Patent Publication Number: US-7222226-B1

Title: System and method for modifying a load operation to include a register-to-register move operation in order to forward speculative load results to a dependent operation

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention is related to the field of microprocessors, and more particularly, to performing data-speculative execution in a microprocessor. 
   2. Description of the Related Art 
   Superscalar microprocessors achieve high performance by executing multiple instructions concurrently and by using the shortest possible clock cycle consistent with their design. However, data and control flow dependencies between instructions may limit how many instructions may be issued at any given time. As a result, some microprocessors support speculative execution in order to achieve additional performance gains. 
   One type of speculation is control flow speculation. Control flow speculation predicts the direction in which program control will proceed. For example, branch prediction may be used to predict whether a branch will be taken. Many types of branch prediction are available, ranging from methods that simply make the same prediction each time to those that maintain sophisticated histories of the previous branches in the program in order to make a history-based prediction. Branch prediction may be facilitated through hardware optimizations, compiler optimizations, or both. Based on the prediction provided by the branch prediction mechanism, instructions may be speculatively fetched and executed. When the branch instruction is finally evaluated, the branch prediction can be verified. If the prediction was incorrect, any instructions that were speculatively executed based on the incorrect predication may be quashed. 
   Another type of speculation that has been proposed is data speculation. For example, value prediction, which predicts the value of data items, may involve observing patterns in data and basing the prediction on those patterns (e.g., an index counter variable&#39;s value may be predicted by observing how prior values of that variable are incremented or decremented). Address prediction involves predicting the location of data. Yet another type of data speculation is called memory system optimism. In multi-processor systems, memory system optimism occurs when a processor speculatively executes an instruction using data from that processor&#39;s local cache before coherency checking is complete. Similarly, another type of data speculation may allow a load to speculatively execute before a store that has an uncomputed address at the time the load executes, even though the store may store data to the same address that the load accesses. In all of these types of data speculation, the underlying conditions are eventually evaluated, allowing the speculation to be verified or undone. If the speculation ends up being incorrect, the instructions that executed using the speculative data may be re-executed (e.g., with updated and/or non-speculative data). 
   Since speculation allows execution to proceed without waiting for dependency checking to complete, significant performance gains may be achieved if the performance gained from correct speculations exceeds the performance lost to incorrect speculations. Accordingly, it is desirable to be able to perform data speculation in a microprocessor and to provide an efficient recovery mechanism for misspeculations. 
   SUMMARY 
   Various embodiments of methods and systems for modifying load operations to forward speculative load results to dependent operations may be implemented. In one embodiment, a system may include a dispatch unit, a scheduler, and an execution core. The dispatch unit may be configured to modify a load operation to include a register-to-register move operation in response to an indication that a speculative result of the load operation is linked to a data value identified by a first tag. The scheduler may be coupled to the dispatch unit and configured to issue the register-to-register move operation in response to availability of the data value. The execution core may be configured to execute the register-to-register move operation by outputting the data value and a tag indicating that the data value is the result of the load operation. 
   One embodiment of a method may involve detecting a link between a speculative result of a load operation and a data value identified by a first tag, and in response to detecting the link, modifying the load operation to include a register-to-register move operation. A source operand of the register-to-register move operation may be identified by the first tag and a destination operand of the register-to-register move operation may be identified by a second tag assigned to identify the result of the load operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
       FIG. 1  shows one embodiment of a microprocessor. 
       FIG. 2A  is a block diagram of one embodiment of a dispatch unit. 
       FIG. 2B  shows an exemplary memory file storage entry that may be used in one embodiment. 
       FIG. 3  is a flowchart showing one embodiment of a method of linking the speculative result of a load operation to a data value identified by a particular tag. 
       FIG. 4  is a block diagram of one embodiment of a scheduler. 
       FIG. 5  is a flowchart of one embodiment of a method of issuing operations and reissuing operations in response to an indication that data speculation was incorrect. 
       FIG. 6  is a block diagram of another embodiment of a dispatch unit. 
       FIG. 7  is a block diagram of yet another embodiment of a dispatch unit. 
       FIG. 8  is a flowchart showing another method of issuing operations with speculative data values and recovering from mispredictions. 
       FIG. 9  shows one embodiment of a computer system. 
       FIG. 10  shows another embodiment of a computer system. 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must). The term “include” and derivations thereof mean “including, but not limited to.” The term “connected” means “directly or indirectly connected,” and the term “coupled” means “directly or indirectly coupled.” 
   DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1  is a block diagram of one embodiment of a microprocessor  100 . Microprocessor  100  is configured to execute instructions stored in a system memory  200 . Many of these instructions operate on data stored in system memory  200 . Note that system memory  200  may be physically distributed throughout a computer system and may be accessed by one or more microprocessors  100 . 
   Microprocessor  100  may include an instruction cache  106  and a data cache  128 . Microprocessor  100  may include a prefetch unit  108  coupled to the instruction cache  106 . A dispatch unit  104  may be configured to receive instructions from instruction cache  106  and to dispatch operations to scheduler(s)  118 . One or more schedulers  118  may be coupled to receive dispatched operations from dispatch unit  104  and to issue operations to one or more execution cores  124 . Execution core(s)  124  may include a load/store unit  126  configured to perform accesses to data cache  128 . Results generated by execution core(s)  124  may be output to a result bus  130 . These results may be used as operand values for subsequently issued instructions and/or stored to register file  116 . A retire queue  102  may be coupled to scheduler(s)  118  and dispatch unit  104 . The retire queue may be configured to determine when each issued operation may be retired. In one embodiment, the microprocessor  100  may be designed to be compatible with the x86 architecture. Note that microprocessor  100  may also include many other components. For example, microprocessor  100  may include a branch prediction unit (not shown). 
   Instruction cache  106  may temporarily store instructions prior to their receipt by dispatch unit  104 . Instruction code may be provided to instruction cache  106  by prefetching code from the system memory  200  through prefetch unit  108 . Instruction cache  106  may be implemented in various configurations (e.g., set-associative, fully-associative, or direct-mapped). 
   Prefetch unit  108  may prefetch instruction code from the system memory  200  for storage within instruction cache  106 . In one embodiment, prefetch unit  108  may be configured to burst code from the system memory  200  into instruction cache  106 . Prefetch unit  108  may employ a variety of specific code prefetching techniques and algorithms. 
   Dispatch unit  104  may output signals including bit-encoded operations executable by the execution core(s)  124  as well as operand address information, immediate data and/or displacement data. In some embodiments, dispatch unit  104  may include decoding circuitry (not shown) for decoding certain instructions into operations executable within execution core(s)  124 . Simple instructions may correspond to a single operation. In some embodiments, more complex instructions may correspond to multiple operations. Upon decode of an operation that involves the update of a register, a register location within register file  116  may be reserved to store speculative register states (in an alternative embodiment, a reorder buffer may be used to store one or more speculative register states for each register). A register map  134  may translate logical register names of source and destination operands to physical register names in order to facilitate register renaming. Register map  134  may track which registers within register file  116  are currently allocated and unallocated. 
   The microprocessor  100  of  FIG. 1  supports out of order execution. A retire queue  102  may keep track of the original program sequence for register read and write operations, allow for speculative instruction execution and branch misprediction recovery, and facilitate precise exceptions. In some embodiments, retire queue  102  may also support register renaming by providing data value storage for speculative register states. In many embodiments, retire queue  102  may function similarly to a reorder buffer. However, unlike a typical reorder buffer, retire queue  102  may not provide any data value storage. In some embodiments, retire queue  102  may be implemented in a first-in-first-out configuration in which operations move to the “bottom” of the buffer as they are validated, thus making room for new entries at the “top” of the queue. As operations are retired, retire queue  102  may deallocate registers in register file  116  that are no longer needed to store speculative register states and provide signals to register map  134  indicating which registers are currently free. By maintaining speculative register states within register file  116  (or, in alternative embodiments, within a reorder buffer) until the operations that generated those states are validated, the results of speculatively-executed operations along a mispredicted path may be invalidated in the register file  116  if a branch prediction is incorrect. 
   Upon decode of a particular operation, if a required operand is a register location, register address information may be routed to register map  134  (or a reorder buffer). For example, in the x86 architecture, there are eight 32-bit logical registers (e.g., EAX, EBX, ECX, EDX, EBP, ESI, EDI and ESP). Physical register file  116  (or a reorder buffer) includes storage for results that change the contents of these logical registers, allowing out of order execution. A physical register in register file  116  may be allocated to store the result of each operation which, upon decode, is determined to modify the contents of one of the logical registers. Therefore, at various points during execution of a particular program, register file  116  (or, in alternative embodiments, a reorder buffer) may have one or more registers that contain the speculatively executed contents of a given logical register. 
   Register map  134  may assign a physical register to a particular logical register specified as a destination operand for an operation. Dispatch unit  104  may determine that register file  116  has one or more previously allocated physical registers assigned to a logical register specified as a source operand in a given operation. The register map  134  may provide a tag for the physical register most recently assigned to that logical register. This tag may be used to access the operand&#39;s data value in the register file  116  or to receive the data value via result forwarding on the result bus  130 . If the operand corresponds to a memory location, the operand value may be provided on the result bus (for result forwarding and/or storage in register file  118 ) through load/store unit  222 . Operand data values may be provided to execution core(s)  124  when the operation is issued by one of the scheduler(s)  118 . Note that in alternative embodiments, operand values may be provided to a corresponding scheduler  118  when an operation is dispatched (instead of being provided to a corresponding execution core  124  when the operation is issued). 
   The bit-encoded operations and immediate data provided at the outputs of dispatch unit  104  may be routed to one or more schedulers  118 . Note that as used herein, a scheduler is a device that detects when operations are ready for execution and issues ready operations to one or more execution units. For example, a reservation station is a scheduler. Each scheduler  118  may be capable of holding operation information (e.g., bit encoded execution bits as well as operand values, operand tags, and/or immediate data) for several pending operations awaiting issue to an execution core  124 . In some embodiments, each scheduler  118  may not provide operand value storage. Instead, each scheduler may monitor issued operations and results available in register file  116  in order to determine when operand values will be available to be read by execution core(s)  124  (from register file  116  or result bus  130 ). In some embodiments, each scheduler  118  may be associated with a dedicated execution core  124 . In other embodiments, a single scheduler  118  may issue operations to more than one of the execution core(s)  124 . 
   Schedulers  118  may be provided to temporarily store operation information to be executed by the execution core(s)  124 . As stated previously, each scheduler  118  may store operation information for pending operations. Additionally, each scheduler may store operation information for operations that have already been executed but may still reissue. Operations are issued to execution core(s)  124  for execution in response to the values of any required operand(s) being made available in time for execution. Accordingly, the order in which operations are executed may not be the same as the order of the original program instruction sequence. Operations that involve data speculation may remain in scheduler(s)  118  until they become non-speculative so that they may be reissued if the data speculation is incorrect. 
   In one embodiment, each of the execution core(s)  124  may include components configured to perform integer arithmetic operations of addition and subtraction, as well as shifts, rotates, logical operations, and branch operations. A floating point unit may also be included to accommodate floating point operations. One or more of the execution core(s)  124  may be configured to perform address generation for load and store memory operations to be performed by load/store unit  126 . 
   The execution core(s)  124  may also provide information regarding the execution of conditional branch instructions to a branch prediction unit so that if the branch was mispredicted, the branch prediction unit may flush instructions subsequent to the mispredicted branch that have entered the instruction processing pipeline and redirect prefetch unit  106 . The redirected prefetch unit  106  may then begin fetching the correct set of instructions from instruction cache  106  or system memory  200 . In such situations, the results of instructions in the original program sequence that occurred after the mispredicted branch instruction may be discarded, including those which were speculatively executed and temporarily stored in register file  116 . Results produced by components within execution core(s)  124  may be output on the result bus  130  to the register file  116  if a register value is being updated. If the contents of a memory location are being changed, the results produced within execution core(s)  124  may be provided to the load/store unit  126 . 
   Load/store unit  126  provides an interface between execution core(s)  124  and data cache  128 . In one embodiment, load/store unit  126  may be configured with a load/store buffer with several storage locations for data and address information for pending loads or stores. The load/store unit  126  may also perform dependency checking for load instructions against pending store instructions to ensure that data coherency is maintained. 
   Data cache  128  is a cache memory provided to temporarily store data being transferred between load/store unit  126  and the system memory  200 . Like the instruction cache  106  described above, the data cache  128  may be implemented in a variety of specific memory configurations, including a set associative configuration. Additionally, data cache  106  and instruction cache  128  may be implemented in a unified cache in some embodiments. 
     FIG. 2A  shows one embodiment of a dispatch unit  104 . In this embodiment, dispatch unit  104  includes a register map  134  and a memory file  132 . As mentioned above, register map  134  may be configured to provide register renaming. Register map  134  may receive logical register names for each source and destination operand and output the physical register names of the physical registers most recently assigned to the logical registers. Memory file  132  includes a memory file controller  202  and memory file storage  204 . 
   Memory file storage  204  includes one or more entries  220 . Each entry  220  may include an addressing pattern  206  and a tag  208  associated with that entry&#39;s addressing pattern. Each tag may identify a data value by indicating where that data value will be stored (e.g., within a reorder buffer or within a register file  116 ) when it is generated. For example, as shown in the illustrated embodiment, each tag may identify the physical register allocated to store that data value, as indicated by register map  134 . Each addressing pattern may include all or some of the information used to specify an address in an operation. For example, looking quickly at  FIG. 2B , an exemplary addressing pattern  206  is shown. In this embodiment, the addressing pattern  206  includes a base logical register name  210 , an index logical register name  212 , and a displacement  214 . Some addressing patterns  206  may include a segment portion used to identify a particular segment in memory. Note that in many embodiments, the addressing pattern  206  stored in memory file storage  204  may include less than all of the addressing information specified for an operation. For example, entries in memory file storage  204  may store fewer than all of the bits used to specify a displacement within displacement field  214 . Memory file storage  204  may be implemented from several registers, latches, flip-flops, or other clocked storage in some embodiments. In alternative embodiments, memory file storage  204  may include one or more RAM (Random Access Memory) cells. 
   Memory file controller  202  may compare the addressing patterns specified in undispatched operations to those stored within entries in memory file storage  204 . If an operation&#39;s addressing pattern does not match any of the addressing patterns currently stored within entries in memory file storage  204  (i.e., the operation&#39;s addressing pattern misses in the memory file storage), memory file controller  202  may allocate a new entry in memory file storage  204  to store all or some of that operation&#39;s addressing pattern. If there are no free entries to allocate within memory file storage  204 , memory file controller  202  may select an entry to overwrite using a replacement scheme such as LRU (Least Recently Used), FIFO (First In, First Out), random replacement, etc. In addition to storing the operation&#39;s addressing pattern within the allocated entry, the memory file controller  202  may also store the a tag (e.g., the name of the physical register) identifying a value being loaded from or stored into the memory location identified by that operation&#39;s addressing pattern. For example, if an operation that loads data from memory is being handled, the entry allocated in response to that operation may store the name of the physical register allocated to store the result of the load operation. If an operation that stores data to a memory location is being handled, memory file controller  202  may store the physical register name of the register storing the value being stored by the store operation in memory file storage  204 . 
   If an operation&#39;s addressing pattern (or a portion of that pattern) is already stored an entry in memory file  204  (i.e., the operation&#39;s addressing pattern hits in the memory file storage), the memory file controller  202  may use or modify the entry containing the matching addressing pattern. If a load operation (an operation that loads a value from a particular address into a register) is being handled, the memory file controller  202  may output the physical register name  208  stored in the matching entry. If a store operation (an operation that stores a value from a register to a particular address) is being handled, the memory file controller  202  may overwrite the tag (e.g., physical register name  208 ) stored in the matching entry with the tag of the data being stored. 
   If a load operation is being handled and the load operation hits in the memory file storage  204 , the tag output by the memory file controller  202  may be used to link the value stored identified by the tag to a speculative result of the load operation. For example, in some embodiments, when the load operation is dispatched to scheduler(s)  118 , the tag output by memory file  132  may also be provided to the scheduler(s) (e.g., as a speculative source operand tag). A scheduler  118  may issue the operation in response to the availability (e.g., in register file  116  or on result bus  130 ) of the value identified by that tag. An execution core  124  may execute the load operation so that the linked value is broadcast as the speculative result of the load operation on result bus  130  (note that the data value produced as the load&#39;s result may not itself be flagged or otherwise identified as a speculative value in some embodiments). In other embodiments, the data value may be linked to the speculative result of the load operation by storing the tag in a speculative map, as described below. 
   As a result of the link, the data value identified by the tag may be forwarded as the speculative result of the load once the data value is available (e.g., in register file  116  or on result bus  130 ) in order to allow dependent operations to execute using the speculative result. In many cases, this may allow dependent operations to execute using the speculative result of the load operation sooner than they could if their execution is delayed until the non-speculative result of the load operation becomes available. In some embodiments, the data value may be forwarded by executing the load operation so that the data value is output onto the result bus  130  as the speculative result of the load operation. For example, in one embodiment, instead of taking three cycles to perform the load non-speculatively (assuming the load hit in the data cache  128 ), the load may be executed in a single cycle by outputting the data value and a tag identifying the data value as the load result. In other embodiments, the data value may be forwarded in a more indirect manner by providing the tag output by memory file controller  202  directly to dependent operations (operations having an operand produced by the load operation) as a speculative operand source at dispatch. Means for forwarding the data value may include one or more of: a dispatch unit configured to modify the load operation to execute as a speculative register-to-register move operation or to provide the tag to a dependent operation as a speculative operand source tag, a scheduler configured to issue the modified load and/or the dependent operation dependent on the availability of the linked data value, and an execution core configured to output the linked data value as the result of the load or to execute the dependent operation using the linked data value. 
   The load store unit  126  (or another means for verifying the speculative link within microprocessor  100 ) may verify the speculative link of the value stored in the physical register identified by memory file  132  to the result of the load operation. If the speculative link is incorrect, the load store unit  126  may cause the load to be reissued and/or broadcast the correct result of the load operation on result bus  130 . Reissuing the load may cause any dependent operations that executed using the speculative result of the load to reissue and execute using the updated, non-speculative value. 
   As shown, memory file  132  tracks the addressing patterns in operations that access data in data cache  128  (or system memory  200 ). As a result, register values stored in physical registers may be linked to values stored in particular addresses in memory. 
   In order to further illustrate the operation of memory file  132 , assume that a sequence of operations to be handled by dispatch unit  104  includes the following operations: 
                                              MOV EBX, [EDX + EAX − displacement A]   (LOAD 1)           . . .           MOV ECX, [EDX + EAX − displacement A]   (LOAD 2)           . . .           MOV [EDX + EAX − displacement A], EAX   (STORE 1).                        
Each of these operations may be separated by one or more intervening operations in program order. As shown, each of these three operations includes the same addressing pattern, EDX+EAX−displacement A.
 
   When LOAD 1&#39;s addressing pattern is provided to memory file  132 , memory file controller  202  may check memory file storage  204  for an addressing pattern that matches LOAD 1&#39;s addressing pattern. Assuming that the addressing pattern misses in the memory file storage  204 , the memory file controller  202  may allocate an entry (either by using an unallocated entry or by overwriting an already allocated entry) to store all or some of the addressing pattern of the load operation and the physical register name of the load operation&#39;s destination physical register as provided by register map  134 . Since the load operation misses in the memory file storage, the memory file controller  202  may not output a tag for that load operation. 
   When LOAD 2 is subsequently handled by memory file  132 , its addressing pattern may match the addressing pattern in the entry allocated in response to LOAD 1 (assuming LOAD 1&#39;s entry has not been already been overwritten in response to an intervening operation). In response to LOAD 2&#39;s addressing pattern hitting in memory file storage  204 , memory file controller  202  may output the physical register name of the physical register allocated to store the result of LOAD 1. This physical register name may be used to link the data value loaded by LOAD 1 to the speculative result of LOAD  2 . 
   When STORE 1 is handled by memory file  132 , its addressing pattern may hit in the entry allocated in response to LOAD 1 (again assuming that no intervening operation has caused this entry to be overwritten). Instead of outputting the physical register name of the physical register allocated to store the result of LOAD 1 (as was done for LOAD 2) however, memory file controller  202  may overwrite the physical register name  208  stored in that entry with the physical register name of the register containing the data being stored by STORE 1. Thus, when subsequent load operations hit in this entry, the memory file controller  202  will output the physical register name of STORE 1&#39;s source physical register instead of outputting the physical register name of LOAD 1&#39;s destination register. 
   Since memory file  132  is being used as a speculative structure, the accuracy of the information stored in memory file storage  204  may not be critical to the correct operation of microprocessor  100  (e.g., mispredictions in memory file  132  may not cause errors in the output of microprocessor  100 ). However, it may be desirable to improve the accuracy of memory file  132  in order to increase the benefits provided by correctly linking the speculative results of load operations to values stored in physical registers and/or to decrease any performance penalties incurred for mispredicted speculative links. In some embodiments, the accuracy of memory file  132  may be increased by invalidating entries in memory file storage  204  when updates to registers used to specify addresses are detected. For example, each addressing pattern may include one or more logical register identifiers used to identify a base and an index for address calculation. If a subsequent operation modifies one of the logical registers specified as part of an entry&#39;s addressing patter  206 , that entry may be invalidated. Thus, logical register names of the destination register of each operation may be input to memory file  132  in order to perform entry invalidations, as shown in  FIG. 2A . 
   Additionally, in some embodiments, entries within memory file storage  204  may be invalidated in response to snooping another device gaining write access to data at a particular address. Similarly, an entry may be invalidated in response to detection of a misprediction. Generally, many conditions such as these that may affect the accuracy of the entries in the memory file storage  204  may be monitored and used to determine when to invalidate entries. However, since memory file  132  is a speculative structure, some embodiments may not implement some of these monitoring methods if the additional hardware cost to implement certain monitoring methods outweighs the potential improvement in memory file accuracy. 
   Note that memory file  132  allows dependencies between operations that are relatively removed from each other in the operation stream to be used to link register values to speculative load results. Thus, a memory file may provide a dependency history between operations that may be separated by several intervening operations. 
     FIG. 3  shows a flowchart of one embodiment of a method of linking the speculative result of a load operation to a register value. At  301 , a tag is associated with an addressing pattern, indicating that both are likely to store the same data value. Both the tag and the addressing pattern are specified for a first load or store operation. For example, if the first operation is a load operation, the tag may identify the physical register allocated to store the result of the load and the addressing pattern may be the used to calculate the address for the load. If instead the first operation is a store operation, the addressing pattern may indicate the address of the store&#39;s destination and the tag may identify the data being stored by the store operation. In some embodiments, the tag and the addressing pattern may be associated by storing both in an entry in a memory file. 
   If a load operation&#39;s addressing pattern matches the addressing pattern of the first operation, the load&#39;s speculative result may be linked to the data value identified by the tag specified for the first operation, as indicated at  303 – 305 . If the load operation&#39;s addressing pattern does not match that of the first operation, the load&#39;s addressing pattern and the tag for the load&#39;s destination may be associated (e.g., by storing both in an entry in a memory file), as indicated at  303 – 311 . Furthermore, the load may be performed normally by accessing the data cache, as shown at  313 . In one embodiment, the load operation&#39;s addressing pattern may be compared to more than one prior operation&#39;s addressing pattern. For example, the load&#39;s addressing pattern may be compared to each addressing pattern currently stored in a memory file. 
   If the load&#39;s speculative result is linked to a data value identified by the tag, that data value may be forwarded to one or more dependent operations as the speculative result of the load operation, as indicated at  307 . The data value may be forwarded through result forwarding or by forwarding an indication that dependent operations may use the data value as a speculative operand source, as will be described in more detail below. In one embodiment, if the data value is forwarded via result forwarding, dependent operations may be executed using the forwarded data value one cycle after an operation that generates the data value completes execution. If the data value is forwarded via an indication that dependent operations may use the physical register as a speculative operand source, dependent operations may be issued as soon as the operation that generates the data value completes execution. The speculative result may be forwarded before the speculative result is verified, as indicated at  309 . The speculative result may be forwarded without accessing the data cache (i.e., the speculative result may be forwarded sooner than the non-speculative result, which is generated by accessing the data cache). 
   If the speculative result is verified to be correct at  309 , the load operation may be completed without performing a data cache access. However, if the speculative result is determined to be incorrect at  309 , the data cache access may be performed in order to obtain the correct result (not shown). If so, any dependent operations that executed using the speculative result of the load may be reexecuted using the load&#39;s correct result. Note that in some situations, the verification (at  309 ) may be performed before the linked data value is forwarded as the speculative result of the load (at  307 ). In such situations, the load may be performed normally or, if the link is determined to be correct, the data value may be forwarded as the non-speculative result of the load operation. 
   Various embodiments may link a load operation&#39;s speculative result to a register data value in many different ways. In some embodiments, values may be linked by identifying two sources for an operand: a speculative source and a non-speculative source. The speculative source may be the linked data value. Speculative sources may be provided for load operations to indicate the data value linked to the speculative result of the load. In some embodiments, speculative sources may also be provided for operations dependent on such a load operation. Accordingly, some operands may have two tags: one identifying the speculative source and one identifying the non-speculative source. In such embodiments, each scheduler  118  may provide tag storage for both the speculative operand and the non-speculative operand, as shown in  FIG. 4 . 
     FIG. 4  shows one embodiment of a scheduler  118  that may be included in a microprocessor. In the illustrated embodiment, the scheduler  118  includes a scheduler controller  502  and an operation storage  504 . In response to dispatch unit dispatching an operation, scheduler controller  502  may allocate an entry within operation storage  504  to store information corresponding to the operation. For example, an entry  522  in operation storage  504  may include an opcode field  510 , one or more operand fields, and a result field  516 . The result field  516  may store a tag identifying the physical register in which the result of that entry&#39;s operation should be stored. When the operation is issued, this tag may be forwarded to each scheduler  118  on one of one or more tag buses  520 . Each scheduler may compare the tags forwarded on tag buses  520  to the operand tags (both speculative and non-speculative, as described below) for pending operations in order to determine when the pending operations&#39; operands will be available. Accordingly, an operation may be issued (or marked as being ready to issue) in response to its source operand tags appearing on tag bus  520 . 
   Each operand field(s) may include storage for a speculative tag identifying a speculative operand source and a non-speculative tag identifying a non-speculative operand source. In the illustrated entry  522 , operand 1&#39;s two sources are identified by non-speculative tag  512  and speculative tag  514 . The scheduler  118  may be configured to issue an operation in response to one or more indications that the operation&#39;s operands are available. An operand is available if it is available from either a speculative source or a non-speculative source. If an operand is available from both a speculative source and a non-speculative source, the operation may be executed using the value available from the non-speculative source. In some embodiments, the scheduler  118  may prioritize issuing operations whose non-speculative operand sources are available over operations for which only speculative operand sources are available. 
   Note that an operation may include some operands that have speculative sources and other operands that do not have speculative sources. Also note that the same source may be a speculative source for one operation and a non-speculative source for another. In some embodiments, when an operation is executed, only one data value may be read for each operand (e.g., the execution core  124  may read either the speculative operand source or the non-speculative operand source, but not both). This may make it unnecessary to add additional ports into the register file  116 . Speculative sources and physical sources may be stored in the same storage locations (e.g., within register file  116 ) and speculative sources may not be flagged or otherwise identified as speculative sources in some embodiments. 
   In many embodiments, scheduler  118  may be configured to keep entries allocated to operations after those operations have been issued to execution core(s)  124 . When an operation is issued by scheduler  118  in response to the availability of one or more speculative operands, the scheduler may keep an entry  522  allocated to that operation so that the operation can be reissued if the speculative link is incorrect. In some embodiments, the load/store unit may be configured to verify speculative links that arise when a speculative load result is linked to a data value stored in a physical register. If the link is correct, the load store unit may be configured to not broadcast a non-speculative result of the load, since the correct result is already available through the link. If so, the scheduler(s)  118  may be configured to reissue an operation if a tag identifying a non-speculative operand source for that operation is broadcast on result bus  130 . 
   Alternatively, the load/store unit may broadcast the result of the store along with an extra status bit that masks the broadcast or indicates that the speculative link was correct and that the load should not be reissued. However, if the speculative link is incorrect, the load/store unit may perform a data cache and/or memory access in order to obtain the correct result for the load and broadcast the result of the load. In embodiments where results are always broadcast, the additional status bit may indicate that the speculative link was incorrect. Thus, in many embodiments, the same tag and result buses already available in a microprocessor may be used to indicate that a speculative link is incorrect. In other embodiments, alternative indication mechanisms (e.g., using separate result buses  130  and/or result tag buses  520  to indicate mispredictions) may be implemented. 
     FIG. 5  shows a flowchart of one embodiment of a method of issuing and reissuing operations that have both speculative and non-speculative operand sources. If an operation&#39;s speculative operand source is available, the operation may be issued, as shown at  801 – 803 . An operation&#39;s speculative operand source may become available when a data value is present in a particular register within the register file or when the data value is output on the result bus. Note that in some situations, the operation&#39;s non-speculative operand source for the same operand may be available before the speculative operand source. In those situations, the operation may be issued before the speculative operand source becomes available. Subsequent availability of the speculative source may not trigger reissue of the operation in some embodiments. 
   The issued operation may be executed using the data value provided by the speculative operand source, as indicated at  805 , and the result of the operation may be broadcast, as indicated at  807 . Broadcasting the operation&#39;s result allows dependent operations to execute. 
   If at some later time the speculative source is determined to be incorrect (e.g., the data value provided by the speculative source and the non-speculative source are not the same, or a speculative link used to generate the tag of the speculative source is not correct), the tag of the non-speculative source may be broadcast as an indication that the speculative source&#39;s value is incorrect. Broadcasting the tag of the non-speculative source involves broadcasting the tag in such a way that the scheduler(s) respond by =reissuing the operation. For example, in some embodiments, a scheduler may respond if the tag is broadcast and a status flag associated with the tag is set to a certain value, while in other embodiments there may not be an associated status flag and a scheduler may be configured to reissue an operation any time the non-speculative tag is broadcast. 
     FIG. 6  shows another embodiment of a dispatch unit  104  that may be included in a microprocessor  100 . In this embodiment, dispatch unit  104  includes a register map  134 , a memory file  132 , and a speculative register map  800 . Like register map  134 , speculative register map  800  may translate logical register names to physical register names. However, speculative register map  800  may speculatively map a logical register name to a physical register name (e.g., in response to memory file  132  linking a value stored in a physical register to a speculative result of a load operation). The speculative register map  800  may allow speculative operand values for operations that do not include addressing patterns to be linked to register data values. For example, if there is a valid speculative map for logical register EAX, an operation having EAX as a source operand may have two source tags: a non-speculative tag provided by register map  134  and a speculative tag provided by speculative register map  800 . Since the operation may issue as soon as its speculative source is available, speculative register map  800  may link data consumers directly to data producers via a speculative operand tag, bypassing any intervening loads and stores. Note that while the speculative map may store tags other than physical register names in some embodiments (e.g., in embodiments having a reorder buffer that includes storage for speculative register states). 
   Speculative register map  800  includes a speculative register map controller  802  and speculative register map storage  804 . Speculative register map storage may include one or more entries  820 . Each entry  820  may be associated with a particular logical register and indicate a physical register identifier  812  of the physical register to which that logical register is currently speculatively mapped. Each speculative register map entry  820  may also include an indication (not shown) as to whether that entry is currently valid or not. In one embodiment, speculative register map storage  804  may include an entry for each logical register. In other embodiments, speculative register map  804  may include fewer entries than there are logical registers. In such embodiments, each entry  820  may include an indication of the logical register to which that entry currently corresponds. 
   Speculative register map controller  802  may be configured to update an entry  820  in response to an indication that a load operation&#39;s speculative result has been linked with a data value identified by a particular physical register name. In the illustrated embodiment, this indication is provided by the memory file  132 . The entry  820  to update is the entry for the logical register specified as the destination of the load operation. The entry may be updated to include the physical register identifier output by the memory file  132  for that load operation. In other embodiments, speculative register map entries may be created in response to indications other than those provided by a memory file  132  (in some of these embodiments, dispatch unit  104  may not include a memory file). For example, dispatch unit  104  may detect a conditional move instruction CMOV EAX, EBX if Z and responsively indicate that the speculative register map entry for EAX should identify the physical register currently mapped to EBX. Generally, speculative register map entries may be created in response to any prediction mechanism that indicates that a logical register should be speculatively mapped to a particular physical register. 
   In some embodiments, operations may be provided to speculative register map  800  during the same cycle that they are provided to register map  134 . As register map  134  performs the non-speculative register renaming for an operation, speculative register map  800  may indicate whether any of the logical registers specified as storing one of the operation&#39;s speculative source operands are linked to a particular physical register. If a valid entry exists in speculative register map storage  804  for one of the operation&#39;s logical register sources, speculative register map controller  802  may output the physical register name stored in that logical register&#39;s entry. Dispatch unit  104  may output this speculative physical register name as a speculative source when the operation is dispatched to a scheduler  118 . Thus, if an ADD operation is provided to speculative register map  800  and one of the ADD&#39;s sources has a valid entry in speculative register map storage  804 , the tag for the physical register identified in that entry may be provided as a speculative source operand tag to scheduler  118 . The scheduler may be configured to store both speculative and non-speculative operand tags, as described above, and may in some embodiments be configured to reissue operations (if already issued) in response to the non-speculative tag being broadcast on a result bus. 
   Entries within the speculative map may be invalidated in response to an indication that a data value for a particular logical register will be modified. For example, if an operation ADD EAX, ECX is handled by the dispatch unit  104 , the speculative register map controller  802  may invalidate the speculative map entry currently assigned to EAX since the ADD operation will modify that register value. 
   Generally, speculative operand tags may be provided to scheduler  118  whenever one operation&#39;s speculative result is linked to a register data value. In some embodiments, a memory file  132  and other structure that tracks dependencies over several cycles (e.g., a speculative register map as described below) may be used to link speculative results to register values. For example, dispatch unit  104  may generate speculative tags for an operation in response to a memory file  132  identifying a link. In some embodiments, speculative tags may be generated without the use of such a speculative map. For example, a sequence of instructions may include: 
                                              ADD EBX, EBX   (ADD 1),           MOV [addressing pattern A], EBX   (STORE 1)           ADD [addressing pattern A], ECX   (ADD 2).                        
These instructions may be contiguous instructions (e.g., they may directly follow each other in program order). These instructions may be separated into the following component operations (shown with logical addresses translated to physical addresses) for execution within execution core(s)  124 :
 
                                              ADD PR2, PR2, PR1   (ADD 1)           MOV [addressing pattern A], PR2   (STORE 1)           MOV PR3, [addressing pattern A]   (load for ADD 2)           ADD PR4, PR3, PR5   (add for ADD 2)           MOV [addressing pattern A], PR4   (store for ADD 2)                        
Before the component load, add, and store operations of ADD 2 are dispatched, a dispatch unit  104  may detect whether there are any dependencies between any of the component operations in the sequence that would allow linking to speculative results. Additionally, the data being stored by STORE 1 may be linked to the load&#39;s speculative result (e.g., by a memory file). Since there are no intervening operations, dispatch unit may have all of the information needed to detect a dependency between the load operation and the add operation (both operations being derived from the same instruction) in the same dispatch cycle. Based on these two dependencies, the dispatch unit  104  may link the tag of the data being stored by STORE 1, PR2, to the speculative result of the load operation performed as part of ADD 2. This speculative link may in turn allow the dispatch unit to link the source of the addition operation performed as part of ADD 2 to the value stored in PR2. Accordingly, dispatch unit may output an indication that PR2 may be speculatively used as the source of one of the operands for the addition but that PR3 is the non-speculative source for that operand. Thus, in one embodiment, the operations and operand identifiers output by dispatch unit may be specified as follows:
 
   
     
       
         
             
             
           
             
                 
             
           
          
             
               ADD PR2, PR2, PR1 
               (ADD 1) 
             
             
               MOV [addressing pattern A], PR2 
               (STORE 1) 
             
             
               MOV PR3, [addressing pattern A] 
               (load for ADD 2) 
             
             
               ADD PR4, PR2*, PR3, PR5 
               (add for ADD 2, where PR2* is a 
             
             
                 
               speculative source for ECX and PR3 
             
             
                 
               is the non-speculative source for 
             
             
                 
               ECX) 
             
             
               MOV [addressing pattern A], PR4 
               (store for ADD 2). 
             
             
                 
             
          
         
       
     
   
   In other embodiments, dispatch unit  104  may not be configured to identify speculative source operands for operations that depend on a load operation. Instead, dispatch unit  104  may include an operation converter  180  configured to convert load operations into one or more operations that include a register-to-register move operation in order to provide speculative load results to dependent operations. The conversion of a load operation may be performed in response to an indication that a link exists between a speculative result of the load operation and a data value identified by a particular physical register name. This indication is provided by the link detector  182 , which may include a memory file  132  in some embodiments. In other embodiments, the link detector  182  may include logic configured to link data values in response to operations such as a conditional move operation, as described above. 
   In one embodiment, the operation converter may receive an input opcode for an operation as well as an indication as to whether a link between a register value and a speculative result of the operation is detected for that operation. If the operation is a load and a speculative link has been detected, the operation converter may output an opcode for a register-to-register move operation. The dispatch unit  104  may dispatch the register-to-register move operation, using the tag output by the link detection unit as the source operand tag for the register-to-register move. 
   In some embodiments, the operation converter may be configured to dispatch the resulting register-to-register move such that the scheduler stores the operand tags needed for both the register-to-register move and the original load operation in the entry allocated to the register-to-register move operation. This may allow the operation to be reissued as the original load operation if the speculative result of the register-to-register move operation is detected to be incorrect. In order to implement this, an additional source operand may be added to each register-to-register move operation that results from modifying a load operation (or, in alternative embodiments, a source operand that is already present may be modified to implement this). In some embodiments, the speculative result of the register-to-register move operation may be verified by performing the address calculation for the original load and/or comparing the linked data value to the actual load result data value. If the speculative result is incorrect, the data cache may be accessed in order to obtain the correct load result. Rebroadcast of the correct load result may cause the scheduler to reissue any dependent operations that were executed using the incorrect value. 
   In some embodiments, the operation converter  180  may be configured to convert a load operation into a dual-nature operation. Like a load operation, this dual-nature operation may involve both address calculation and data movement. Unlike a load, the data movement initiated by the dual-nature operation is a register-register move. Furthermore, the data movement initiated by the dual-nature operation may occur before the address calculation has completed. The address calculation may be used to verify whether the speculative link was correct. If the speculative link was incorrect, the dual-purpose operation may be reissued as a normal load operation and its result may be rebroadcast to dependent operations upon completion of a data cache access. 
   The following examples show how different embodiments may convert this exemplary sequence of operations: 
                                              ADD PR2, PR1, PR1   (ADD 1)           . . .           STORE [addressing pattern A], PR2   (STORE 1)           . . .           LOAD PR3, [addressing pattern A]   (LOAD 1)           . . .           ADD PR4, PR3, PR3   (ADD 2)                        
In this sequence, it is possible that the specified operations may be separated by one or more intervening operations. However, assuming that no intervening operations appear to modify the values used in addressing pattern A or to modify the data values stored at the address calculated from addressing pattern A and in PR2, a speculative link may be detected between the data values stored in PR2 and at the address calculated from addressing pattern A.
 
   In one embodiment, in response to the detection of the speculative link by speculative link detector  182 , operation converter  180  may convert LOAD 1 into a dual-purpose move operation: MOV PR3, PR2. In addition to specifying the register source and destination, this dual-purpose move operation may also specify addressing pattern A so that the address calculation for LOAD 1 may be performed. However, the move portion of the dual-purpose move operation may issue as soon as ECX is available. As soon as the result of the move portion of the dual-portion operation is broadcast, ADD 2 may issue, using the speculative result of the move operation as an operand. When the address calculation is performed, the speculative link may be verified. If the speculative link is incorrect, the load/store unit may provide an indication to the scheduler that causes the scheduler to reissue the dual-purpose move operation as a load operation. The result of the load operation may be broadcast, causing any dependent operations, such as ADD 2, which may have executed using the speculative result of the move to reissue. Note that this dual-purpose operation may be scheduled using a single scheduler entry and that a scheduler  118  may select the dual-purpose operation for issue twice: once for the load&#39;s address calculation and once for the register-to-register move. Scheduler  118  may thus select the dual-purpose move operation for issue at least twice. Scheduler  118  may also select the dual-purpose move operation for issue more than twice. For example, as just described, if the speculative link is incorrect the scheduler may reissue the dual-purpose move operation as a load operation. 
   In an alternative embodiment, operation converter  180  may convert LOAD  1  into two linked operations, LOAD PR3, [addressing pattern A] and MOV PR3, PR2, in response to detection of the speculative link. Unlike the earlier example, which involved a dual-purpose operation, these linked operations may each take up an entry in a scheduler  118  instead of sharing a single scheduler entry. When the register-to-register move operation issues, dependent operations such as ADD 2 may issue using the speculative value of PR3 as an operand value. The LOAD operation may be tagged as a special type of load operation so that, if the speculative link is determined to be correct, the load&#39;s result tag is not broadcast (or is broadcast with an indication that the move&#39;s result is correct and that the dependent operations should not reissue). Also, if both the load and the move operation broadcast their results on the same cycle, an indication may be provided so that the load result is used for dependent operations instead of the move result. In some embodiments, this may occur by adding an extra bit to each tag bus within the microprocessor  100  that indicates to which tag a scheduler should respond. If the speculative link is incorrect (e.g., as indicated by the broadcast of the load&#39;s result tag), the move operation may be cancelled (e.g., the scheduler  118  that schedules the move operation may deallocate the scheduler entry currently allocated to the register-to-register move so that the operation cannot reissue). In many embodiments, the scheduler may be configured to reissue any dependent operations that executed using the speculative load result in response to the load result tag reappearing on the result bus. 
     FIG. 8  illustrates one embodiment of a method of converting load operations to include a speculative register-to-register move operation. As shown, if a link between a register data value identified by a particular tag and a speculative result of a load operation is detected (e.g., by a memory file) at  1401 , the load operation may be modified to include a register-to-register move operation having a source operand tag equal to the tag of the data value linked to the speculative load result, as shown at  1403 . Execution of the register-to-register move operation may involve outputting the data value identified by the tag onto the result bus along with the tag of the load&#39;s result at  1405 . 
   The speculative result may be verified (e.g., by performing the address calculation for the original load) at  1409 . If the speculative result is correct, the load&#39;s result may not be rebroadcast on the result bus (or, alternatively, the load&#39;s result may be rebroadcast along with an indication that dependent operations should not reissue in response to the rebroadcast result). If the speculative result is incorrect, the data cache may be accessed to retrieve the correct load result and the correct load result may be broadcast on the result bus. This may in turn cause any dependent operations that issued using the speculative result value to be reissued. Note that in some situations, the speculative result may be verified before performance of the register-to-register move operation. If the speculative result is incorrect, the register-to-register move operation may not be performed. 
   If no link between the speculative result of the load operation and a register data value is detected (e.g., the load&#39;s addressing pattern misses in a memory file), the load may not be modified. The load may execute normally, accessing the data cache and broadcasting its result on the result bus, as shown at  1417  and  1419 . 
     FIG. 9  shows a block diagram of one embodiment of a computer system  400  that includes a processor  100  coupled to a variety of system components through a bus bridge  402 . Processor  100  may include an embodiment of a dispatch unit  104 , a memory file  132 , a scheduler  118 , and/or speculative register map  800  as described above. Other embodiments of a computer system are possible and contemplated. In the depicted system, a main memory  200  is coupled to bus bridge  402  through a memory bus  406 , and a graphics controller  408  is coupled to bus bridge  402  through an AGP bus  410 . Several PCI devices  412 A– 412 B are coupled to bus bridge  402  through a PCI bus  414 . A secondary bus bridge  416  may also be provided to accommodate an electrical interface to one or more EISA or ISA devices  418  through an EISA/ISA bus  420 . In this example, processor  100  is coupled to bus bridge  402  through a CPU bus  424  and to an optional L2 cache  428 . In some embodiments, the processor  100  may include an integrated L1 cache (not shown). 
   Bus bridge  402  provides an interface between processor  100 , main memory  404 , graphics controller  408 , and devices attached to PCI bus  414 . When an operation is received from one of the devices connected to bus bridge  402 , bus bridge  402  identifies the target of the operation (e.g., a particular device or, in the case of PCI bus  414 , that the target is on PCI bus  414 ). Bus bridge  402  routes the operation to the targeted device. Bus bridge  402  generally translates an operation from the protocol used by the source device or bus to the protocol used by the target device or bus. 
   In addition to providing an interface to an ISA/EISA bus for PCI bus  414 , secondary bus bridge  416  may incorporate additional functionality. An input/output controller (not shown), either external from or integrated with secondary bus bridge  416 , may also be included within computer system  400  to provide operational support for a keyboard and mouse  422  and for various serial and parallel ports. An external cache unit (not shown) may also be coupled to CPU bus  424  between processor  100  and bus bridge  402  in other embodiments. Alternatively, the external cache may be coupled to bus bridge  402  and cache control logic for the external cache may be integrated into bus bridge  402 . L2 cache  428  is shown in a backside configuration to processor  100 . It is noted that L2 cache  428  may be separate from processor  100 , integrated into a cartridge (e.g., slot  1  or slot A) with processor  100 , or even integrated onto a semiconductor substrate with processor  100 . 
   Main memory  200  is a memory in which application programs are stored and from which processor  100  primarily executes. A suitable main memory  200  comprises DRAM (Dynamic Random Access Memory). For example, a plurality of banks of SDRAM (Synchronous DRAM) or Rambus DRAM (RDRAM) may be suitable. 
   PCI devices  412 A– 412 B are illustrative of a variety of peripheral devices such as network interface cards, video accelerators, audio cards, hard or floppy disk drives or drive controllers, SCSI (Small Computer Systems Interface) adapters and telephony cards. Similarly, ISA device  418  is illustrative of various types of peripheral devices, such as a modem, a sound card, and a variety of data acquisition cards such as GPIB or field bus interface cards. 
   Graphics controller  408  is provided to control the rendering of text and images on a display  426 . Graphics controller  408  may embody a typical graphics accelerator generally known in the art to render three-dimensional data structures that can be effectively shifted into and from main memory  200 . Graphics controller  408  may therefore be a master of AGP bus  410  in that it can request and receive access to a target interface within bus bridge  402  to thereby obtain access to main memory  200 . A dedicated graphics bus accommodates rapid retrieval of data from main memory  404 . For certain operations, graphics controller  408  may further be configured to generate PCI protocol transactions on AGP bus  410 . The AGP interface of bus bridge  402  may thus include functionality to support both AGP protocol transactions as well as PCI protocol target and initiator transactions. Display  426  is any electronic display upon which an image or text can be presented. A suitable display  426  includes a cathode ray tube (“CRT”), a liquid crystal display (“LCD”), etc. 
   It is noted that, while the AGP, PCI, and ISA or EISA buses have been used as examples in the above description, any bus architectures may be substituted as desired. It is further noted that computer system  400  may be a multiprocessing computer system including additional processors (e.g., processor  100   a  shown as an optional component of A computer system  400 ). Processor  100   a  may be similar to processor  100 . More particularly, processor  100   a  may be an identical copy of processor  100 . Processor  100   a  may be connected to bus bridge  402  via an independent bus (as shown in  FIG. 9 ) or may share CPU bus  224  with processor  100 . Furthermore, processor  100   a  may be coupled to an optional L2 cache  428   a  similar to L2 cache  428 . 
   Turning now to  FIG. 10 , another embodiment of a computer system  400  that may include a dispatch unit  104 , a memory file  132 , a scheduler  118 , and/or speculative register map  800  as described above is shown. Other embodiments are possible and contemplated. In the embodiment of  FIG. 10 , computer system  400  includes several processing nodes  612 A,  612 B,  612 C, and  612 D. Each processing node is coupled to a respective memory  614 A– 614 D via a memory controller  616 A– 616 D included within each respective processing node  612 A– 612 D. Additionally, processing nodes  612 A– 612 D include interface logic used to communicate between the processing nodes  612 A– 612 D. For example, processing node  612 A includes interface logic  618 A for communicating with processing node  612 B, interface logic  618 B for communicating with processing node  612 C, and a third interface logic  618 C for communicating with yet another processing node (not shown). Similarly, processing node  612 B includes interface logic  618 D,  618 E, and  618 F; processing node  612 C includes interface logic  618 G,  618 H, and  6181 ; and processing node  612 D includes interface logic  618 J,  618 K, and  618 L. Processing node  612 D is coupled to communicate with a plurality of input/output devices (e.g., devices  620 A– 620 B in a daisy chain configuration) via interface logic  618 L. Other processing nodes may communicate with other I/O devices in a similar fashion. 
   Processing nodes  612 A– 612 D implement a packet-based link for inter-processing node communication. In the present embodiment, the link is implemented as sets of unidirectional lines (e.g., lines  624 A are used to transmit packets from processing node  612 A to processing node  612 B and lines  624 B are used to transmit packets from processing node  612 B to processing node  612 A). Other sets of lines  624 C– 624 H are used to transmit packets between other processing nodes as illustrated in  FIG. 10 . Generally, each set of lines  624  may include one or more data lines, one or more clock lines corresponding to the data lines, and one or more control lines indicating the type of packet being conveyed. The link may be operated in a cache coherent fashion for communication between processing nodes or in a non-coherent fashion for communication between a processing node and an I/O device (or a bus bridge to an I/O bus of conventional construction such as the PCI bus or ISA bus). Furthermore, the link may be operated in a non-coherent fashion using a daisy-chain structure between I/O devices as shown. It is noted that a packet to be transmitted from one processing node to another may pass through one or more intermediate nodes. For example, a packet transmitted by processing node  612 A to processing node  612 D may pass through either processing node  612 B or processing node  612 C as shown in  FIG. 10 . Any suitable routing algorithm may be used. Other embodiments of computer system  400  may include more or fewer processing nodes then the embodiment shown in  FIG. 10 . 
   Generally, the packets may be transmitted as one or more bit times on the lines  624  between nodes. A bit time may be the rising or falling edge of the clock signal on the corresponding clock lines. The packets may include command packets for initiating transactions, probe packets for maintaining cache coherency, and response packets from responding to probes and commands. 
   Processing nodes  612 A– 612 D, in addition to a memory controller and interface logic, may include one or more processors. Broadly speaking, a processing node comprises at least one processor and may optionally include a memory controller for communicating with a memory and other logic as desired. More particularly, each processing node  612 A– 612 D may comprise one or more copies of processor  100 . External interface unit  18  may includes the interface logic  618  within the node, as well as the memory controller  616 . 
   Memories  614 A– 614 D may comprise any suitable memory devices. For example, a memory  614 A– 614 D may comprise one or more RAMBUS DRAMs (RDRAMs), synchronous DRAMs (SDRAMs), static RAM, etc. The address space of computer system  400  is divided among memories  614 A– 614 D. Each processing node  612 A– 612 D may include a memory map used to determine which addresses are mapped to which memories  614 A– 614 D, and hence to which processing node  612 A– 612 D a memory request for a particular address should be routed. In one embodiment, the coherency point for an address within computer system  400  is the memory controller  616 A– 616 D coupled to the memory storing bytes corresponding to the address. In other words, the memory controller  616 A– 616 D is responsible for ensuring that each memory access to the corresponding memory  614 A– 614 D occurs in a cache coherent fashion. Memory controllers  616 A– 616 D may comprise control circuitry for interfacing to memories  614 A– 614 D. Additionally, memory controllers  616 A– 616 D may include request queues for queuing memory requests. 
   Interface logic  618 A– 618 L may comprise a variety of buffers for receiving packets from the link and for buffering packets to be transmitted upon the link. Computer system  400  may employ any suitable flow control mechanism for transmitting packets. For example, in one embodiment, each interface logic  618  stores a count of the number of each type of buffer within the receiver at the other end of the link to which that interface logic is connected. The interface logic does not transmit a packet unless the receiving interface logic has a free buffer to store the packet. As a receiving buffer is freed by routing a packet onward, the receiving interface logic transmits a message to the sending interface logic to indicate that the buffer has been freed. Such a mechanism may be referred to as a “coupon-based” system. 
   I/O devices  620 A– 620 B may be any suitable I/O devices. For example, I/O devices  620 A– 620 B may include devices for communicate with another computer system to which the devices may be coupled (e.g., network interface cards or modems). Furthermore, I/O devices  620 A– 620 B may include video accelerators, audio cards, hard or floppy disk drives or drive controllers, SCSI (Small Computer Systems Interface) adapters and telephony cards, sound cards, and a variety of data acquisition cards such as GPIB or field bus interface cards. It is noted that the term “I/O device” and the term “peripheral device” are intended to be synonymous herein. 
   As used herein, the terms “clock cycle” or “cycle” refer to an interval of time in which the various stages of the instruction processing pipelines complete their tasks. Instructions and computed values are captured by memory elements (such as registers or arrays) according to a clock signal defining the clock cycle. For example, a memory element may capture a value according to the rising or falling edge of the clock signal. 
   Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.