modified page is X86 text, then any translated TAXi code, and any profile information describing the page, are purged, and the corresponding PIPM entry  640  is released. 
     Referring to  FIG. 7   a , the physical address space is divided into 4K pages in the conventional manner. The pages are grouped into contiguous blocks called sectors. In the embodiment of  FIG. 7   a , thirty-two contiguous, naturally-aligned pages form one sector. In this embodiment, which allows for a maximum of 256 MB of physical memory, bits &lt;27:17&gt;  702  designate the sector. In other embodiments, more physical memory can be accommodated by extending the number of bits  702  to designate a sector. Bits &lt;16:12&gt;  704  designate the page number within a sector  702 . Bits &lt;11:00&gt; designate a byte within a page. 
     VII.D. Interface and Status Register 
       FIG. 7   b  illustrates the DMU interface. Writing to DMU_Command register  790  provides the sector address  702  and page address  704  (which in turn, is the bit address for the page&#39;s MPF bit within the SMR  707 ) and a DMU  700  command from the G-bus data. The low six bits of a datum are written to DMU_Command register  790  designates the command. The six bits of the command portion are designated D, E, R, A, M and X  791   a - 796   a . (The meaning of these bits is discussed in detail in section VII.H, infra.) When a DMA device issues a write to memory, the command value is D, E, R equal to Zero and A, M, X equal to One. From the D, E, A, M, X and R signals, several predicates are derived. Enable signal  714  means that the DMU is currently enabled. Allocate signal  715  is asserted on a bus transaction in which memory is written from a DMA device, and thus an SMR register must match, or be newly allocated to track the write. MPF modify signal  716  is asserted when the setting of the command bits specifies that the contents of an MPF bit  710  is to be written. MPF data signal  717  carries a datum to be written to an MPF bit  710  when MPF modify  716  is asserted. Reset signal  718  is asserted when the R reset command  794   a  is asserted on the bus. Read signal  719  is asserted as a distinct line of the G-bus  FIG. 7   b  also shows the Enable and Overrun flip-flops and the interrupt generation logic. The meanings of the six command bits  791   a - 796   a  are discussed in more detail infra, in connection with  FIGS. 7   i  and  7   j.    
     When DMU  700  is enabled  714 , DMU  700  requests an interrupt anytime there is at least one SMR  707  whose SMR.Active bit  711  is One or whenever the DMU Overrun flag  728  is set. The value of the active  711  SMR  707  is exposed in DMU_Status register  720 . 
     Referring to  FIG. 7   c , DMU_Status register  720  is 64 bits wide. The sector bits are located at their natural position, bits &lt;30:17&gt;, within a physical address, allowing for implementations with up to 2 GB of physical memory. The DMU_Status.Active bit  723  (bit &lt;31&gt;) is One when an active  711  SMR  707  is selected and Zero when all SMR&#39;s  707  are inactive. The least significant Modified Page Frame bit (SMR&lt;32&gt;)  724  corresponds to the page frame at the lowest address within a sector. Successive MPF bits  710  correspond to successively higher page frames. When DMU_Status.Active bit  723  is One, then the value of SMR# field  725  (SMR&lt;02:00&gt;) identifies the SMR  707  being returned. When DMU_Status.Active bit  723  is Zero, the Modified Page Frame bits  710 , Sector bits  722  and SMR# 725  are all Zero. 
     The Enable bit  727  and Overrun bit  728  are not actually part of any specific SMR  707 . Rather they summarize the overall state of DMU  700  and all SMR&#39;s  707 . Monitoring of DMA activity occurs only when DMU Enable  714  is set (DMU_Status.Enable  727  reflects the value of DMU Enable  714 , which in turn is set by writing to DMU_Command.Enable  795 , see  FIGS. 7   i  and  7   j ). Overrun bit  728  is provided at the time that an SMR  707  is read out, to allow recognition of cases when DMU  700  has shut down in response to a catastrophic overrun condition. The position of Overrun bit  728  as bit &lt;15&gt; (the sign bit of a 16-bit segment of DMU_Status register  720 ) simplifies testing it. 
     DMU_Status register  720  is described further in section VII.G in connection with  FIG. 7   h.    
     VII.E. Operation 
     Referring to  FIG. 7   d , the following steps occur on each DMA write transaction. In step  730 , DMU Enable  714 ,  727  is tested. If the DMU is disabled, no further processing occurs. In step  731 , the target physical address of the DMA bus transaction is captured into DMU_Command register  790 . Bits &lt;27:17&gt;  702  of the target address are captured as the sector number, and bits &lt;17:12&gt;  704  are captured as the page number index into an SMR of 32 MPF bits  710 , as shown in  FIG. 7   a . In step  740 , SMR sector CAM address tags  708  are searched associatively using the sector number. (This search will be elaborated further in the discussion of  FIG. 7   e  in section VII.F.) If the search succeeds (arrow  732 ), control skips forward to step  737 . If there is no match with any sector CAM address tag  708  (arrow  733 ), in step  750 , an inactive SMR  707  (one whose SMR.Active bit  711  is Zero) is allocated. (Allocation is discussed further in connection with  FIG. 7   f ). If no inactive SMR  707  is available, then a catastrophic overflow has occurred, and in step  734 , DMU Overrun  728  is set. On an overrun  728 , TAXi processing is aborted, and all translated code segments are purged (it is known that the DMA write that caused the overrun  728  may have overwritten a page of X86 code that had corresponding TAXi code, but the identity of that page cannot be identified, so all pages of TAXi code are considered suspect). Once the TAXi “cache” is purged, TAXi operation can resume. If an inactive SMR  707  can be located (arrow  735 ), then in step  736  within the allocated SMR  707 , all MPF bits  710  are Zeroed. Sector CAM address tag  708  of the allocated SMR  707  is loaded with the search key, sector number  702 . With SMR  707  thus allocated and set, it now satisfies the associative search criteria, so control flows to step  737  as though the search of step  740  had succeeded. 
     In step  737 , within matching SMR  707 , the MPF bit  710  corresponding to the modified page frame is tested. If the MPF bit  710  is already set to One (arrow  738 ), then no further processing is necessary. Otherwise (arrow  739 ), in step  760 ,  778 , the appropriate MPF bit  710  and the SMR.Active bit  711  are set to One (Active bit  711  may already be set). 
     VII.F. Circuitry 
     Referring to  FIG. 7   e , sector match hardware  740  performs the associative search of the sector CAM address tags  708  to determine whether the sector  702  of the current DMA write transaction already has an SMR  707  associated. Sector compare circuit  741  simultaneously compares the sector address  702  from DMU_Command register  790  with each of the four CAM address tag values  708  of the four SMR&#39;s  707  in the SMR file. Sector compare circuit  741  puts the result of this comparison on four bit bus  742 : each line of bus  742  is set to One if the corresponding SMR address tag  708  matches the bus sector address  702 . If any one of the four lines of bus  742  is One, then there was a match; OR gate  743  OR&#39;s together the four lines to determine whether a match occurred. Since the sector value in an inactive SMR  707  is undefined, more than one SMR  707  could match the incoming sector address  702 . Unary priority function  745  resolves this ambiguity by deterministically selecting at most one of the four asserted lines from bus  742 . Thus, the “matched SMR” 4-bit bus  746  will always have at most one line set to One. 
     Referring to  FIG. 7   f , SMR allocation hardware  750  allocates one of the inactive SMR&#39;s  707  out of the pool for writing into when none of the current SMRs&#39; address tags  708  match sector address  702 . Inactive SMR function  751  selects one of the inactive SMRs  707  (those whose SMR.Active bits  711  are Zero) if one is available. If the current bus transaction writes into a memory sector  702  that has no SMR  707  with a corresponding address tag  708  (indicated by matched  744  being Zero), and no SMR  707  is inactive  711  to accept the write (indicated by Allocate  715  being One), then the Overrun  728  condition has occurred. Otherwise, the SMR-to-write mask  753  (a four bit bus, with the one line asserted corresponding to the SMR register to be written) is generated from the SMR-to-read mask  787  (a four bit bus, with the one line asserted corresponding to the SMR register to be read), the matched SMR mask  746  (a four bit bus, with the one line asserted corresponding to the SMR register whose CAM sector address tag matches the bus address sector  702 ) and the inactive SMR mask  754  (the complement of the four SMR.Active bits  711  of the four SMR registers  707 ). 
     After sector match circuitry  740  or allocation circuitry  750  has selected an SMR  707 , MPF update logic  760 ,  772 ,  778  updates the appropriate MPF bits  710  and SMR.Allocate bits  711  in the selected SMR  707 . (Part of MPF update logic  760 , the portions  772 ,  778  that update the SMR address tags  708  and SMR.Active bits  711 , are shown in  FIGS. 7   e  and  7   f  and omitted from  FIG. 7   g .) The MPF bits  710  to modify are selected by MUX  761 , whose select is the SMR-to-write mask  753 . If the sector address  702  matched  744  none of the address tags  708  of any SMR  707 , then this is a newly-allocated, empty SMR  707 ; AND gate  762  generates all Zeros so that all MPF bits  710  of the new SMR  707  will be Zeroed. MPF bit update function  763  generates a new 32-bit value  764  for the MPF portion  710  of the selected SMR  707 . The inputs to MPF bit update function  763  are the 5-bit page address  704  within the sector  702  (these five bits select one of the 32=2 5  bits of MPF), the old contents of the MPF  710 , and the MPF modify signal  716 . The outputs  764 ,  766  of MPF bit update function  763  are chosen according to table  765 . If the old MPF bit  710  value was Zero and the new bit  710  value is One, then a Zero-to-One MPF transition  766  signal is asserted. The 32 bits of new MPF value  764  are OR&#39;ed together to generate MPF-all-Zeros signal  767 . Write logic  768  determines which MPF bit  710  to update, using as inputs the Reset  718 , Allocate  715 , matched  744 , MPF modify  716 , and SMR-to-write  753  signals. The outputs  770 ,  771  of write logic  768  are chosen according to table  769 . If column  770  is a One, then the MPF bits  710  of the SMR  707  selected by SMR-to-write mask  753  are written with 32-bit value  764 . If column  771  is a One, then the other SMR&#39;s  707  are written as well. Thus, the last line of table  769  indicates that a Reset  718  writes the all-Zeros value generated by AND gate  762  to all MPF registers  710 . 
     Referring again to  FIG. 7   f , write logic  772  determines a new SMR.Active bit  711  value to write according to table  773 . The inputs to write logic  772  are Read  719 , MPF all Zero&#39;s signal  767  and Zero-to-One MPF transition signal  766 . Column  774  tells whether to write the SMR.Active bit  711  of the SMR  707  selected be SMR-to-write  753  when the data inputs to write logic  772  match columns  719 ,  767 ,  766 . If column  774  is One, then column  775  tells the data value to write into that SMR.Active bit  711 . Similarly, column  776  tells whether or not to write the SMR.Active bits  711  of the unselected SMR registers, and column  777  tells the datum value to write. 
     Referring again to  FIG. 7   e , the sector tag  708  of a newly-allocated  750  SMR  707  is written as determined by write logic  778  (write logic  778  is intertwined with write logic  768 ,  772 , and is presented here simply for expository reasons). Write logic  778  accepts as input Allocate signal  715  and matched signal  744 , and computes its outputs according to table  779 . As indicated by the center row of the table, when an empty SMR is allocated by allocate logic  750  (the new allocation is indicated by Allocate  715  being One and the emptiness is indicated by matched  744  being Zero), then the sector address tag  708  of SMR indicated by SMR-to-write mask  753  is written. Else, as indicated by the top and bottom rows of table  779 , no SMR  707  is written. 
       FIGS. 7   d - 7   g  are merely representative of one embodiment. Known techniques for associative cache or TLB address tag matching, cache line placement policies, and inter-processor modified and dirty bits are generally applicable to managing SMR&#39;s  707 . (One difference should be noted. In a software-managed TLB, on a TLB miss, the PTE in memory is updated, and then the PTE is copied into the TLB. Thus, there is always a reliable backing copy of the TLB. In the DMU design presented here, there is no backing memory for the SMR registers  707 .) 
     In an alternative embodiment, in  FIG. 7   d , an additional step is performed in parallel with step  740 : TLB  116  is consulted to determine the ISA bit  182  and XP bit  184 ,  186  for the page being written. Unless the ISA bit  182  and XP bit  184 ,  186  are both One (indicating a page of protected X86 code), the entire rest of the DMU can be bypassed. The DMU exists only to track the validity of the TAXi code “cache” over the original X86 code, and if no such TAXi code can exist, then the remaining functions can be omitted. 
     Whenever an MPF bit undergoes a Zero-to-One transition, that is, when one or more page frames have been modified, a DMU interrupt is raised. The handler for the DMU interrupt identifies the modified page frame(s) by retrieving the state of all the active  711  SMR&#39;s  707 . The search for an active SMR  707  is performed in hardware, as described next. 
     VII.G. DMU_Status Register 
     Referring to  FIG. 7   h  in conjunction with  FIG. 7   c , DMU_Status register  720  is a 64-bit register on the G-bus. It is the only source of DMU information used in normal TAXi operation. If DMU Enable  714  (reflected in DMU_Status.Enable  727 , bit &lt;14&gt; of DMU_Status register  720 ) is Zero, then all reads of DMU_Status register  720  will return a result that is entirely Zero. Such a read does not re-enable DMU  700 ; DMU re-enablement is only accomplished by reinitialization. If DMU Enable  714  is One and no SMRs  707  are active  711 , then all reads of DMU_Status  720  will return a result that is entirely Zero except for a One in DMU_Status.Enable bit  727 . If DMU Enable  714  is One and there is at least one SMR  707  whose SMR.Active bit  711  is One, then reading DMU_Status  720  will return a snapshot of one of the active  711  SMRs  707 . This snapshot will have at least one MPF bit  710  set, DMU_Status.Active bit  723  set (reflecting SMR.Active bit  711  of the SMR  707 ) and DMU_Status.Enable bit  727  set. Reading the DMU_Status register  720  has the side effect of Zeroing SMR.Active bit  711  of the SMR  707  currently reflected in the DMU_Status register  720 , leaving the SMR  707  ready for reallocation  750 , but the address tag  708  and MPF bits  710  are left intact. Thus, further DMA writes into the same page will not induce a new Zero-to-One transition reducing the interrupt overhead induced by intensive I/O to I/O buffers. That SMR  707  will become active  711  again only if it gets reallocated  750  or if a DMA write occurs within the sector  702  that it monitors to a page frame whose MPF bit  710  is Zero. Similarly, a DMU interrupt will only be raised for that page if the MPF bit for the page is explicitly cleared (using a command where the M command bit is One, and all other command bits are Zero, see the commands discussed in section VII.H). 
     DMU_Status register  720  is driven by inputs from the file of SMR&#39;s  707 . The SMR select function  782  chooses an SMR  707  whose SMR.Active bit  711  is One. The selection  783  of the active SMR is used to select  784  the corresponding sector tag  708  and MPF bit  710  portions of the selected SMR  707 . When there is no active  711  SMR  707  (computed by OR gate  785 ), or the DMU is disabled  714 , then AND gates  786  ensure that all outputs are Zero. The selection  783  is gated by an AND gate to generate SMR-to-read signal  787 , which is used in  FIG. 7   f  to select one SMR register to be read. 
     Returning to the operation of the interrupt handler software, the act of reading DMU_Status register  720  is taken by DMU  700  as an implicit acknowledgment of the notification and hence a sign that the SMR(s)  707  involved can be reassigned. The DMU interrupt handler checks ISA bit  180 ,  182  and XP bit  184 ,  186  for the page to see whether the page written by the DMA write is a protected X86 page (this can be done in hardware before raising the interrupt, or in software). If the page is a protected X86 page, then the interrupt handler consults PIPM  602  to see whether any translated TAXi code exists corresponding to the modified page, and whether any profile information  430 ,  440  exists describing the modified page. If TAXi code is found, then it is released, and PIPM  602  is updated to reflect the release. If profile information is found, then it is released. 
     The DMU interrupt has higher priority than the probe exception, so that a probe will not transfer control to a page that has recently been invalidated. 
     VII.H. DMU_Command Register 
     Referring to  FIGS. 7   i ,  7   j  and Table 5 in conjunction with  FIG. 7   b , software controls DMU  700  through the DMU_Command register  790 . Bits &lt;05:00&gt;  791 - 796  control initializing DMU  700 , response after an overrun, re-enabling reporting of modifications to a page frame for which a modification might already have been reported, and simulating DMA traffic. The functions of the bits  791  are summarized in the following table 5. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 command 
                 bit 
                   
               
               
                 bit 
                 position 
                 Meaning 
               
               
                   
               
             
            
               
                 D 
                 5 
                 Disable monitoring of DMA writes by Zeroing the 
               
               
                   
                   
                 DMU enable flag 
               
               
                 E 
                 4 
                 Enable monitoring of DMA writes by setting the 
               
               
                   
                   
                 DMU Enable flag to One 
               
               
                 R 
                 3 
                 Reset all SMR&#39;s: Zero all A and MPF bits and 
               
               
                   
                   
                 Zero the DMU overrun flag 
               
               
                 A 
                 2 
                 Allocate an inactive SMR on a failed search 
               
               
                 M 
                 1 
                 Allow MPF modifications 
               
               
                 X 
                 0 
                 New MPF bit value to record on successful 
               
               
                   
                   
                 search or allocation 
               
               
                   
               
            
           
         
       
     
     D command bit  796   a ,  796   b ,  796   c  Zeros DMU Enable  714 ,  727 , thereby disabling any further changes to the SMRs  707  due to DMA traffic. If DMU Enable  714 ,  727  is already Zero, D bit  796  has no effect. 
     E enable command bit  795   a ,  795   b ,  795   c  sets DMU Enable  714 ,  727  to One, thereby enabling monitoring of future DMA traffic and DMA interrupts. If DMU Enable  714 ,  727  is already set, E bit  795  has no effect. 
     R command bit  794   a ,  794   b ,  794   c  resets DMU  700 . It does this by Zeroing the SMR.Active bit  711  and all MPF bits  710  in every SMR  707  and also Zeroing DMU Overrun flag  728 . The R command bit  794  has no effect on the values in the sector address CAM address tags  708 . The R command  794  takes precedence over the A, M and X commands  793 ,  792 ,  791 , and resets DMU  700  whether or not DMU  700  is enabled. 
     The high order bits (bits &lt;27:12&gt;)  797  of DMU_Command register  790  identify a page frame. Whenever a write occurs to DMU_Command register  790 , the page frame address  797  is presented to the SMR sector CAM address tags  708 . The A, M and X command bits  793 ,  792 ,  791  control what happens under various conditions:
         1. If the sector match hardware ( 740  of  FIG. 7   e ) fails to find a match  744 , and A command bit  793  is Zero, then do nothing. If there is no match  744 , and A command bit  793  is One, then normal allocation  750  is performed, as described in connection with  FIGS. 7   d  and  7   f . (Recall that normal allocation  750  can lead to an overrun condition  728  and hence to a DMU interrupt).   2. If either sector matching  740  or sector allocation  750  succeeds, then the M and X command bits  792 ,  791  define three possible actions according to table 6:       

     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 M 
                 X 
                 Action 
               
               
                   
                   
               
             
            
               
                   
                 0 
                 — 
                 Inhibit modification of the MPF bit 
               
               
                   
                 1 
                 0 
                 Zero the corresponding MPF bit 
               
               
                   
                 1 
                 1 
                 set the corresponding MPF bit to One 
               
               
                   
                   
               
            
           
         
       
     
     Writing a page frame address  702 ,  704 ,  797  to DMU_Command register  790  with the M command bit  792  set to One and the rest of the command bits  791 ,  793 - 796  to Zero searches  740  the sector CAM address tags  708  for a match. If a match  744  is found, the corresponding MPF bit  710  is Zeroed (because M bit  792  is One and X bit  791  is Zero, matching the second line of table  6 ). This is how TAXi enables monitoring of a page that is about to be turned from a page whose ISA bit  180 ,  182  is One and XP bit  184 ,  186  is Zero (unprotected X86 code) into a page whose XP bit  184 ,  186  is One (protected X86 code). If the MPF bit  710  that is cleared by such a command was the only MPF bit  710  set in the SMR  707 , then the SMR  707  reverts to inactive  711  and can be reallocated  750  to monitor a different sector. SMR.Active bit  711  is only affected by an MPF transition from Zero to One, or a transition of the last MPF bit from One to Zero. Otherwise, SMR.Active bit  711  is unaffected by changes to the MPF bits  710 . 
     It is software&#39;s responsibility never to enable DMU  700  until the sector CAM address tags  708  contain mutually distinct values. Once an overrun  728  occurs this condition is no longer assured. Hence the safest response to an overrun is reinitialization: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 DMU_Command = D+R 
                 // Disable DMU, reset SMRs 707 
               
               
                 for (i = 0, i &lt; #SMRs, i++) { 
                 // Initialize all SMRs 707 
               
            
           
           
               
               
            
               
                   
                 // Initialize each SMR 707 to a distinct address, by 
               
               
                   
                 // allocating a new SMR (A + M command bits) with 
               
               
                   
                 // sector “j” (j&lt;&lt;17) and page zero (1&lt;&lt;12) within the sector 
               
               
                   
                 DMU_Command = (i&lt;&lt;17) + (1&lt;&lt;12) + A + M 
               
            
           
           
               
            
               
                 } 
               
            
           
           
               
               
            
               
                 DMU_Command = E+R 
                 // Enable DMU, free all SMRs 
               
               
                   
               
            
           
         
       
     
     If not properly initialized the behavior of DMU  700  is undefined, guaranteed only not to harm the chip nor to introduce any security holes. 
     In an alternative embodiment, DMU  700  is more closely integrated with TLB  116 . In these embodiments, DMU  700  has access to ISA bit  182  and XP bit  186  (see section I.F, supra), and only raises an interrupt when a protected X86 page is written, or if the written page has no entry in TLB  116 . 
     VIII. Managing Out-of-Order Effects 
     Requiring all memory references (memory loads, memory stores, and instruction fetches) to be in-order and unoptimized limits the speed-up achievable by TAXi. Often the only barrier to optimization is knowing whether or not a load references well-behaved memory or some un-memory-like object. Recovering the original order of side effects, and preserving perfect X86 behavior, in spite of reordering and optimization by the TAXi translator, is discussed in section VIII. 
     VIII.A. Ensuring In-Order Handling of Events Reordered by Optimized Translation 
     Binary translator  124  is allowed to use code optimization techniques that reorder memory read instructions, floating-point instructions, integer divides, and other instructions that may generate exceptions or other side effects, in spite of the fact that the TAXi execution model of perfect emulation of the X86 maintains the order of side-effects. (“Side-effects” are permanent state changes, such as memory writes, exceptions that the X86/Windows architecture exposes to the application program, etc. Thus, a memory write and a divide-by-zero are each side-effects whose order is preserved relative to other side effects.) For instance, all memory references (memory reads, memory writes, and instruction fetches) are assumed to be “well-behaved,” free of both exceptions and hidden side-effects. All side-effects are kept ordered relative to each other. Correct execution is then ensured by catching any violations of these optimistic assumptions before any side-effect is irreversibly committed. 
     When profile information (see section V) tells TAXi translator  124  that a memory read can have a side-effect, for instance a read to I/O space (see section VII.B, infra), then the X86 code is translated using more conservative assumptions, assumptions that disallow memory references from being optimized to eliminate redundant loads, or to be reordered. This conservative code is annotated as having been generated under conservative assumptions. When conservative code accesses I/O space, the memory reference is allowed to complete, because the annotation assures the run-time environment that the code was generated with no optimistic assumptions. References to well-behaved memory from conservative code complete normally, simply at the cost of the foregone optimization. 
     Conversely, if no I/O space reference appears in the profile, then the TAXi code will be optimized under the optimistic assumption that all references are to well-behaved (that is, ASI Zero) memory—memory reads may be reordered or eliminated. The code is annotated to record the optimistic assumptions. All references to well-behaved memory complete normally, regardless of the value of the annotation. When optimistic TAXi code is running, and a memory reference violates the optimistic assumption by referencing I/O space (ASI not Zero) from optimistic code, then the reference is aborted by a TAXi I/O exception. In TAXi code references to I/O space are allowed to complete only if the code is annotated as following conservative assumptions. When a TAXi I/O exception occurs, the exception handler will force execution to resume in the converter. 
     When TAXi translator  124  generates native code, it may make the optimistic assumption that all memory references are to safe, well-behaved (ASI Zero) memory and can be optimized: e.g., that loads can be moved ahead of stores, if it can be proved that the memory locations don&#39;t overlap with each other, that memory reads can be reordered with respect to each other and with respect to instructions that do have side-effects, and that redundant loads from the same location, with no intervening store, can be merged together (CSE&#39;d—common sub-expression). TAXi translator  124  preserves all memory writes—memory writes are neither removed by optimization nor reordered relative to each other. However, references to I/O space, even mere reads, may have unknown side-effects (e.g., successive reads may return distinct values, and/or trigger separate side effects in an I/O device—recall, for instance from section VII.G, that a read of the DMU_Status register  720  invokes a state change in DMU  700 , so the next read of DMU_Status  720  will give a different result). 
     TAXi translator  124  relies on the safety net to protect references to non-well-behaved I/O space, that is, to intervene when the well-behaved translate-time optimistic assumption is violated at run time. The TAXi system records a static property of each memory reference, annotating whether that memory reference (specifically, a load) is somehow optimized. 
     TAXi translator  124  conveys to the hardware whether a memory reference involves optimistic assumptions or not. Those references that involve no optimistic assumptions are always allowed to complete. Those that do involve the optimistic assumption that the target is well-behaved memory will have this assumption verified on every execution and are aborted if the assumption cannot be guaranteed correct. 
     In one embodiment, one bit of each load or store instruction (or one bit of each memory operand descriptor in an instruction, if a single instruction makes multiple loads or stores) is reserved to annotate whether or not that particular load or store is optimized. 
     The following embodiment eliminates the need to dedicate one instruction opcode bit for this purpose. 
     The optimistic/conservative annotation is recorded in the “TAXi Optimized Load” bit  810  of a segment descriptor. 
     Because every X86 load is based off a segment register (the reference to a segment register may be explicitly encoded in the load operation, or it may be implicit in the instruction definition), and every segment has a segment descriptor, the segment register is a useful place to annotate the optimized property, and to monitor memory references. As each X86 load operation is decoded into micro-ops to send down the Tapestry pipeline, the segment register is explicitly materialized into the micro-op. 
     When TAXi code is running (that is, when PSW.TAXi_Active  198  is asserted), and in TAXi translated code a load occurs in-order with respect to other memory references, then the effect will be identical to the original X86 instruction stream irrespective of the nature of memory referenced by that load. When memory references are not reordered, it is preferable that a TAXi Optimized Load  810  Zero segment be used, so that no exceptions will be raised. 
     Referring to  FIG. 8   a , a Tapestry segment register  800  encodes a superset of the functions encoded in an X86 segment descriptor, and adds a few bits of additional functionality. Bit &lt;61&gt; of Tapestry segment register  800  is the “TAXi Optimized Load bit”  810 . (The segment descriptor TAXi Optimized Load bit  810  is distinct from the TAXi_Control.tio bit  820 ). When the segment descriptor TAXi Optimized Load bit  810  is One, all memory references off of this segment register are viewed as having been optimized under the optimistic assumptions. If a memory reference goes through a segment descriptor whose TAXi Optimized Load bit  810  is One, and the reference resolves to non-well-behaved memory (D-TLB.ASI, address space ID, not equal to Zero), and PSW.TAXi_Active  198  is true, then a TAXi I/O exception is raised. The handler for the TAXi I/O exception rolls the execution context back to the last safety net checkpoint and restarts execution in converter  136 , where the original unoptimized X86 instructions will be executed to perform the memory references in their original form and order. 
     The X86 has six architecturally-accessible segment descriptors; Tapestry models these six for the use of converter  136 , and provides an additional ten segment descriptors  800  accessible to native Tapestry code and TAXi code. The six X86-visible registers are managed by exception handlers in emulator  316 —when X86 code reads or writes one of the segment descriptors  800 , the exception handler intervenes to perform both the X86-architecturally-defined management and the management of the Tapestry extended functions. Converter  136  and emulator  316  ignore the value of the segment descriptor TAXi Optimized Load bits  810 ; during execution of X86 code in converter  136 , the value of bits  810  could be random. Nonetheless, converter  136  maintains bits  820  for the benefit of TAXi—in these six segment descriptors, the value of the segment descriptor TAXi Optimized Load bit  810  always matches Taxi_Control.tio ( 820  of  FIG. 4   g ). 
     The hardware format of a Tapestry segment register  800  differs from the architecturally-specified format of an X86 segment descriptor. Special X86-to-Tapestry hardware is provided to translate from one form to the other. When X86 code writes a segment descriptor value into a segment register, emulator  316  takes the segment descriptor value and writes it into a special X86-to-Tapestry conversion register. Hardware behind the special conversion register performs shifting and masking to convert from the X86 form to Tapestry form, copying the X86 segment descriptor bits into different bit positions, and gathering the Tapestry extended bits from elsewhere in the machine. In particular, the cloned segment descriptor&#39;s TAXi Optimized Load bit  810  is copied from TAXi_Control.tio  820 . Emulator  316  then reads the special conversion register, and that value is written into one of the Tapestry segment registers  800 . 
     At any particular software release, the value of TAXi_Control.tio  820  will always be set to the same value, and the TAXi translator  124  will rely on that value in translating X86 code. 
     Referring to  FIGS. 8   b  and  8   c , the segment descriptor TAXi Optimized Load bit  810  is managed by the TAXi translator  124 , as follows. 
     For the six segment registers visible to the X86, the default value of TAXi Optimized Load  810  is programmable at the discretion of the implementer. Recall that TAXi Optimized Load  810  is ignored by converter  136 . Hence, each time the converter  136  loads a segment descriptor register (a complex operation that in reality is performed in emulator  316 ), TAXi Optimized Load can be set arbitrarily. The conversion of X86 format segment descriptor values into Tapestry internal segment descriptor format is performed by hardware. This hardware must provide some value to TAXi Optimized Load. Rather than hardwire the value, the Tapestry system makes the value of the TAXi Optimized Load bit  810  programmable via TAXi_Control.tio  820 . 
     At system boot TAXi_Control.tio  820  is initialized to reflect the form of loads most likely to be emitted by the current TAXi translator. If translator  124  is not especially mature and rarely or never optimizes loads, then TAXi_Control.tio  820  is initialized to Zero. This means that the segment descriptors mapped to the six architecturally visible X86 segment registers will always have TAXi Optimized Load  810  Zero. Then code to clone the descriptor and set TAXi Optimized Load need only be generated in the prolog when a optimized load is actually generated. 
     The default registers will all be in one state, chosen to be the more common case so that those registers can be the defaults for use by TAXi. When TAXi wants the other semantics, the descriptor cloning at the beginning of the TAXi segment will copy the descriptor used by converter  136 , using a copy of TAXi_Control.tio  820  into the new segment descriptor&#39;s TAXi Optimized Load bit  810 . The opposite sense for bit  810  will be explicitly set by software. For instance, if the default sense of the segment descriptor is TAXi Optimized Load of Zero (the more optimistic assumption that allows optimization), then all optimized memory references must go through a segment descriptor that has TAXi Optimized Load bit  810  set to One, a new descriptor cloned by the TAXi code. This cloned descriptor will give us all the other descriptor exceptions, the segment limits, all the other effects will be exactly the same, with the additional function of safety-net checking for loads. 
     Referring to  FIG. 8   b , as the TAXi optimizer  124  translates the binary, it keeps track of which memory load operations are optimized, and which segment descriptors are referenced through loads that counter the default optimization assumption.  FIG. 8   b  shows the actions taken in a near-to-last pass of translator  124 , after all optimization has been completed, but before final emission of the new Tapestry binary. The upper half  840  of  FIG. 8   b  covers the case of relatively early releases of TAXi optimizer  124 , when optimization that reorders the side-effects is the exception rather than the rule. Lower half  850  reflects the later case, when optimization is more common, in which case the value of a segment&#39;s TAXi Optimized Load  810  would default to One, which in turn is controlled by setting TAXi_Control.tio  820  to One. For memory references that are reordered, commoned, or otherwise optimized on the optimistic assumption that only well-behaved, side-effect-free memory will be addressed (steps  841 ,  851 ), TAXi translator  124  forces the memory references to go through a segment descriptor whose TAXi Optimized Load  810  value is One (steps  843 ,  852 ). If the assumption is violated, that is, if at run time the memory reference through a TAXi Optimized Load  810  One segment is found to access I/O space, then that memory reference will raise a TAXi I/O exception, and execution of the translated code will be aborted into the safety net of converter  136 . If the TAXi translator  124  is willing to adopt conservative assumptions and not forgo opportunities to optimize this memory reference (for instance, if the profile indicates that this load referenced I/O space, as discussed in section VIII.B) (steps  844 ,  853 ), then the memory reference can go through a segment descriptor whose TAXi Optimized Load  810  bit is Zero (step  845 ,  855 ), thus guaranteeing that this memory reference will complete and never generate a TAXi I/O exception, even if to non-well-behaved memory. 
     In steps  842  and  854 , TAXi translator records which segment descriptors are used in a non-default manner. The overhead of a cloning a descriptor, and setting a non-default value of TAXi Optimized Load  810 , is only borne when required. 
     Referring to  FIG. 8   c , at the beginning of each translated hot spot, TAXi translator  124  inserts code that creates a cloned copy of any of the segment descriptors that were marked by steps  842 ,  854 , as being used in a non-default way, into one of the ten extra segment descriptors (step  866 ). This cloned descriptor will be used for some of the memory references made by the translated code, those that match the assumption embedded in the current release&#39;s value of TAXi_Control.tio  820 . The prolog code copies (step  866 ) the segment descriptor, and sets (step  868 ) the TAXi Optimized Load bit  810  to the sense opposite to the value of TAXi_Control.tio  820 , for use by memory references that assume opposite to the assumption embedded in the current release&#39;s value of TAXi_Control.tio  820 . 
     TAXi Optimized Load bit  810  has the following run-time behavior. 
     When converter  136  is running (that is, when PSW.TAXi_Active bit  198  is Zero), the TAXi optimized load bit  810  has no effect. Therefore converter  136  can issue loads through a segment irrespective of the value of the TAXi Optimized Load bit  810 . Whatever the value of TAXi Optimized Load bit  810 , the converter will be allowed to perform arbitrary memory references to arbitrary forms of memory and no TAXi optimized load exception will be induced. 
     When PSW.TAXi_Active  198  is One, the TAXi Optimized Load bit  810  determines whether a load from a non-zero ASI (i.e. memory not known to be well-behaved) should be allowed to complete (TAXi Optimized Load is Zero) or be aborted (TAXi Optimized Load is One). A TAXi I/O exception is raised when all three of the following are true:
         1. PSW.TAXi_Active  198  is One   2. a memory reference goes through a segment whose TAXi optimized Load bit  810  is One   3. the memory reference touches I/O space, that is, the ASI is not Zero       

     Given a mention of an X86 segment in some X86 code, the TAXi translator will sometimes want to use a descriptor with TAXi Optimized Load of One and sometimes with TAXi Optimized Load  810  Zero. Given an ability to read and write the descriptor register file, and one or more spare segment descriptor locations, a properly configured descriptor can be constructed by reading the original X86 descriptor location and setting or clearing TAXi Optimized Load  810  as appropriate. 
     Consider an example, where the TAXi translator uses optimistic assumptions and CSE&#39;s two loads together, so that only one load instruction actually exists in the TAXi instruction stream. The load that is actually optimized is the later load—but it no longer exists in the optimized instruction stream. Therefore, the remaining load is annotated, even if that load was not itself reordered relative to other side effects. When a load actually occurs to I/O space, off a TAXi Optimized Load  810  segment, then execution is rolled back to an instruction boundary, where all extended Tapestry state is dead. The TAXi code is abandoned, and the original X86 code is executed in converter  136 . Converter  136  will execute the X86 instructions exactly as it sees them and it will execute every one of the loads (the X86 instruction stream will still be in its original unoptimized form, even if the TAXi instruction stream was optimized) so that there will be no loads dropped from the stream as emitted by converter  136 . 
     The TAXi I/O fault is recognized before any side effects of the instruction are committed. 
     All TAXi code is kept in wired memory. Thus, no page fault can occur in fetching an instruction of TAXi code, and any page fault must necessarily involve a data reference. 
     As the TAXi code executes, as it crosses from a region translated from one page of X86 text to another page, it “touches” (a load without use of the result) the corresponding pages of X86 instruction text. (The page boundary crossings of the original X86 instruction text, were noted in the profile using the mechanism discussed in connection with  FIGS. 4   e  and  4   f  in section V.D.) This induces page faults in the original X86 code, to provide faithful emulation of the execution of the original X86 code. 
     After servicing a TAXi I/O exception in the Tapestry operating system  312  and emulator  316 , execution is restarted. In a simple embodiment, the X86 is restored to a previous X86 instruction boundary, and the restart is always at an X86 instruction boundary. Thus, if a single X86 instruction has two loads, then translator  124  must take one of two strategies, either (1) neither load can be optimized, or (2) both have to be annotated as optimized. This avoids a situation in which the first load is to non-well-behaved memory and is then re-executed if the second load raises a TAXi I/O exception. 
     VIII.B. Profiling References to Non-Well-Behaved Memory 
     Referring again to  FIG. 4   b , memory loads that are directed to anything other than address space ID (ASI) zero are recorded in the execution profile (see section V, supra) with a profile entry whose event code is 1.1100. ASI-non-zero references are typically (and conservatively assumed to be) directed to I/O space, that is, memory that is not well-behaved, as discussed in section I.D, supra. This indication provides a good heuristic for the TAXi translator  124  to choose between generating aggressive, optimized code and generating conservative, in-order code. 
     The initial assumption is that all memory reads are directed to well-behaved (address space zero) memory. When converter  136  is running (PSW.ISA indicates X86 mode), and profiler  400  is active (TAXi_State.Profile_Active  482  is One, see section V.E and V.F, infra), load instructions to I/O space (D-TLB.ASI not equal Zero) that complete cause a “I/O space load” profile entry to be stored in a register. The TAXi translator will interpret this profile entry to indicate that the optimistic assumption does not hold, and that at least this load must be treated under pessimistic assumptions by translator  124 , and can be marked with the “safe” setting of the segment descriptor “TAXi optimized load” bit discussed in section VIII.A, supra. 
     The implementation of this feature somewhat parallels the mechanism used for branch prediction. Recall that converter  134 ,  136  decomposes each X86 instruction into a plurality of native Tapestry RISC instructions for execution by Tapestry pipeline  120 . When a single X86 instruction has several memory references, each memory reference is isolated into a discrete Tapestry instruction. Even though the Zero/non-Zero ASI value is recorded in the D-TLB, and thus can be determined without actually initiating a bus cycle, the address space resolution occurs relatively late in the pipeline. Thus, when a reference to a non-zero ASI is detected, the Tapestry instructions following the load in the pipeline are flushed. TAXi_State.Event_Code_Latch  486 ,  487  (see section V.E, infra) is updated with the special I/O load converter event code 1.1100 of  FIG. 4   b . A TAXi instruction to record the I/O space profile entry is injected, and the normal profile collection hardware then records an “I/O space load” profile entry, in the manner discussed in connection with  FIGS. 5   a  and  5   b  in section V.F, supra. Note that this TAXi instruction may be injected in the middle of the recipe for a single X86 instruction, where the other TAXi instructions discussed in section V.F are injected at X86 instruction boundaries. Normal X86 instruction execution resumes in converter  136 , and the remainder of the instructions in the converter recipe are reinitiated. 
     Alternative embodiments might select other classes of instructions for profiling, typically those instructions that have a high likelihood of raising a synchronous exception, or that have some other property of interest to hot spot detector  122  or TAXi translator  124 . The number of such profiled instructions is kept relatively small, so as not to substantially reduce the density of the information made available to hot spot detector  122  or TAXi translator  124 . 
     VIII.C. Reconstructing Canonical Machine State to Arrive at a Precise Boundary 
     The code generated by TAXi translator  124  is annotated with information that allows the recovery of X86 instruction boundaries. If a single X86 instruction is decomposed into many Tapestry instructions, and those Tapestry instructions are reordered by the TAXi optimizer, then the annotation allows the end of particular X86 instructions to be identified. The information stored is similar to that emitted by optimizing compilers for use by debuggers. There, the instructions of a single source statement are annotated so that source statements can be recovered. In TAXi, the annotation allows the recovery of X86 instruction boundaries from a tangled web of Tapestry instructions. Thus, when a synchronous exception is to be exposed to the virtual X86, the TAXi run time system establishes a system state equivalent to that which would prevail at an X86 instruction boundary. Once state is restored to a precise instruction boundary, execution can be tendered to converter  136 , which in turn can resume execution from that instruction boundary. 
     In some instances, this annotation mechanism may roll back execution by a considerable number of instructions, in order to establish a “safe” state, where all X86 instructions can either be assumed to have not started, or completed completely. The rollback mechanism avoids resuming execution from a state where a single side-effect may be applied twice. 
     The code may “checkpoint” itself, capturing a self-consistent state snapshot somewhat in the manner of a database system. Then, in the event of a fault in the TAXi code, execution can be rolled back to the checkpoint, and resumed in converter  136 . 
     VIII.D. Safety Net Execution 
     Referring again to  FIG. 3   j , in one alternative embodiment, if this is an asynchronous interrupt, case  351  or  354  can allow X86 emulator  316  or converter  136 , respectively, to progress forward to the next X86 instruction boundary, before delivering the interrupt. In another alternative embodiment, case  354  can roll back X86 emulator  316  to the previous X86 instruction boundary. After state is secured to an X86 boundary, execution proceeds through X86 operating system  306  as in case  351 . In other alternative embodiments, in the case of asynchronous interrupts in cases  351 ,  353 , and  354 , the code can be allowed to progress forward to the next safety net checkpoint before delivering the exception. Each of these are conceptually similar, in that the virtual X86  310  is “brought to rest” at a stable point at which all Tapestry extended context is dead and discardable, and only events whose order is not guaranteed by the X86 architecture are allowed to be reordered with respect to each other. 
     When an exception occurs in TAXi code and the exception handler determines that it must materialize the exception in the x86 virtual machine, it jumps to a common entry in emulator  316  that is responsible for setting the x86 state—establishing the interrupt stack frame, accessing the IDT and performing the control transfer. When this function is invoked, it must first determine if TAXi code was being executed by examining PSW.TAXi_Active and if so jump to a TAXi function that reconstructs the X86 machine state and then re-executes the X86 instruction in the converter to provoke the same exception again. Re-executing the X86 instruction establishes the correct X86 exception state. Anytime the converter is started to re-execute an x86 instruction, the exception handler uses the RFE with probe failed, reload probe timer event code to prevent a recursive probe exception from occurring. 
     The only exceptions that may not be exposed to the X86 are those that can be completely executed by native Tapestry code, e.g. TLB miss that is satisfied without a page fault, FP incomplete with no unmasked x86 floating-point exceptions, etc. 
     IX. Interrupt Priority 
     The TAXi system uses five exceptions, and one software trap. DMU  700  introduces one new interrupt sub-case. These interrupts are summarized in the following table 7: 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 name 
                 description 
                 type 
                 priority 
                 discussion 
               
               
                   
               
             
            
               
                 VECT_TAXi_UNPROTECTED 
                 starting profile on a TAXi 
                 note 1 
                 4.0 
                 I.F 
               
               
                   
                 unprotected page 
               
               
                 VECT_TAXi_PROBE 
                 probe for translated code 
                 note 2 
                 4.1 
                 VI 
               
               
                   
                 exception 
               
               
                 VECT_TAXi_PROFILE 
                 profile packet complete 
                 note 2 
                 4.2 
                 V 
               
               
                   
                 exception 
               
               
                 VECT_TAXi_PROTECTED 
                 writing to a TAXi protected 
                 fault 
                 5.4 
                 I.F 
               
               
                   
                 page 
               
               
                 VECT_TAXi_IO 
                 read from (ASI != 0) in 
                 fault 
                 5.5 
                 VIII.A 
               
               
                   
                 translated code 
               
               
                 VECT_TAXi_EXIT 
                 restart converter on TAXi 
                 software 
                 2.4 
                 VI.F 
               
               
                   
                 code completion 
                 trap 
               
               
                 DMU_INVALIDATE 
                 DMU invalidation event 
                 interrupt 
                 2.0 
                 VII 
               
               
                   
               
               
                 note 1. 
               
               
                 This fault is raised on the first native instruction in an X86 converter recipe. 
               
               
                 note 2. 
               
               
                 This fault is raised as a trap on the TAXi instruction, i.e. the TAXi instruction completes writing its data to the GPR. 
               
            
           
         
       
     
     To achieve performance, TAXi code does not keep X86 state in the canonical locations assumed by converter  136  and emulator  316 . Therefore, when TAXi code is interrupted, the converter is not allowed to resume without first recovering the canonical picture of the X86 machine&#39;s state. 
     The exception strategy described supra is intended to achieve correctness through simplicity, to have a single common strategy for processing all exceptions, to ensure that exceptions raised in TAXi code are processed by exactly the same code as exceptions raised by the converter, to maximize performance, to delay abandoning TAXi code until it is known that an exception must be surfaced to the X86, and to allow TAXi code to forego maintaining the X86 FP exception state. 
     For the convenience of the reader, the above description has focused on a representative sample of all possible embodiments, a sample that teaches the principles of the invention and conveys the best mode contemplated for carrying it out. The description has not attempted to exhaustively enumerate all possible variations. Other undescribed variations or modifications may be possible. For example, where multiple alternative embodiments are described, in many cases it will be possible to combine elements of different embodiments, or to combine elements of the embodiments described here with other modifications or variations that are not expressly described. Many of those undescribed variations, modifications and variations are within the literal scope of the following claims, and others are equivalent. 
     The following volumes are incorporated by reference. I NTEL  A RCHITECTURE  S OFTWARE  D EVELOPER&#39;S  M ANUAL, VOL.  1-3, Intel Corp. (1997); G ERRY  K ANE , PA-RISC 2.0 A RCHITECTURE , Hewlett-Packard Professional Books, Prentice-Hall (1996); R ICHARD  L. S ITES AND  R ICHARD  T. W ITEK , T HE  A LPHA  AXP A RCHITECTURE  R EFERENCE  M ANUAL,  2d ed., Digital Press, Boston (1995); D AVID  A. P ATTERSON AND  J OHN  L. H ENNESSEY , C OMPUTER  A RCHITECTURE : A Q UANTITATIVE  A PPROACH , Morgan Kaufman Publ., San Mateo, Calif. (1990); T IMOTHY  L EONARD, ED ., VAX A RCHITECTURE  R EFERENCE  M ANUAL , Digital Equipment Corp. (1987); P ETER  M. K OGGE , THE A RCHITECTURE OF  P IPELINED  C OMPUTERS , Hemisphere Publ., McGraw Hill (1981); J OHN  M ICK AND  J AMES  B RICK , B IT -S LICE  M ICROPROCESSOR  D ESIGN , McGraw-Hill (1980).