# Patent

## Title
Mechanism for extending the number of registers in a microprocessor

## Abstract

An apparatus and method are provided, for accessing extended registers within a microprocessor. The apparatus includes translation logic and extended register logic. The translation logic translates an extended instruction into corresponding micro instructions for execution by the microprocessor. The extended instruction has an extended prefix and an extended prefix tag. The extended prefix specifies register address extensions, the register address extensions indicating the extended registers, where the extended registers cannot be specified by an existing instruction set, and where the existing instruction set includes the x86 instruction set. The extended prefix tag indicates the extended prefix, where the extended prefix tag is an otherwise architecturally specified opcode within the existing instruction set, and where the extended prefix tag includes opcode F1 (ICE BKPT) in the x86 instruction set. The extended register logic is coupled to the translation logic. The extended register logic receives the corresponding micro instructions, and for accesses the extended registers.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority based on U.S. Provisional Application Ser. No. 60/369,586, filed Apr. 2, 2002, entitled “MECHANISM FOR EXTENDING THE NUMBER OF REGISTERS IN A MICROPROCESSOR.”
This application is related to the following co-pending U.S. patent applications, which are filed on the same day as this application, and which have a common assignee and common inventors.












SERIAL
DOCKET



NUMBER
NUMBER
TITLE







10/144595
CNTR.2176
APPARATUS AND METHOD FOR




EXTENDING A MICROPROCESSOR




INSTRUCTION SET


10/144592
CNTR.2186
APPARATUS AND METHOD FOR




CONDITIONAL INSTRUCTION




EXECUTION


10/144593
CNTR.2188
APPARATUS AND METHOD FOR




SELECTIVE CONTROL OF




CONDITION CODE WRITE BACK


10/144589
CNTR.2198
APPARATUS AND METHOD FOR




SELECTIVE CONTROL OF




RESULTS WRITE BACK











BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to the field of microelectronics, and more particularly to a technique for incorporating additional addressable registers into an existing microprocessor instruction set architecture.
2. Description of the Related Art
Since microprocessors were fielded in the early 1970's, their use has grown exponentially. Originally applied in the scientific and technical fields, microprocessor use has moved over time from those specialty fields into commercial consumer fields that include products such as desktop and laptop computers, video game controllers, and many other common household and business devices.
Along with this explosive growth in use, the art has experienced a corresponding technology pull that is characterized by an escalating demand for increased speed, expanded addressing capabilities, faster memory accesses, larger operand size, more types of general purpose operations (e.g., floating point, single-instruction multiple data (SIMD), conditional moves, etc.), and added special purpose operations (e.g., digital signal processing functions and other multi-media operations). This technology pull has resulted in an incredible number of advances in the art which have been incorporated in microprocessor designs such as extensive pipelining, super-scalar architectures, cache structures, out-of-order processing, burst access mechanisms, branch predication, and speculative execution. Quite frankly, a present day microprocessor is an amazingly complex and capable machine in comparison to its 30-year-old predecessors.
But unlike many other products, there is another very important factor that has constrained, and continues to constrain, the evolution of microprocessor architecture. This factor—legacy compatibility—furthermore accounts for much of the complexity that is present in a modern microprocessor. For market-driven reasons, many producers have opted to retain all of the capabilities that are required to insure compatibility with older, so-called legacy application programs as new designs are provided which incorporate new architectural features.
Nowhere has this legacy compatibility burden been more noticeable than can be seen in the development history of x86-compatible microprocessors. It is well known that a present day virtual-mode, 32-/16-bit x86 microprocessor is still capable of executing 8-bit, real-mode, application programs which were produced during the 1980's. And those skilled in the art will also acknowledge that a significant amount of corresponding architectural “baggage” is carried along in the x86 architecture for the sole purpose of supporting compatibility with legacy applications and operating modes. Yet while in the past developers have been able to incorporate newly developed architectural features into existing instruction set architectures, the means whereby use of these features is enabled—programmable instructions—are becoming scarce. More specifically, there are no more “spare” instructions in certain instruction sets of interest that provide designers with a means to incorporate newer features into an existing architecture.
In the x86 instruction set architecture for example, there are no undefined 1-byte opcode states that have not already been used. All 256 opcode states in the primary 1-byte x86 opcode map are taken up with existing instructions. As a result, x86 microprocessor designers must presently make a choice between providing new features and abandoning legacy compatibility. If new programmable features are to be provided, then they must be assigned to opcode states in order for programmers to exercise those features. And if spare opcode states do not remain in an existing instruction set architecture, then some of the existing opcode states must be redefined to provide for the new features. Thus, legacy compatibility is sacrificed in order to provide for new feature growth.
On area of growth that continues to plague microprocessor designers involves the number and use of addressable registers within a microprocessor. Early microprocessor designs provided for one or two general purpose 8-bit registers. Then, as computations within application programs became more complex, both the number and size of the general purpose registers grew. The present state of the art in microprocessors that are employed in desktop/laptop computing applications provides for less than 10 general purpose 32-bit registers. But even at present, there are application programming areas that are disadvantageously impacted because present day microprocessors do not provide more addressable registers for general purpose operations.
Therefore, what is needed is an apparatus and method that incorporate additional general purpose registers into an existing microprocessor instruction set architecture that has a completely full opcode set, and where incorporation of the technique additionally allows a conforming microprocessor to retain the capability to execute legacy application programs.
SUMMARY OF THE INVENTION
The present invention, among other applications, is directed to overcoming these and other problems and disadvantages of the prior art. The present invention provides a superior technique for extending a microprocessor instruction set beyond its current capabilities to provide for additional general purpose registers whose contents can be operated upon by programmable instructions in the microprocessor instruction set. In one embodiment, an apparatus is provided, for accessing extended registers within a microprocessor. The apparatus includes translation logic and extended register logic. The translation logic translates an extended instruction into corresponding micro instructions for execution by the microprocessor. The extended instruction has an extended prefix and an extended prefix tag. The extended prefix specifies register address extensions, the register address extensions indicating the extended registers, where the extended registers cannot be specified by an existing instruction set, and where the existing instruction set is the x86 instruction set. The extended prefix tag indicates the extended prefix, where the extended prefix tag is an otherwise architecturally specified opcode within the existing instruction set, and where the extended prefix tag is opcode F1 (ICE BKPT) in the x86 instruction set. The extended register logic is coupled to the translation logic. The extended register logic receives the corresponding micro instructions, and for accesses the extended registers.
One aspect of the present invention contemplates a mechanism, for extending an existing microprocessor instruction set to provide for additional operand registers. The mechanism includes an extended instruction and a translator. The extended instruction prescribes the additional operand registers, where the additional operand registers correspond to a prescribed operation, and where the extended instruction comprises a selected opcode in the existing microprocessor instruction set followed by an n-bit extended prefix. The selected opcode indicates the extended instruction and the n-bit extended prefix indicates the additional operand registers, where the additional operand registers cannot otherwise be prescribed according to the existing microprocessor instruction set and where the existing microprocessor instruction set is the x86 microprocessor instruction set, and where the selected opcode includes opcode ICE BKPT (i.e., opcode F1) in the x86 microprocessor instruction set. The translator receives the extended instruction, and generates a micro instruction sequence directing the microprocessor to access the additional operand registers during execution of the prescribed operation.
Another aspect of the present invention comprehends an instruction set extension apparatus, for providing extended register addressing capabilities to an existing instruction set. The instruction set extension apparatus has an escape tag, an extended registers specifier, and extended register logic. The escape tag is received by translation logic, and indicates that accompanying parts of a corresponding instruction prescribe an extended operation to be performed by a microprocessor, wherein the escape tag is a first opcode within the existing instruction set and where the existing instruction set is the x86 instruction set, and where the first opcode include the ICE BKPT opcode (i.e., opcode F1) in the x86 instruction set. The extended registers specifier is coupled to the escape tag and is one of the accompanying parts. The extended registers specifier prescribes a plurality of address extensions that corresponds to extended registers required by the extended operation. The extended register logic is coupled to the translation logic and accesses the extended registers during execution of the extended operation, where the existing instruction set provides only for addressing existing registers, and where the extended registers specifier enables addressing of the, extended registers.
A further aspect of the present invention provides a method for extending an existing instruction set architecture to provide for programmable addressing of additional registers within a microprocessor. The method includes providing an extended instruction, the extended instruction including an extended tag along with an extended prefix, where the extended tag is a first opcode entity according to the existing instruction set architecture, and where the providing includes selecting the first opcode entity according to the x86 microprocessor instruction set architecture, and where the selecting includes choosing the x86 ICE BKPT opcode (i.e., opcode F1) as the extended tag; prescribing, via the extended prefix and remaining parts of the extended instruction, the extended registers, where the extended registers are to be accessed during execution of a specified operation, and where the existing instruction set architecture only provides instructions for addressing of existing registers according to the instruction set architecture; and accessing the extended registers during execution of the specified operation.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features, and advantages of the present invention will become better understood with regard to the following description, and accompanying drawings where:
FIG. 1 is a block diagram illustrating a related art microprocessor instruction format;
FIG. 2 is a table depicting how instructions in a instruction set architecture are mapped to logic states of bits in an 8-bit opcode byte within the instruction format of FIG. 1;
FIG. 3 is a block diagram featuring an extended instruction format according to the present invention;
FIG. 4 is a table showing how extended architectural features are mapped to logic states of bits in an 8-bit extended prefix embodiment according to the present invention;
FIG. 5 is a block diagram illustrating a pipeline microprocessor for accessing extended registers according to the present invention;
FIG. 6 is a block diagram featuring one embodiment of an extended prefix for addressing additional registers in a microprocessor according to the present invention;
FIG. 7 is a block diagram featuring details of translate stage logic within the microprocessor of FIG. 5;
FIG. 8 is a block diagram illustrating extended register stage logic within the microprocessor of FIG. 5; and
FIG. 9 is a flow chart depicting a method for translating and executing instructions that address extended registers in a microprocessor according to the present invention.



DETAILED DESCRIPTION
The following description is presented to enable one of ordinary skill in the art to make and use the present invention as provided within the context of a particular application and its requirements. Various modifications to the preferred embodiment will, however, be apparent to one skilled in the art, and the general principles discussed herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described herein, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.
In view of the above background discussion on the techniques employed within present day microprocessors to extend the architectural features of those microprocessors beyond the capabilities of their associated instruction sets, a related art example will now be discussed with reference to FIGS. 1-2. The discussion highlights the problems that microprocessor designers routinely face today. On the one hand, they desire to incorporate more recently developed architectural features into a microprocessor design and, on the other hand, market conditions dictate that they must retain the capability to execute legacy application programs. In the example of FIGS. 1-2, a completely defined opcode map rules out the possibility of defining new opcodes for the exemplary architecture. Thus, the designers are compelled to choose either to incorporate the new features and sacrifice legacy compatibility to some extent, or to forego more recent architectural advances altogether in favor of maintaining the ability to execute older application programs. Following the related art discussion, a discussion of the present invention will be provided with reference to FIGS. 3-9. By employing an existing, yet archaic, opcode as a prefix tag for an extended instruction that follows, the present invention enables microprocessor designers to overcome the limitations of completely full instruction set architectures, thereby allowing them to provide additional general purpose registers for programmer use while concurrently retaining all the features that are required to run legacy application programs.
Turning to FIG. 1, a block diagram is presented illustrating a related art microprocessor instruction format 100. The related art instruction 100 has a variable number of instruction entities 101-103, each set to a specified value, that together make up a specific instruction 100 for a microprocessor. The specific instruction 100 directs the microprocessor to perform a specific operation such as adding two operands together, or moving an operand from/to memory to/from an internal register. In general, an opcode entity 102 within the instruction 100 prescribes the specific operation to be performed, and optional address specifier entities 103 follow the opcode 101 prescribing additional information about the specific operation such as how the operation is to be performed, where the operands are located, etc. The instruction format 100 additionally allows a programmer to prefix an opcode 102 with prefix entities 101. The prefixes 101 direct the application of specified architectural features during the execution of the specific operation prescribed by the opcode 102. Typically, these architectural features can be applied to most of the operations prescribed by any of the opcodes 102 in the instruction set. For example, prefixes 101 in many present day microprocessors direct operations to be executed using different operand sizes (e.g., 8-bit, 16-bit, 32-bit). Accordingly ,these processors are programmed to a default operand size (say, 32-bit), and prefixes 101 are provided in their respective instruction sets enabling programmers to selectively override the default operand size (say, to perform 16-bit operations) on an instruction-by-instruction basis. Selectable operand size is merely one example of an architectural feature that spans a significant number of the operations (e.g., add, subtract, multiply, Boolean logic, etc.) prescribed by opcodes 102 within many present day microprocessors.
One well-known instance of the instruction format 100 shown in FIG. 1 is the x86 instruction format 100, which is employed by all present day x86-compatible microprocessors. More specifically, the x86 instruction format 100 (also known as the x86 instruction set architecture 100) uses 8-bit prefixes 101, 8-bit opcodes 102, and 8-bit address specifiers 103. The x86 architecture 100 has several prefixes 101 as well, two of which override default address/data sizes of an x86 microprocessor (i.e., opcode states 66H and 67H), another which directs the microprocessor to interpret a following opcode byte 102 according to alternative translation rules (i.e., prefix value 0FH, which causes translation to be performed according to the so-called 2-byte opcode rules), and other prefixes 101 which cause particular operations to be repeated until repetition criteria are satisfied (i.e., the REP opcodes: F0H, F2H, and F3H).
Referring now to FIG. 2, a table 200 is presented depicting how instructions 201 in an instruction set architecture are mapped to values of bits in an 8-bit opcode byte 102 within the instruction format of FIG. 1. The table 200 presents an exemplary 8-bit opcode map 200 that associates up to 256 values of an 8-bit opcode entity 102 with corresponding microprocessor opcode instructions 201. The table 200 maps a particular value of an opcode entity 102, say value 02H, to a corresponding instruction opcode 201 (i.e., instruction I02201). In the particular case of the x86 opcode map, it is well known in the art that opcode value 14H is mapped to the x86 Add With Carry (ADC) instruction opcode, which directs that an 8-bit immediate operand be added to the contents of architectural register AL. One skilled in the art will also appreciate that the x86 prefixes 101 alluded to above (i.e., 66H, 67H, 0FH, F0H, F2H, and F3H) are actually opcode values 201 that contextually specify the application of certain architectural extensions to the operation prescribed by a following opcode entity 102. For example, preceding opcode 14H (normally, the ADC opcode discussed above) with prefix OFH results in an x86 processor executing an Unpack and Interleave Low Packed Single-Precision Floating-Point Values (UNPCKLPS) operation instead of the Add With Carry (ADC). Features such as described in this x86 example are enabled in part in a present day microprocessor because instruction translation/decoding logic in the microprocessor interprets the entities 101-103 of an instruction 100 in order. Hence, the use of specific opcode values as prefixes 101 in instruction set architectures has, in past times, allowed microprocessor designers to incorporate a significant number of advanced architectural features into a complying microprocessor design without disadvantageously impacting the execution of older programs which do not employ those specific opcode states. For example, a legacy program that never uses x86 opcode 0FH will still run on a present day x86 microprocessor. And a newer application program, by employing x86 opcode 0FH as a prefix 101, can utilize a substantial number of x86 architectural features that have been more recently incorporated such as single instruction multiple data (SIMD) operations and conditional move operations.
The incorporation of architectural feature advances has been accomplished in the past through the designation of available/spare opcode values 201 as prefixes 101 (also known as architectural feature tags/indicators 101 or escape instructions 101). Yet, many instruction set architectures 100 have run into a brick wall in terms of providing enhancements for a very straightforward reason: all of the available/spare opcode states have been used up, that is, all of the opcode values in the opcode map 200 have been architecturally specified. When all of the available opcode values have been assigned as either opcode entities 102 or prefix entities 101, then there are no more values left to provide for the incorporation of new features. This significant problem exists in many microprocessor architectures today and consequently forces designers to choose between adding architectural features to a design and retaining compatibility with older programs.
It is notable that the instructions 201 shown in FIG. 2 are depicted generically (i.e., I24, 186) rather than specifically (i.e., Add With Carry, Subtract, Exclusive-OR). This is because fully occupied opcode maps 200 are presently precluding the incorporation of more recent architectural advances in a number of different microprocessor architectures. And although an 8-bit opcode entity 102 is alluded to in the example of FIG. 2, one skilled in the art will appreciate that the specific size of the opcode 102 is irrelevant in any sense other than its use as a specific case to teach the problem of a full opcode structure 200. Accordingly, a fully populated 6-bit opcode map would exhibit 64 architecturally defined opcodes/prefixes 201 and would likewise provide no available/spare opcode values for expansion.
One alternative that falls short of entirely obliterating an existing instruction set and replacing it with a new format 100 and opcode map 200 is to substitute new instruction meanings for only a subset of existing opcodes 201, say opcodes 40H through 4FH in FIG. 2. Under this hybrid technique, a conforming microprocessor operates exclusively in one of two operating modes: a legacy-compatible mode, where opcodes 40H-4FH are interpreted according to legacy rules, or an enhanced mode, where opcodes 40H-4FH are interpreted according to enhanced architectural rules. This technique indeed enables designers to incorporate new features into a design, but when the conforming microprocessor is running in an enhanced mode it excludes execution of any application program that uses opcodes 40H-4FH. Hence, from the standpoint of retaining legacy compatibility, the legacy-compatible/enhanced mode technique is not optimum.
The present inventors, however, have noted the frequency of use of certain opcodes 201 in instruction sets 200 having fully-populated opcode spaces over the breadth of application programs composed for execution on compliant microprocessors. They have accordingly observed that there are some opcodes 202 which, although they are architecturally defined, are not employed within application programs that are capable of being executed by the microprocessors. Instruction IF1202 is depicted in FIG. 2 as such an example of this singularity. In fact, the very same opcode value 202 (i.e., F1H) maps to a valid instruction 202 in the x86 instruction set architecture that is not presently employed by any extant application program. While the unused x86 instruction 202 is a valid x86 instruction 202 that directs an architecturally specified operation on an x86 microprocessor, it is not employed in any application program that can be executed on any present day x86 microprocessor. The particular x86 instruction 202 is known as In Circuit Emulation Breakpoint (i.e., ICE BKPT, opcode value F1H), and was formerly employed exclusively in a class of microprocessor emulation equipment that no longer exists today. ICE BKPT 202 was never employed in an application program outside of an in-circuit emulator, and the form of in-circuit emulation equipment that formerly employed ICE BKPT 202 no longer exists. Hence, in the x86 case, the present inventors have identified a means within a completely occupied instruction set architecture 200 whereby they can exploit a valid, yet obsolete, opcode 202 to allow for the incorporation of advanced architectural features in a microprocessor design without sacrificing legacy compatibility. In a fully-occupied instruction set architecture 200, the present invention employs an architecturally specified, yet unemployed, opcode 202 as a indicator tag for in an n-bit prefix that follows, thus allowing microprocessor designers to incorporate up to 2n more recently developed architectural features into an existing microprocessor design, while concurrently retaining complete compatibility with all legacy software.
The present invention exploits the prefix tag/extended prefix concept by providing an n-bit extended registers specifier prefix whereby programmers are enabled to specify the addresses of additional general purpose registers that have been provided in a microprocessor beyond those provided for by an existing instruction set architecture for the microprocessor. The present invention will now be discussed with reference to FIGS. 3-9.
Turning to FIG. 3, a block diagram is presented featuring an extended instruction format 300 according to the present invention. Very much like the format 100 discussed with reference to FIG. 1, the extended instruction format 300 has a variable number of instruction entities 301-305, each set to a specified value, that together make up a specific instruction 300 for a microprocessor. The specific instruction 300 directs the microprocessor to perform a specific operation such as adding two operands together, or moving an operand from memory to a register within the microprocessor. Typically, an opcode entity 302 in the instruction 300 prescribes the specific operation to be performed, and optional address specifier entities 303 follow the opcode 302 prescribing additional information about the specific operation such as how the operation is to be performed, where the operands are located, etc. The instruction format 300 also allows a programmer to prefix an opcode 302 with prefix entities 301 that direct the application of existing architectural features during the execution of the specific operation prescribed by the opcode 302.
The extended instruction 300 according to the present invention, however, is a superset of the instruction format 100 described above with reference to FIG. 1, having two additional entities 304, 305 which are optionally provided as an instruction extension to precede all remaining entities 301-303 in a formatted extended instruction 300. The purpose of the two additional entities 304, 305 is to provide the capabilities for programmers to specify extended addresses of additional, or extended, general purpose registers within a conforming microprocessor for the purpose of executing an operation on operands contained therein, where the extended addresses are not otherwise programmable by an existing instruction set for the conforming microprocessor. The two additional entities 304, 305 provide for the incorporation of extended register features in a microprocessor design which would otherwise be not specifiable within a fully populated instruction set architecture. The optional entities 304, 305 are an extended instruction tag 304 and an extended registers specifier prefix 305. The extended instruction tag 305 is an otherwise architecturally specified opcode within a microprocessor instruction set. In an x86 embodiment, the extended instruction tag 304, or escape tag 304, is opcode state F1H, the formerly used ICE BKPT instruction. The escape tag 304 indicates to microprocessor logic that the extended registers specifier prefix 305, or extended features specifier 305, follows, where the extended registers specifier 305 prescribes extended addresses for internal microprocessor registers that correspond to operands/results required/generated by execution of a specified operation. In one embodiment, the escape tag 304 indicates that accompanying parts 301-303, 305 of a corresponding instruction 300 prescribe an extended operation to be performed by the microprocessor. The extended registers specifier 305, or extended prefix 305, prescribes a plurality of address extensions that correspond to extended registers required by the extended operation. Extended register logic in the microprocessor accesses the extended registers during execution of the extended operation.
To summarize the conditional execution extension technique according to the present invention, an extended instruction is configured to prescribe additional operand registers in an existing microprocessor instruction set, where the additional operand registers cannot otherwise be prescribed according to the existing microprocessor instruction set. The extended instruction includes one of the opcodes/instructions 304 in the existing instruction set and an n-bit extended features prefix 305. The selected opcode/instruction serves as an indicator 304 that the instruction 300 is an extended features instruction 300 (that is, it prescribes extensions to the microprocessor architecture), and the n-bit features prefix 305 indicates the additional operand registers. In an alternative embodiment, the extended prefix 305 is 8-bits in size, providing for the specification of up to 256 different values that are configured to specify a plurality of register address extensions that correspond to extended registers required by an associated extended operation. An n-bit prefix embodiment provides for the specification of up to 2n different address extensions.
Now turning to FIG. 4, a table 400 is presented showing how register extensions are mapped to logic states of bits in an 8-bit extended prefix embodiment according to the present invention. Similar to the opcode map 200 discussed with reference to FIG. 2, the table 400 of FIG. 4 presents an exemplary 8-bit register specifier prefix map 400 that associates up to 256 values of an 8-bit extended prefix entity 305 with corresponding register address extensions 401 (e.g., E34, E4D, etc.) of a conforming microprocessor. In the case of an x86 embodiment, the 8-bit extended feature prefix 305 according to the present invention serves to provide for register specifiers 401 (i.e., E00-EFF) which are not provided for by the current x86 instruction set architecture.
The extended features 401 shown in FIG. 4 are depicted generically rather than specifically because the technique according to the present invention is applicable to a variety of different architectural extensions 401 and specific instruction set architectures. One skilled in the art will appreciate that many different architectural features 401, including those noted above, can be incorporated into an existing instruction set according to the escape tag 304/extended prefix 305 technique described herein. The 8-bit prefix embodiment of FIG. 4 provides for up to 256 different features 401, however, an n-bit prefix embodiment can allow for programming of up to 2n different features 401.
Varying embodiments can be configured according to the manner in which a particular existing microprocessor instruction set provides for addressing of its existing registers. For example, one embodiment contemplates a plurality of source register and destination register specifier fields within the extended prefix 305 that entirely supercede existing register specification fields within remaining parts 301-303 of an extended instruction 300. A field extending embodiment comprises a plurality of source register and destination register address extension fields whose contents serve as address extensions to corresponding source/destination register addresses specified in existing register specification fields in the remaining parts 301-303 of the extended instruction 300. One form of this embodiment utilizes the address extensions as a plurality of upper register address bits which are combined with corresponding lower register address bits in the remaining parts 301-303 to specify the extended registers. A different embodiment does not employ separate fields for specification of operand registers, but rather employs a specifically encoded value of the n-bit extended prefix 305 to prescribe a corresponding set of source/destination operand registers that are to be applied to the prescribed operation. There are a number of different ways in which to encode addresses for extended registers in an n-bit extended prefix 305 according to the present invention. One skilled in the art will appreciate, however, that a specific form of encoding to prescribe register extensions in the n-bit extended prefix 305 is dependent upon the specific microprocessor architecture and instruction set to which the present invention is applied. Because occurrence of the selected escape instruction 304 indicates a following n-bit extended prefix 305, the size of the n-bit extended prefix 305, the size of the n-bit extended prefix 305 can be optimally established to comport with any means of extended register specification.
Now referring to FIG. 5, a block diagram is presented illustrating a pipeline microprocessor 500 for accessing extended registers according to the present invention. The microprocessor 500 has three notable stage categories: fetch, translate, and execute. The fetch stage has fetch logic 501 that retrieves instructions from an instruction cache 502 or external memory 502. The retrieved instructions are provided to the translate stage via an instruction queue 503. The translate stage has translation logic 504 that is coupled to a micro instruction queue 506. The translation logic 504 includes extended translation logic 505. The execute stage has execution logic 507 having extended execution logic 508 therein.
In operation, the fetch logic 501 retrieves formatted instructions according to the present invention from the instruction cache/external memory 502, and places these instructions in the instruction queue 503 in execution order. The instructions are retrieved from the instruction queue 503 and are provided to the translation logic 504. The translation logic 504 translates/decodes each of the provided instructions into a corresponding sequence of micro instructions that direct the microprocessor 500 to perform the operations prescribed by the instructions. The extended translation logic 505 detects those instructions having an extended prefix tag according to the present invention and also provides for translation/decoding of corresponding extended register specifier prefixes. In an x86 embodiment, the extended translation logic 505 is configured to detect an extended prefix tag of value F1H, which is the x86 ICE BKPT opcode. Extended micro instruction fields are provided in the micro instruction queue 506 to allow for the specification of additional internal registers within the microprocessor 500.
The micro instructions are provided from the micro instruction queue 506 to the execution logic 507, wherein the extended execution logic 508 is configured to access internal microprocessor registers therein according to specification by the extended micro instruction fields. A plurality of source operands are retrieved from source operand extended registers as prescribed for employment during execution of a specified operation. The extended execution logic 508 performs the operation prescribed by the micro instructions and generates corresponding results. Following generation of the corresponding results, the extended execution logic 508 writes back the corresponding results into destination operand extended registers as prescribed by the extended micro instruction fields.
One skilled in the art will appreciate that the microprocessor 500 described with reference to FIG. 5 is a simplified representation of a present day pipeline microprocessor 500. In fact, a present day pipeline microprocessor 500 comprises upwards to 20-30 pipeline stages. However, these stages can be generally categorized into those three stage groups shown in the block diagram and thus, the block diagram 500 of FIG. 5 serves to teach the essential elements that are required to implement embodiments of the present invention as described hereinabove. Those elements of a microprocessor 500 which are extraneous to the present discussion, for clarity, are not depicted.
Turning now to FIG. 6, a block diagram is presented featuring one embodiment of an extended prefix 600 for addressing additional registers in a microprocessor according to the present invention. The extended registers specifier prefix 600 is an 8-bit extended prefix 600 and includes a source address extension field 601 (S3), a destination address extension field 602 (D3), and a spare field 603. The S3 field 601 comprises bit 0 of the prefix 600 and is employed by extended register logic according to the present invention as bit 3 of a 4-bit extended first source register address. The remaining three bits (bits 2:0) of the 4-bit first source register address are specified by remaining opcode and address specifier entities 604 provided according to register specification conventions of the existing microprocessor instruction set architecture. The D3 field 602 comprises bit 1 of the prefix 600 and is employed by extended register logic according to the present invention as bit 3 of a 4-bit extended second source register address. The remaining three bits (bits 2:0) of the 4-bit second source register address are specified by remaining opcode and address specifier entities 605 provided according to register specification conventions of the existing microprocessor instruction set architecture. The second source register address is employed as well as a destination register address for results of an executed operation.
The exemplary embodiment of an extended prefix 600 according to the present invention as shown in FIG. 6 reflect a register extension embodiment that is suited to application in the x86 instruction set architecture. Presently the x86 architecture provides for eight general purpose registers which are specified in existing x86 instructions according to well known encoding formats within an opcode byte and address specifier bytes (known as ModR/M and SIB bytes of an x86 instruction). By using the S3601 and D3602 fields of an extended prefix 600 as shown, the number of addressable registers in an x86 microprocessor can be increased from eight to 16. One skilled in the art will appreciate that providing for 2-bit source and destination fields 601, 602 in the extended prefix would allow 4-fold increase in the number of addressable registers in an existing architecture.
Turning now to FIG. 7, a block diagram is presented featuring details of translate stage logic 700 within the microprocessor of FIG. 5. The translate stage logic has an instruction buffer 704 that provides an extended instruction to translation logic 705. The translation logic 705 is coupled to a machine specific register 702 that has an extended features field 703. The translation logic 705 has a translation controller 706 that provides a disable signal 707 to an escape instruction detector 708 and an extended decoder 709. The escape instruction detector 708 is coupled to the extended decoder 709 and an instruction decoder 710. The extended decoding logic 709 and the instruction decoding logic 710 access a control read-only memory (ROM) 711, wherein are stored template micro instruction sequences that correspond to some of the extended instructions. The translation logic 705 also has a micro instruction buffer 712 having a micro opcode field 714, a destination field 715, a source field 716, and a displacement field 717.
Operationally, during power-up of the microprocessor, the state of the extended field 703 within the machine specific register 702 is established via signal power-up state 701 to indicate whether the particular microprocessor is capable of translating and executing extended instructions according to the present invention for addressing additional registers in the microprocessor. In one embodiment, the signal 701 is derived from a feature control register (not shown) that reads a fuse array (not shown) configured during fabrication of the part. The machine specific register 702 provides the state of the extended features field 703 to the translation controller 706. The translation control logic 706 controls whether or not instructions from the instruction buffer 704 are translated according to extended translation rules or according to conventional translation rules. Such a control feature is provided to allow supervisory applications (e.g., BIOS) to enable/disable extended execution features of the microprocessor. If extended features are disabled, then instructions having the opcode state selected as the extended features tag would be translated according to the conventional translation rules. In an x86 embodiment having opcode state F1H selected as the tag, an occurrence of F1H under conventional translation would result in an illegal instruction exception. With extended translation disabled, the instruction decoder 710 would translate/decode all provided instructions 704 and would configure all fields 714-717 of the micro instruction 712. Under extended translation rules, however, occurrence of the tag would be detected by the escape instruction detector 708. The escape instruction detector 708 would accordingly allow the instruction decoder 710 to translate/decode the remaining parts of the extended instruction and configure the micro opcode field 714 and the displacement field 717 of the micro instruction 712 and the extended decoder 709 would decode/translate the extended prefix and applicable parts of the extended instruction to configure the source and destination fields 716, 715 of the micro instruction 712. Certain instructions would cause access to the control ROM 711 to obtain corresponding micro instruction sequence templates. Configured micro instructions 712 are provided to a micro instruction queue (not shown) for subsequent execution by the processor.
Now referring to FIG. 8, a block diagram is presented illustrating extended register stage logic 800 within the microprocessor of FIG. 5. The extended register stage logic 800 has register logic 802 that retrieves an extended micro instruction according to the present invention from a micro instruction buffer 801 or micro instruction queue 801. The register logic 802 has an extended register file 803 comprising existing architectural registers and additional registers. In an x86 embodiment, registers R0-R7 comprise the eight existing architectural registers and registers R8-R15 comprise eight added registers. The registers R0-R15 are accessed for retrieval of source operands by extended read logic 806 and are accessed for storage of result operands by extended write back logic 807. The extended read logic 806 outputs the source operands OP1, OP2, to two operand buffers 809, 810. Result operands RS1, RS2 are provide to the extended write back logic 807 via two result buffers 812, 813.
In operation, extended micro instructions are provided from the micro instruction queue 801 to the register logic 802 in synchronization with a pipeline clock (not shown). During a clock cycle, the extended read logic 802 decodes source address fields 804, 805 of the extended micro instruction to determine which of the registers R0-R15 contain source operands for employment by a prescribed operation. The source operands OP1, OP2 are retrieved from and are provided to source operand registers 809, 810. In addition, the extended micro instruction is piped to buffer 808 so that it will be available for employment by subsequent pipeline stages (not shown) in the microprocessor. During the same clock cycle, results RS1, RS2 of a recently executed operation are written back into destination registers as prescribed by destination register fields (not shown) in completed micro instruction buffer 811. The corresponding result operands RS1, RS2 are provided in buffers 812, 813.
The register stage logic 800 depicted in FIG. 8 provides the ability to coherently access two source registers and two result registers during a single clock cycle. An alternative embodiment provides for two source operands and a single destination operand. To ensure coherency of the registers R0-R15, the extended register logic 802 accesses source operands OP1, OP2 prior to performing write back of results RS1, RS2.
Now referring to FIG. 9, a flow chart 900 is presented depicting a method for translating and executing instructions that address extended registers in a microprocessor according to the present invention. Flow begins at block 902 wherein a program configured with extended register addressing instructions is provided to a microprocessor. Flow then proceeds to block 904.
At block 904, a next instruction is fetched from cache/memory. Flow then proceeds to block 906.
At block 906, the next instruction is decoded/translated according to conventional translation rules that employ only existing architectural registers in an existing instruction set architecture. Flow then proceeds to block 908
At block 908, the instruction's opcode and address specifier fields are employed to determine register addresses of operands corresponding to a prescribed operation. Flow then proceeds to block 910.
At block 910, a micro instruction sequence is configured to specify the prescribed operation along with its corresponding operand register addresses. Flow then proceeds to decision block 912.
At decision block 912, the next instruction fetched in block 904 is evaluated to determine whether or not it contains an extended escape tag/code. If not, then flow proceeds to block 918. If the extended escape code is detected, then flow proceeds to block 914.
At block 914, because an extended escape tag has been detected in block 906, translation/decoding is performed on an extended registers specifier prefix to determine extended register addresses that correspond to the prescribed operation. In one embodiment, the extended registers specifier prefix provides all address bits that are required to determine the extended register addresses. In an alternative embodiment, the prefix provides only a portion of the address bits and remaining bits of the addresses are provided in the other fields translated/decodes in block 906. Flow then proceeds to block 916.
At block 916, the operand address fields of the micro instruction sequence that were configured in block 910 are modified to indicate extended operand register addresses determined in block 914. Flow then proceeds to block 918.
At block 918, the micro instruction sequence is provided to a micro instruction queue for execution by the microprocessor. Flow then proceeds to block 920.
At block 920, the micro instruction sequence is retrieved by extended register logic according to the present invention. The extended register logic retrieves operands corresponding to the prescribed operation from specified extended registers. Flow then proceeds to block 922.
At block 922, extended execution logic executes the prescribed operation using the operands accessed in block 920 and generates result operands. Flow then proceeds to block 924.
At block 924, the result operands are provided to the extended register logic and are written back into extended registers as prescribed by the micro instruction sequence. Flow then proceeds to block 926.
At block 926, the method completes.
An alternative embodiment of the method 900 initially detects the extended escape code (decision block 912) prior to translation operations (blocks 906 and 914) and micro instruction sequence is configured in a single step that does not require the replacement step 916.
Although the present invention and its objects, features, and advantages have been described in detail, other embodiments are encompassed by the invention as well. For example, the present invention has been described in terms of a technique that employs a single, unused, opcode state within a completely full instruction set architecture as a tag to indicate that an extended feature prefix follows. But the scope of the present invention is not limited in any sense to full instruction set architectures, or unused instructions, or single tags. On the contrary the present invention comprehends instruction sets that are not entirely mapped, embodiments having used opcodes, and embodiments that employ more than one instruction tag. For example, consider an instruction set architecture where there are no unused opcode states. One embodiment of the present invention comprises selecting an opcode state that is presently used as the escape tag, where the selection criteria is determined according to market-driven factors. An alternative embodiment comprehends employing a peculiar combination of opcodes as the tag, say back-to-back occurrences of opcode state 7FH. The essential nature of the present invention thus embodies use of a tag sequence followed by an n-bit extension prefix that allows a programmer to specify additional operand registers in an extended instruction which are not otherwise provided for by existing instructions in a microprocessor instruction set.
Furthermore, although a microprocessor setting has been employed to teach the present invention and its objects, features, and advantages, one skilled in the art will appreciate that its scope extends beyond the boundaries of microprocessor architecture to include all forms of programmable devices such as signal processors, industrial controllers, array processors, and the like.
Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention, and 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.

## Claims

1. An apparatus, for accessing extended registers within a microprocessor, the apparatus comprising:
translation logic, for translating an extended instruction into corresponding micro instructions for execution by the microprocessor, wherein said extended instruction comprises: 
an extended prefix, for specifying register address extensions, said register address extensions indicating the extended registers, wherein the extended registers cannot be specified by an existing instruction set, and wherein said existing instruction set comprises the x86 instruction set; and 
an extended prefix tag, for indicating said extended prefix, wherein said extended prefix tag is an otherwise architecturally specified opcode within said existing instruction set, and wherein said extended prefix tag comprises opcode F1 (ICE BKPT) in the x86 instruction set; and 

extended register logic, coupled to said translation logic, for receiving said corresponding micro instructions, and for accessing the extended registers.

2. The apparatus as recited in claim 1, wherein said extended instruction further comprises instruction entities according to said existing instruction set.

3. The apparatus as recited in claim 2, wherein said instruction entities prescribe an operation to be executed by the microprocessor, and wherein operands corresponding to said operation are retrieved/stored from/to the extended registers.

4. The apparatus as recited in claim 3, wherein said instruction entities further prescribe architectural register addresses.

5. The apparatus as recited in claim 4, wherein said register address extensions are employed by said translation logic to determine the extended registers.

6. The apparatus as recited in claim 4, wherein said translation logic combines said register address extensions with said architectural register addresses to determine the extended registers.

7. The apparatus as recited in claim 1, wherein said extended prefix comprises 8 bits.

8. The apparatus as recited in claim 1, wherein said extended prefix comprises:
a source register address extension, for prescribing a first extended register containing a first source operand; and 
a destination register address extension, coupled to said source register address extension, for prescribing a second extended register containing a second source operand, and for prescribing said second extended register for storage of a result operand.

9. The apparatus as recited in claim 1, wherein said translation logic comprises:
escape instruction detection logic, for detecting said extended prefix tag; 
instruction decoding logic, for determining an operation to be performed along with architectural registers according to said existing instruction set; and 
extended decoding logic, coupled to said escape instruction detection logic and said instruction decoding logic, for determining the extended registers, and for prescribing the extended registers within said corresponding micro instructions.

10. A mechanism, for extending an existing microprocessor instruction set to provide for additional operand registers, the mechanism comprising:
an extended instruction, configured to prescribe the additional operand registers, the additional operand registers corresponding to a prescribed operation, wherein said extended instruction comprises a selected opcode in the existing microprocessor instruction set followed by an n-bit extended prefix, said selected opcode indicating said extended instruction and said n-bit extended prefix indicating the additional operand registers, wherein the additional operand registers cannot otherwise be prescribed according to the existing microprocessor instruction set, and wherein the existing microprocessor instruction set is the x86 microprocessor instruction set, and wherein said selected opcode comprises opcode ICE BKPT (i.e., opcode F1) in the x86 microprocessor instruction set; and 
a translator, configured to receive said extended instruction, and configured to generate a micro instruction sequence directing said microprocessor to access the additional operand registers during execution of said prescribed operation.

11. The mechanism as recited in claim 10, wherein said extended instruction further comprises:
remaining instruction entities, configured to specify said prescribed operation, wherein said prescribed operation is to be executed by the microprocessor, and wherein operands corresponding to said prescribed operation are provided from/to the additional operand registers.

12. The mechanism as recited in claim 11, wherein said n-bit prefix comprises:
register extension fields, each of said fields configured to designate upper address bits of an address for each of the additional operand registers.

13. The mechanism as recited in claim 12, wherein said lower address bits of the address are provided by said remaining instruction entities within said extended instruction.

14. The mechanism as recited in claim 10, wherein said n-bit extended prefix comprises 8 bits.

15. The mechanism as recited in claim 10, wherein said translator comprises:
an escape instruction detector, for detecting said selected opcode within said extended instruction; 
an instruction decoder, for decoding remaining parts of said extended instruction to determine said prescribed operation; and 
an extended prefix decoder, coupled to said escape instruction detector and said instruction decoder, for decoding said n-bit extended prefix, and for specifying the additional operand registers within said micro instruction sequence.

16. An instruction set extension apparatus, for providing extended register addressing capabilities to an existing instruction set, the instruction set extension apparatus comprising:
an escape tag, for reception by translation logic, and for indicating that accompanying parts of a corresponding instruction prescribe an extended operation to be performed by a microprocessor, wherein said escape tag is a first opcode within the existing instruction set, and wherein the existing instruction set is the x86 instruction set, and wherein said first opcode comprises the ICE BKPT opcode (i.e., opcode F1) in the x86 instruction set; 
an extended registers specifier, coupled to said escape tag and being one of said accompanying parts, for prescribing a plurality of address extensions that corresponds to extended registers required by said extended operation; and 
extended register logic, coupled to said translation logic, for accessing said extended registers during execution of said extended operation, wherein the existing instruction set provides only for addressing existing registers, and wherein said extended registers specifier enables addressing of said extended registers.

17. The instruction set extension apparatus as recited in claim 16, wherein remaining ones of said accompanying parts comprise a second opcode and an optional plurality of address specifiers, for specifying said extended operation and a plurality of addresses, wherein said plurality of address extensions is combined with said plurality of addresses to generate extended addresses for said extended registers.

18. The instruction set extension apparatus as recited in claim 16, wherein said extended registers specifier comprises 8 bits.

19. The instruction set extension apparatus as recited in claim 16, wherein said translation logic translates said escape tag and said accompanying parts into corresponding micro instructions that direct said extended register logic to access said extended registers during execution of said extended operation for retrieval/storage of operands.

20. The instruction set extension apparatus as recited in claim 16, wherein said translation logic comprises:
escape tag detection logic, for detecting said escape tag, and for directing that said accompanying parts be translated according to extended translation conventions; and 
decoding logic, coupled to said escape tag detection logic, for performing translation of instructions according to conventions of the existing instruction set, and for performing translation of said corresponding instruction according to said extended translation conventions to enable addressing of said extended registers.

21. A method for extending an existing instruction set architecture to provide for programmable addressing of additional registers within a microprocessor, the method comprising:
providing an extended instruction, the extended instruction including an extended tag along with an extended prefix, wherein the extended tag is a first opcode entity according to the existing instruction set architecture, and wherein said providing comprises selecting the first opcode entity according to the x86 microprocessor instruction set architecture, and wherein said selecting comprises choosing the x86 ICE BKPT opcode (i.e., opcode F1) as the extended tag; 
prescribing, via the extended prefix and remaining parts of the extended instruction, the extended registers, wherein the extended registers are to be accessed during execution of a specified operation, and wherein the existing instruction set architecture only provides instructions for addressing of existing registers according to the instruction set architecture; and 
accessing the extended registers during execution of the specified operation.

22. The method as recited in claim 21, wherein said prescribing comprises:
first specifying the specified operation, wherein said first specifying comprises employing a second opcode entity according to the existing instruction set architecture.

23. The method as recited in claim 21, wherein said providing comprises employing an 8-bit entity to configure the extended prefix.

24. The method as recited in claim 21, further comprising:
translating the extended instruction into micro instructions that direct extended execution logic to access the extended registers.

25. The method as recited in claim 24, wherein said translating comprises:
within translation logic, detecting the extended tag; and 
decoding the extended prefix and the remaining parts according to extended translation rules to identify the extended registers for said translating.