Abstract:
An apparatus for integer exception register (XER) renaming and methods of using the same are implemented. In a central processing unit (CPU) having a pipelined architecture, integer instructions that use or update the XER may be executed out-of-order using the XER renaming mechanism. Any instruction that updates the XER has an associated instruction identifier (IID) stored in a register. Subsequent instructions that use data in the XER use the stored IID to determine when the XER data has been updated by the execution of the instruction corresponding to the stored IID. As each instruction that updates XER data is executed, the data is stored in an XER rename buffer. Instructions using XER data then obtain the updated, valid, XER data from the rename buffer. In this way, these instructions can obtain valid XER data prior to completion of the preceding instructions. The XER data is retrieved from the XER rename buffer by indexing into the buffer by using an index derived from the stored IID. Because the updated XER data is available in the rename buffer before the updating instruction completes, out-of-order execution of instructions using or updating XER data is thereby realized.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to U.S. patent application Ser. No. 09/024,804 which is hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to a data processing system and, in particular, to a data processing system performing out-of-order execution. 
     BACKGROUND INFORMATION 
     As computers have been developed to perform a greater number of instructions at greater speeds, many types of architectures have been developed to optimize this process. For example, a reduced instruction set computer (RISC) device uses fewer instructions in greater parallelism in executing those instructions to ensure that computational results will be available more quickly than the results provided by more traditional data processing systems. In addition to providing increasingly parallel execution of instructions, some data processing systems implement out-of-order instruction execution to increase processor performance. Out-of-order instruction execution increases processor performance by dynamically allowing instructions dispatched with no data dependencies to execute before previous instructions in an instruction stream that have unresolved data dependencies. In some data processing systems, instructions are renamed and instruction sequencing tables, also referred to as re-order buffers, facilitate out-of-order execution by re-ordering instruction execution at instruction completion time. 
     Re-order buffer devices are also used to allow speculative instruction execution. Therefore, data processing systems which support speculative instruction execution can be adapted for out-of-order execution with the addition of relatively minimal hardware. A portion of this added hardware includes logic which is used to determine a time and order that instructions should be issued. Such issue logic can be extremely complex since the dependencies and instructions in a state of a pipeline in which the instructions are being executed must be examined to determine a time at which the instruction should issue. If the issue logic is not properly designed, such issue logic can become a critical path for the data processing system and limit the frequency of instruction execution such that performance gains which could be achieved by out-of-order issue are eliminated. 
     The out-of-order instruction execution implemented by many prior art systems increases processor performance by dynamically allowing instructions dispatched with no data dependencies to execute before previous instructions in the instruction stream that have unresolved data dependencies. Register file renaming, renaming selected bits of architected facilities, for example registers accessible by software, and instruction sequencing tables (re-order buffers) facilitate out-of-order execution by re-ordering instruction execution at instruction completion time. For more information on such structures, refer to &#34;An Efficient Algorithm for Exploiting Multiple Arithmetic Units,&#34; by R. M. Tomasulo, published in IBM JOURNAL, January 1967, pp. 25-33. It should be noted that these devices are also used to allow speculative instruction execution. Therefore, system architecture supporting speculative instruction execution can be adapted for out-of-order execution with the addition of relatively &#34;little&#34; hardware and few overhead expenses. Thus, register file renaming may support out-of-order execution without modification from a speculative instruction execution architecture. 
     However, instructions that alter or use an architected integer exception register (XER) must be executed one at a time in a processor that includes only one such register. In these instances, data dependencies can only be resolved when the updating instruction completes, and the architected register then is valid. Fixed-point instructions that use or alter the contents of the XER are representative of such instructions. The XER contains data values reporting on and conditioning integer instruction execution. Thus, when there is more than one instruction that needs to update or use the XER, then those instructions must be executed in order according to the software program, and can cause bottlenecks in fixed-point operations. The methods discussed above are inadequate and cannot be simply modified to permit out-of-order execution of instructions updating or using the XER. Because the XER includes sticky status data values, conventional rename methods are not useable to implement out-of-order instructions using or updating the XER. Sticky status data are data values that are set by an executing instruction and are retained until reset by software. 
     Thus, there is a need in the art for a renaming apparatus and method that permits each instruction that needs to update or use the XER to execute out-of-order, thereby increasing instruction execution parallelism and processor performance. 
     SUMMARY OF THE INVENTION 
     The previously mentioned needs are addressed by the present invention. Accordingly, there is provided in a first form, a method of increasing instruction execution parallelism in a data processor. The method includes the step of writing an instruction identifier into a first data storage buffer. When an instruction corresponding to the instruction identifier finishes execution, one or more execution condition data values generated thereby are written to a second data storage buffer. In response to a next instruction, the set of execution condition data values is retrieved from the second data storage buffer using the instruction identifier stored in the first data storage buffer. 
     Additionally, there is provided, in a second form, a data processing apparatus for increasing instruction execution parallelism. The data processing apparatus includes a first data storage buffer containing an instruction identifier. Circuitry for accessing the first data storage buffer is coupled thereto, and to a second data storage buffer containing one or more sets of execution condition data values, each generated in response to an instruction associated with previous ones of the instruction identifier. Circuitry coupled to the second data storage buffer and to the circuitry for accessing the first data storage buffer, retrieves one of the sets of execution condition data values from the second data storage buffer. 
     Finally, there is provided, in a third form, a data processing system that includes an input circuit for communicating a plurality of instructions. A condition logic circuit generates one or more sets of condition exception values in response to one or more of the plurality of instructions. Rename logic is circuitry coupled to the condition logic circuit, wherein the rename logic circuitry includes a data storage buffer for containing the one or more sets of execution exception values. The data processing system also includes an exception register coupled to the rename logic circuitry wherein the exception register receives one or more sets of execution condition values contained in the first data storage buffer. 
     These and other features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to note that the drawings are not intended to represent the only form of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates, in block diagram form, a data processing system in accordance with one embodiment of the present invention; 
     FIG. 2 illustrates, in block diagram form, a central processing unit in accordance with an embodiment of the present invention; 
     FIGS. 3, 3A and 3B illustrate, in block diagram form, an out-of-order XER rename apparatus implemented in accordance with an embodiment of the present invention; 
     FIG. 4 illustrates, in flow chart form, a method of XER update execution in accordance with an embodiment of the present invention; and 
     FIGS. 5, 5A and 5B illustrates, in flow chart form, out-of-order XER use in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides an XER renaming mechanism that supports out-of-order integer unit instruction execution in which there is more than one instruction that needs to update or use the XER. The present invention allows all instructions that need to update or use the XER to execute simultaneously. 
     Operation of the present invention will subsequently be described in greater detail. Prior to that discussion, however, a description of connectivity of the elements of the present invention will be provided. 
     In the following description, numerous specific details are set forth, such as specific word or byte lengths, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art. 
     Refer now to the drawings wherein depicted elements are not necessary shown to scale and wherein like or similar elements are designated by the same referenced numeral through the several views. 
     Referring first to FIG. 1, an example is shown of a data processing system 100 which may be used for the invention. The system has a central processing unit (CPU) 110, such as a POWERPC microprocessor (&#34;PowerPC&#34; is a trademark of IBM Corporation) according to &#34;The PowerPC Architecture: A Specification for a New Family of Risk (RISC) Processors&#34;, 2d ed., 1994, Cathy May, et al., ed., which is hereby incorporated by reference. A more specific implementation of a PowerPC microprocessor is described in the &#34;PowerPC 604 Risk Microprocessor User&#39;s Manual&#34;, 1994, IBM Corporation, which is hereby incorporated herein by reference. The XER renaming mechanism of the present invention is included in CPU 110. The CPU 110 is coupled to various other components by system bus 112. Read only memory (&#34;ROM&#34;) 116 is coupled to the system bus 112 and includes a basic input/output system (&#34;BIOS&#34;) that controls certain basic functions of the data processing system 100. Random access memory (&#34;RAM&#34;) 114, I/O adapter 118, and communications adapter 134 are also coupled to the system bus 112. I/O adapter 118 may be a small computer system interface (&#34;SCSI&#34;) adapter that communicates with a disk storage device 120. Communications adapter 134 interconnects bus 112 with an outside network enabling the data processing system to communicate with other such systems. I/O devices are also connected to system bus 112 via user interface adapter 122 and display adapter 136. Keyboard 124, track ball 132, mouse 126, and speaker 128 are all interconnected to bus 112 via user interface adapter 122. Display monitor 138 is connected to system bus 112 by display adapter 136. In this matter, a user is capable of inputting to the system through the keyboard 124, track ball 132, or mouse 126 and receiving output from the system via speaker 128 and display 138. Additionally, an operating system such as AIX (&#34;AIX&#34; is a trademark of the IBM Corporation) is used to coordinate the functions of the various components shown in FIG. 1. 
     Note that the invention describes terms such as comparing, validating, selecting, or other terms that could be associated with a human operator. However, for at least a number of the operations described herein which form part of the present invention, no action by a human operator is desirable. The operations described are, in large part, machine operations processing electrical signals to generate other electrical signals. 
     FIG. 2 illustrates a portion of CPU 110 in greater detail. The portion of CPU 110 comprises an instruction cache (I-cache) 202, an instruction unit/branch unit 204, a fixed point execution unit (FXU) 206, a load/store unit 208, a floating point unit (FPU) 210, a data cache (D-cache) 212, and a bus interface unit (BIU) 214, which interfaces with bus 112 in FIG. 1. I-cache 202 is coupled to instruction unit/branch unit 204 to communicate control information in a plurality of instructions. Instruction unit/branch unit 204 is coupled to each of FXU 206, load/store unit 208, and FPU 210 to provide a plurality of dispatched instructions. I-cache 202 is coupled to bus interface unit 214 to communicate data and control information. FXU 206 is coupled to load/store unit 208 to communicate a load data value, a store data value, and a forwarding data value. Load/store unit 208 is coupled to FPU 210 to communicate a store data value and a load data value. Load/store unit 208 is also coupled to D-cache 212 to communicate a request for a load/store signal, a plurality of data values, and an address value. D-cache 212 is coupled to bus interface unit 214 to communicate a data in signal, a data out signal, and a control signal. 
     FIG. 3 illustrates an XER rename mechanism 300 according to the principles of the present invention. XER rename mechanism 300 is incorporated in FXU 206. Instructions targeted for FXU 206 are retrieved from I-cache 202 and loaded into instruction queue 301 by issue/branch unit 204. Instruction queue 301 contains a stack of integer instructions to be performed by FXU 206. Instruction queue 301 may be a predetermined value, M, in depth. Each entry in instruction queue 301 contains a portion 302 holding the machine instruction, usually in binary form. Each instruction includes an instruction identification (IID) portion that is assigned by issue unit 204. If the instruction, on executing, updates the XER, its IID is included in another portion of the instruction queue entry, the field IID 303. The IID is also stored in XER status register 304. Within XER status register 304, the IID is stored in the field XER IID 305. If the instruction does not update the XER, XER IID 303 and XER IID 305 contain the IID of the last previous instruction that does update the XER. XER status register 304 also includes a validity field, valid flag 306. Valid flag 306 may be one bit wide. Thus, for simplicity, valid flag 306 may be referred to as validity bit 306. However, it would be understood that in an alternative embodiment, according to the principles of the present invention, valid flag 306 may be a predetermined number of bits, M, wide. This field will be subsequently discussed in further detail. 
     CPU 110 performs operations in pipeline fashion. Multiple instructions are dispatched from I-cache 202 by issue unit/branch unit 204 to the execution units, such as FXU unit 206. The execution units maintain a pipelined instruction buffer whereby, an instruction is sent to execution as soon as all of its operands are available. Instruction queue 301 is such a pipelined queue. The next instruction for execution is sent to multiplexer (MUX) 307. The XER IID corresponding to that instruction is, at the same time, sent to MUX 308. MUX 307 and MUX 308 also receive instructions and the corresponding IID, respectively, from issue unit 204. If instruction queue 301 is empty, MUX 307 selects the instruction from issue unit 204 and MUX 308 selects the passed-through IID of the last instruction to modify the XER, which may be the passed-through instruction. Instruction select logic 309 controls MUXs 307 and 308. 
     The instruction selected for execution is passed by MUX 307 to decode logic/operand register 310. Decoded instructions and operands are then sent to fixed-point execution engine 311. Any XER data that the currently executing instruction requires is also sent to fixed-point execution engine 311 from XER data register 319. Integer data output from fixed-point execution engine 311 is sent to load/store unit 208 and is also sent back to decode logic/operand register 310. 
     The XER data required by the executing instruction may be derived from one of several sources. XER selection logic 312 receives IIDs from MUX 308 and the IID of the finishing instruction from fixed-point execution engine 311. XER selection logic 312 outputs a control signal to MUX 313. The operation of XER select logic 312 and MUX 313 in selecting the XER data to send to fixed-point execution engine 310 subsequently will be described in detail. 
     MUX 313 receives XER data from several sources. Instruction queue 301 includes a XER data field 314. When an instruction is loaded into instruction queue 301, it receives the XER data from the then-finishing instruction from fixed-point execution engine 311, if this instruction updates the XER. If the currently executing instruction does not update the XER, XER rename buffer 315 is searched for the required XER data. This is effected by a content addressable memory (CAM) read using finished IID buffer 315. Finished IID buffer 316 contains IIDs of finished instructions that have modified XER data values. The CAM read into XER rename buffer 315 is implemented by CAM logic 317. 
     The queued instruction next ready to execute brings this XER data in XER data 314. MUX 313 also receives XER data from XER rename buffer 315. As each instruction finishes execution, fixed-point execution engine 311 sends the XER data updated by the finishing instruction to XER rename buffer 315. The XER data is also sent to instruction queue 301, where it is received in the field XER data 314. The IID of the corresponding instruction is stored in finished IID buffer 316. The depth of finished IID buffer 316 and XER rename buffer 315 may be the same, and may be a predetermined value, N. The XER data corresponding to a finished instruction and the IID for that instruction are stored in the same relative location in XER rename buffer 315 and finished IID buffer 316, respectively. When an instruction completes, the XER data associated with that instruction is then sent to XER 318. XER register 318 also provides XER data to MUX 313. If the last instruction affecting the XER data required by a pending instruction at the input to MUX 307 has completed, the state of XER register 318 is valid, and may be used by the pending instruction. In response to an output from XER select logic 312, MUX 313 outputs XER data from one of the input sources. Select logic 312 compares finishing IIDs with the XER IID received via MUX 308, to select the XER data source. Select logic 312 effectively allows the next instruction to execute to snoop the output of fixed-point engines 311 in order to resolve its XER data dependencies. In this way, the XER rename mechanism according to the principles of the present invention alleviates the need for the instruction to wait in a queue until valid XER data appears in a rename buffer. Moreover, if XER 318 has valid data, validity bit 306 informs select logic 312 to select that data. This data is latched into XER data register 319 where it is available to the currently executing instruction in fixed-point engine 311. 
     Issue/branch unit 204 may issue two instructions in each instruction cycle. Thus, FXU 206 may retire two instructions in each cycle, whence two fixed-point engines, 311a and 311b, are included in XER rename mechanism 300. Likewise, MUXs 307a and 308a are duplicated in MUXs 307b and 308b, instruction select logic 309a in instruction logic 309b, decode logic/operand register 310a in decode logic operand register 310b. MUX 313a and MUX 313b receive XER data from both fixed-point engines 311a and 311b. Likewise, XER select logic 312a receives finishing IIDs from fixed-point engine 311a and engine 311b and XER select logic 312b receives finishing IIDs from both fixed-point engine 311a and engine 311b. Similarly, XER rename buffer 315 receives XER data from fixed-point engines 311. 
     The operation of XER rename mechanism 300 may be better appreciated by now referring to FIG. 4. FIG. 4 depicts, in flow chart form, the operation of XER rename mechanism 300. If the instruction dispatched, in step 400, to fixed point unit 206 alters the XER, step 401, in step 402 its IID is written into XER IID 305 in XER status register 304. At the same time, valid flag 306 is turned on. If the instruction is not ready to execute, step 403, in step 404 the instruction is written to instruction queue 301. If, in step 403, the instruction is ready to execute, it is sent to fixed-point instruction engine 311, in step 405. In step 406, an instruction is held in instruction queue 301 until it is ready to execute. When an instruction is ready to execute, it is sent to the fixed-point execution engine 311 and executed in step 405. When the instruction finishes execution, in step 407, the resulting XER data is written into XER rename buffer 315 and the IID of the instruction is written into finished IID buffer 316. The XER data is, at the same time, written into XER data field 314 of the instruction in instruction queue 301 next ready to execute. However, instruction queue 301 may be empty, wherein the next instruction to execute may pass through to MUX 307. This instruction may use XER data, and this will be discussed below. When an instruction completes, in step 408, its XER data is read from XER rename buffer 315 and written into the architected XER 318. 
     An instruction that does not update XER data may nonetheless use XER data. Refer now to FIG. 5 in which is illustrated, in flow chart form, the operation of XER rename mechanism 300 for an instruction using XER data. In step 500, an instruction that neither updates nor uses XER data executes without implicating the XER rename process, in step 501. Otherwise, in step 502, the instruction reads XER rename register 304 to obtain the IID of the last preceding instruction to update XER data, and it also retrieves the value of validity bit 306. If, in step 503, the XER data is not in rename buffer 315, that is, validity bit 306 is off, then the XER data in the architected XER 316 is valid and the instruction does not retrieve XER data from rename buffer 315. The XER data is read from architected XER 317, in step 504. This may be effected by providing validity bit 306 to select logic 312. 
     If, however, validity bit 306 is set, in step 503, a CAM read is made in step 505. The CAM read is made into finish IID buffer 316, using the XER IID retrieved in step 502. If the IID is in finished IID buffer 316, then the relative address of that ID is used to access the corresponding XER data in XER rename buffer 315, in step 506. If, in step 505, the CAM read does not find the XER IID in finished IID buffer 316, the XER IID retrieved in step 502 is tested, in step 507, to determine if it is the IID of the instruction currently finishing. XER select logic 312 may implement this operation. If the XER IID retrieved in step 502 is the finished IID, then, in step 508, the XER data from fixed-point execution engine 311, through MUX 313, is used. If, in step 509, the instruction is ready to execute, the XER data is latched for use in the next execution cycle, step 510. 
     Otherwise in step 511, the instruction is written into instruction queue 301. If in step 512, the XER data is available from either steps 504, 506, or 508, in step 512, it is written into instruction queue 301 in field 314. Otherwise, in step 514, XER IID 303 in instruction queue 301 is compared with incoming finished IIDs using XER select logic 312. If a match is not obtained, then the process returns to step 512. If, however, in step 514, a match is obtained, and in step 515, the instruction depending on the finishing IID is ready to execute, then XER data is latched for use in the next execution cycle, in step 510. Then the instruction is executed in step 517. However, if in step 515, the instruction is not ready to execute, and the XER data has not been written to the instruction queue, in step 516, then the XER data is written to the instruction queue, in step 513. Otherwise, in step 516, the process returns to step 515, until the instruction is ready to execute, in step 515. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.