CA2053941A1

CA2053941A1 - System for preparing instructions for instruction parallel processor and system with mechanism for branching in the middle of a compound instruction

Info

Publication number: CA2053941A1
Application number: CA 2053941
Authority: CA
Inventors: Stamatis Vassiliadis; Bartholomew Blaner; Thomas L. Jeremiah
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1991-03-29
Filing date: 1991-10-22
Publication date: 1992-09-30

Abstract

??9-90-040 SYSTEM FOR PREPARING INSTRUCTIONS FOR INSTRUCTION PARALLEL
PROCESSOR AND SYSTEM WITH MECHANISM FOR BRANCHING
IN THE MIDDLE OF A COMPOUND INSTRUCTION

ABSTRACT OF THE DISCLOSURE
An instruction processor system for decoding compound instructions created from a series of base instructions of a scalar machine, the processor generating a series of compound instructions with an instruction format text having appended control bits in the instruction format text enabling the execution of the compound instruction format text in said instruction processor with a compounding facility which fetches and decodes compound instructions which can be executed as compounded and single instructions by the arithmetic and logic units of the instruction processor while preserving intact the scalar execution of the base instructions of a scalar machine which were originally in storage. The system nullifies any execution of a member instruction unit of a compound instruction upon occurrence of possible conditions, such as branch. which would affect the correctness of recording results of execution of the member instruction unit portion based upon the interrelationship of member units of the compound instruction with other instructions. The resultant series of compounded instructions generally executes in a faster manner than the original format which is preserved due to the parallel nature of the compounded instruction stream which is executed.

Description

I,N9-9()-()4() I 2 ~ 5 3 9 4 ~

SYSTi-:M FOR PREPARIN~ IN~TRUCTIONS FOR INSTRUCTION PARALLEL
PROCESSOR AND SY~TEM WITH MECHANISM FOR BRANCHING
IN THE MIDDL.E OF A COMPOUND INSTRUCTION

FIELD OF THE INVENTION

This invention relates to digital computers and digltal data processors, and particularly to digita, computers and data processors capable of executing 1wo or more instructions in parallel, and provides a mechanism for branching in the middie o~ a compound instruction. Particularly also, the system thus provided enables a set of instructions which may be combined into compound instructiom tu be statically determined, and allows the appending of control inforrnation to the Instructions In the program to be executed with branching in l:he middle of a compound instruction enabled.

BACKGROUND OF THE INVENTION.

Traditional computers which receive a sequence of instructions and execute the sequence one instruction at a time are known, and are referred to as scalar computers. With each new generation of computing machines, new acceleration mechanlsms must be dlscovered for tradltional scalar machines. A recent mechanlsm for acceleratlng computatlonal speed is found In reduced instruction set architecture (RISC) that employs a limlted set of very simple inst! uctlons executing at a high rate of speed.
Alternatively, another acceleration mechanism may be to add more complex instructions to the architecture to provlde more computation function per 2~5334~
r;~is-s~ 4() - 2 -instruction and thus re~ ce the number of instructions in a program.
Application of either of these approaches to an existing scalar computer would require a fundament~l alleration of the instruction set and architecture of the machine. Such a far-reaching transformation is fraught with expense, downtime, and an initial reduction in the machine's reliability and availability.

Recently, "superscalar" computers have been developed which further accelerate computation speed. These machines are essentially scalar machines whose performance is increased by adapting them to execute more than one instruction at a time from an instruction stream including a sequence of single scalar instructions. These machines typically decide at instruction execution time whether two or more instructions in a sequence of scalar instructions may be executed in parallel. The decision is based upon the operation codes (OP codes) of the instruc~ions and on data dependencies which may exist between instructions. An OP code signifies the computational hardware required for an instruction. In general, it is not possible to concurrently execute two or more instructions which utilize the same hardware (a hardware dependency) or the same operand (a data dependency). These hardware and data dependencies prevent the parallel execution of some instruction combinations. In these cases, the affected instructions are executed serially. This, of course, reduces the performance of a superscalar machine.

Superscalar computers suffer from disadvantages which it is desirable to minimize. A concrete amount of time is consumed in deciding at instruction execution time which instructions can be executed in parallel. This time cannot be readily masked by overlapping with other machine operations.
This disadvantage becomes more pronounced as the complexity of the instruction set architecture increases. Also, the parallel execution decision must be repeated each time the same instructions are to be executed.

In extending the useful lifetime of existlng scalar computers, every means of acceleratlng execution is vit31. However, acceleration by means of 2 ~ ~ 3 9 4 1 reduced instruction set architecture, complex instruction se~ architecture, or traditional superscalar t~cllniques is potentially too costly or too disadvantageous to consider for an existing scalar rnachine~

It would be preferred tO accelerate the speed of execution of such a computer by parallel, or concurrent, execution of instructions in an existing instruction set without requiring change of the instruction set, change of machine architec4ure, or extension of the time required for instruction execution .

SUMMARY OF THE INVENTION

It is to this object that the inventions which are here stated are addressed as a system which enables original programs to be processed as parallel and single instructions as fits the original program functions which are implemented for execution by a machine capable of instruction-level parallel processing. We have provided a way for existing programs written in existing high level languages or existing assembly language programs to be processed by a preprocessor which can identify sequences of instructions which can be executed as a single compound instruction in a computer designed to execute compound instructions.

The instruction processor system provides compounding decoding for a series of base instructions of a scalar machine, generates a series of compound instructions, provides for fetching of the compound instructions and for decoding the fetched compound instructions and necessary single instructions, and provides a compound instruction program which preserves intact the scalar execution of the base instructions of a scalar machine when the program with compound instructions Is executed on the system. This system also provldes a mechanism for branching in the middle of a compounding instruction. For branching in the middle of a compound instruction, compound instructions are examined for appended control bits 2~539~1 r;~'9 9()-04() - 4 -and a nullification mechanism responsive to the control bits is provided for nullifying the execution of;`member units of the branched-to or subsequent compound instructions if Iheir execution would cause incorrect program behavior.

The instruction processor described herein provides decoding of compound instructions created from a series of base instructions of a scalar machine, the processor generating a series of compound instructions with instructlon text having appended control bits enabling the execution of the compound instruction in said instruction processor with a compounding facility which is provided in the system. The system fetches and decodes compound instructions which can be executed as compounded and single instructions by the functional instruction units of the instruction processor while preserving intact the scalar execution of the base instructions of a scalar machine which were originally in storage. The resultant series of compounded instructions yenerally executes in a faster manner than the original format due to the parallel nature of the compounded instruction stream which is executed.

For a better understan~ing of the invention, together with its advantages and features, reference should be made to the following description and the below-described drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
-FIGURE 1 is h,gh-level schematic diagram of a computing system which is capable of compounding instructions in a sequence of scalar instructions for concurrent execution.

FIGURE 2 is a timing dlagram for a uni-processor implementation showing !
the parallel execution of certain Instructions which have been selectively grouped in a compound instruction stream.

T;~9-s()-()4() ~ 2 ~ ~ 3 9 41 FIGURE 3 is a block diagram of a hierarchical memory organization in a scalable compound instru~tion set machine with in-memory processing shown as an alternative preferred embodiment for the operational envlronrnent .

FIGURE 4-A is a high level schematic diagram of the process which will be described pictorially for providing means for processing existing programs to identify sequences of instructions which can be executed as a single compound instruction in a computer designed to execute compound instructions, while FIGURE 4-B illustrates the preferred operational environment of the inventions and the inventions' location in the environment.

FIGURE 5 shows a path taken by a program from original program code to actual execution.

FIGURE 6 is a flow diagram showing generation of a compound Instructlon set program frorn an assembly language program; while FIGURE 7 is a flow diagram showing execution of a compound instruction set.

FIGURE 8 illustrates a situation where a branch target may be at the beginning or middle of a compound instruction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
. _ Background 1 ~'9-9()-04() 1, 2053941 A scalable compound instruction set machine (SCISM) architecture method is a system in which Instruction level parallelism is achieved by staticaily analyzing a sequence of scalar instruction at a time prior to instruction execution to generate compound instructions formed by grouping adjacent instructions in a sequence which are capable of parallel execution.
Relevant control information in the form of compounding tags is added to the instruction stream to indicate where a compound instruction starts, as well as to indicate the number of instructions which are incorporated into a compound instruction. i~<elatedly, when used herein, the term "compounding"
refers to the grouping of instructions contained in a sequence of instructions, the grouping being for the purpose of concurrent or parallel execution of the grouped instructions. At minimum, compounding is satisfied by "pairing" of two instructions for simultaneous execution. Preferably, compounded instructions are unaltered from the forms they have when presented for scalar execution.

In a digital computer system which includes a means for executing a plurality of instructions in parallel, a particularly advantageous embodiment of the invention of this ai~plication is based upon a memory architecture which provides for compounding of instructions prior to their issue and execution. This memory structure provides instructions to the CPU (central processing unit) of a computer. Typically, a hierarchical memory structure includes a high-speed cache storage containing currently accessed instructions, a medium speed main memory connected to the cache, and a low-speed, high-capacity auxiliary storage. Typically, the cache and main storage (referred to collectively as "real storage") contain instructions which can be directly referenced for execution. Access to instructions in the auxiliary storage is had through an Input/output (1/0) adaptor connected between the main memory and the auxiliary storage.

An in-memory preprocessor for a SCISM architecture is a system in which an instruction compounding mechanlsm in real storage produces compounding tag information for a sequence of scalar instructions, the J:~9_9()_~)4() 7 205~941 compounding tag information indicaSing instructions of the sequence which may be executed in parallen The instruction compounding unit produces the cornpounding tag information as a page of instructions is being prefetched and stored in the main memory. Although the particular embodiment of that patent application teaches compounding of up to two instructions, it is contemplated that up to N instructions may be compounded for concurrent execution in a scalar computer.

A compounding preprocessor for a cache in a hierarchical memory is a system in which the instruction compounding unit is located between the main memory and cache of a hierarchical storage organi~ation. The instruction compounding Ul1it produces compounding tag information for the instructions in a line of instructions being fetched into the cache from the main memory.

Instructions in a cache have accompanying compounding tags to which a plurality of parallel execution units respond by executing one or more of the instructions in a group of up to N instructions in parallel. In these related systems, there is proposed in the context of a computer system capable of concurrently executing up to N instructions in a sequence of scalar instructions, the sequence including compounding tags which accompany the set of ordered scalar instructions and which are activated to indicate the instructions which are to be concurrently executed. There is a mechanism for managing the compounding tags of scalar instructions which are stored in the real storage of the computer system, and it includes a merging unit connected to the real memory for merging a modified instruction from the real memory with non-modified instructions in the real memory. A tag reduction unit connected to the merglng unit and to the real memory deactivates the compounding tags of the modified Instruction and N-1 instructions in the real mernory with which the modifled instruction could be compounded.

~ ~9-9~)-04() - x - 2 0 ~ 3 9 4 1 However, in all of ~he~;e developments there is an unfulfilled need to determine in the process`lng of a set of instructlons or program to be executed by a computer exactly which instructions may be combined into compound instructions, and in what manner there shall be accomplished the appending of cor,trol information to the instructions in the program to be executed .

Overview Referring to FIGURE 1 of the drawings, there is shown a representative embodlment of a portion of a digital computer system for a digital data processing system constructed in accordance with the present invention.
This computer system is capable of executing two or more instructions in parallel. In order to support the concurrent execution of a group of two or more instructions, the computer system includes a plurality of functional units which operate in parallel in a concurrent manner; each on its own, is capable of processing one or more types of machine-level instructions. The system Includes the capabllity of compounding Instructlons for parallel or concurrent executlon. In this regard, "compounding" refers to the grouping of a plurality of instructions in a sequence of scalar instructions, wherein the size of the grouping is scalable from 1 to N. The sequence of scalar instructions could, for example, be drawn from an existlng set of scalar instructions such as that used by the IE~M~ System/S70 products. (IBM is a registered trade mark of International Business Machines Corporation.) However, it is understood that compounding is intended to facilitate the parallel issue and execution of instructions in all computer architectures that can be adapted to process multlple instructions per cycle.

When an instruction stream is compounded, adlacent scalar Instructions are selectively grouped for the purpose of concurrent or parallel execution.
In general, an instruction compoundlng facllity will look for classes of instructions that may be executed in parallel. When compatible sequences of instructions are found, a compound instruction is created.

I;~9-9n-n4() (~ 20~3941 Instructi~)n Compounding anci Execution As is generally shown in FIGURE 1, an instruction compounding unit 20 takes a stream of binary scalar Instructions 21 and selectively groups some of the adjacent scalar instructions to form encoded compound instructions. A
resulting compounded instruction stream 22, therefore, provides scalar instructions to be executecl singly or in compound instructions formed by groups of scalar instructions to be exeGuted in parallel. When a scalar instruction is presented to an instructlon processing unit 24, it is routed to the appropriate one of a plurality of execution units for serial execution.
When a compound instruction is presented to the instruction processing unit 24, its scalar components are each routed to an appropriate execution unit for simultaneous parallel e~ecution. Typical functional units include, but are not limited to, an arithmetic logic unit (ALU) 26, 28 a floating point arithmetic unit (FP) 30 and a storage address generation unit (AU) 32.

Referring now to FIGURE 2, compounding can be implemented in a uni-processor environment where each functional unit executes a scalar Instruction !S) or, alternatively a compound instruction (CS). As shown in the drawing, an instruction stream 33 containing a sequence of scalar and compounded scalar instructions has control bits or tags (T) associated with each compound instruction. Thus, a first scalar instruction 34 could be executed sin~ly by functional unit A in cycle 1; a triplet compound instruction 36 identified by tag T3 could have its 3 compounded scalar instructions executed in parallel by functional units A, C, and D in cycle 2; another compound instruction 38 identified by tag T2 could have its pair of compounded scalar instructions executed In parallel by functional units A
and B in cycle 3; a second scalar instructlon 40 could be executed slngly by functional unit C in cycle ~i; a large group compound instruction 42 could have its 4 compounded scalar instructions executed in parallel by functional ' ~Is-sn-n4~) 1() 2 0 5 3 9 41 units A-D in cycle 5; and a third scalar instruction 44 could be executed singly by functional unit A in cycle 6.

One example of a c~mputer architecture which can be adapted for handling compound instructions is an IBM System/370 instruction level architecture in which multiple scalar instructions can be issued for execution in each machine cycle, in accordance with FIGURE 2. In this context, a machine cycle refers to a single pipeline stage required to execute a scalar instruction.

Generally, it is useful to provide for compounding at a time prior to instruction issue 50 that the process carl be done once for an instruction or instructions that may be executed many times. It has been proposed to locate the instruction compounding functions in the real memory of a computer system in order to implement compounding in hardware, affer compile time, yet prior to instruction issue. Such compounding is considered to be a preferred alternative to other alternatives described herein and referred to as "in-memory compounding".

Generally, in-memory compounding is illustrated in FIGUF<E 3. In FIGURE

3, a hierarchical memory organization includes an l/O adaptor 40 which interfaces with auxiliary storage devices and with a computer real memory.
The real memory of the organization includes a medium speed, relatively high capacity main memory 46 and a high-speed, relatively low capacity instruction cache 48. (The main memory and cache collectively are referred to herein as "real memory", "real storage", or, simply "memory".) A stream of instructions is brought in from auxiliary storage devices by way of the l/O
adaptor 40, and stored In biocks called "pages" in the main memory 46. Sets of conti~uous instructions called "lines" are moved from the main memory 46 to the instruction cache 48 where they are available for hlgh-speed relerence for processing by the instruction fetch and Issue unit 50. Instructlons which are fetched from the cache are issued, decoded at 52, and passed to the functional units 56, 58, ..., 60 for execution.

~ ()~()4() Il~ 23~3941 During execution, when`reference is made to an instruction which is in the program, the instruction's address is provided to a cache management unit 62 which uses the address to fetch one or more instructions, including the addressed instruction, from the instruction cache 48 into the queue in the unit 50. If the addressed instruction is in the cache, a cache "hit" occurs.
Otherwise, a cache "miss' occurs. A cache miss will cause the cache management unit 62 to sen~i the line address of the requested instruction to a group of storage management functions 64. These functions can include, for example, real storage management functions which use the line address provided by the cache management unit 62 to determine whether the page containing the addressed line is in the main memory 46. If the page is in main memory, the real storage management will use the line address to transfer a line containing the missing instruction from the main memory 46 to the instruction cache 48. If the line containing the requested instruction is not in the main memory, the operating system will activate another storage management function, providing it with the identification of the page containing the needed line. Such a storage management function will send to the 1/0 adaptor 40 an address identlfylng the page contalning the line. The 1/0 adaptor 40 will bring the page from auxiliary storage and provide it to the main memory 46. To make room for the fetched page, the storage management function selects a page in the main memory 46 to be replaced by the fetched page. In ~CISM architecture, it is contemplated that the replaced page is returned to auxiliary storage through 1he 1/0 adaptor without compounding tag information. In this manner, those instructions most likely to be immediately required during execution of an instruction sequence are adjacent to the functional units in the instruction cache 48. The hierarchical memory organlzation provides the capabillty for fast retrleval of Instructions that are require!d but not in the cache.

In the context of SCISM architecture, in-memory instruction compounding can be provided by an instruction compounding unit 70 which is located functionally between the 1/0 adaptor 40 and the main memory 46 so that ~Ns-90-()4() - 12 2 0 ~ 3 9 4 1 compoundin~ of the scalar instruction stream can take place at the input to, or in, the maln memory 46~ In this location, instructlons can be compounded during an ongoing page fetch.

Alternatively, the instruction compounding unit can occupy the position 72 between the main memory 46 and the instruction cache 48 and compound instructions are formed line-by-line as they are fetched into the instruction cache 48, as may be considered a preferred embodiment.

Example of Compounding The particular technique for compounding is a matter of design choice.
However, for purposes of illustration, one technique for creating compound instructions formed from adjacent scalar instructions can be stated. By way of example, instructions may occupy 6 bytes (3 half words), 4 bytes (2 half words), or 2 bytes (1 half word) of text. For this exampie, the rule for compounding a set of instructions which includes variable instruction lengths provides that all instructions which are 2 bytes or 4 bytes long are compoundable with each other. That Is, a 2 byte instruction is capable of parallel execution in this particular example with another 2 byte or another 4 byte instruction and a 4 byte instruction is capable of parallel execution with another 2 byte or another 4 byte instruction. The rule further provides that all instructions which are 6 bytes long are not compoundable. Thus, a 6 byte instruction is only capable of execution singly by itself. Of course, compounding is not limited to these exemplary rules, but can embrace a plurality of rules which define the criteria for parallel execution of existing instructions in a specific configuration for a given computer architecture.

The instruction set used for this example is taken from the Systeml370 archltecture. By examining the OP code for each Instruction, the length of each Instruction can be de~ermined from an instructlon length code (ILC) in the op code. The Instruction's type Is further defined in other op code bits.
Once the type and length ot the instruction is determined, a compounding tag 1 ~9-9()-()4() 1~l 2 ~ 5 3 9 4 1 containing tag bits is then generated for that specific instruction to denote whether it is to be comp~unded with one or more other instructions for par~llel execution, or to be ~xecuted singly by Itself.

In the tag format of th5s example twhich is not limiting), if 2 adjacent instructions can be compounded, the tag bits, which are generated in memory, are "1" for the first compounded instruction and "zero" for the second compounded instruction. However, if the first and second instructions cannot be compounded, in this first tag format the tag bit for the first instruction is "zero" and the second and third instructions are then considered for compounding. Once an instruction byte stream has been processed in accordance with the chosen compounding technique and the compounding bits encoded for various scalar instructions, more optimum results for achieving parallal execution may be obtained by using a bigger window for looking at larger groups of instructions and then picking the best combination of N instructions for compounding.

Compounding Programs However, taking the above by way of example of the problem solved here, it may be considered that we have generally a need to provide the system and process illustrated generally by FIGURE 4-A. Existing programs written in existing high level languages, as described with reference 10 FIGURE 5, or existing assembly languagé programs to be processed, as described with reference to FIGURE 6, need to be processed, and we have provided a system which has the capability to identify sequences of instructions which can be executed as a single compound instruction in a computer deslgned to execute compound instructlons.

Turning now to FIGUR~ 4-A, it will be seen that there is illustrated pictorially the sequence ir~ which a program is provided as an input to a compounding facility that produces a compound Instruction program based on a set of rules which reflect both the system and hardware archltecture.

I-Ng-90-040 14- 20~3941 These rules are hereafter referred to as compounding rules. The programproduceci by the compou`~$~ing facility can then be executed directly by a compound instruction execution engine generally illustrated by FIGURE 7.

However, FIGURE 5 shows a typical path taken by a program from higher level source code to actual execution, and may also be considered as one of the possible organizations suggested by FIGURE 4-A. An alternative related to assembly level programs is discussed with respect to FIGURE 6.

As will have been appreciated, referring to FIGURE 5, there are many possible locations in a computer system where compounding may occur, both in software and in hardware. Each has unique advantages and disadvantages. As shown in FIGURE 5, there are various stages that a program typically takes from source code to actual execution. During the compilation phase, a sour~e program is translated into machlne code and stored on a disk 46. During the execution phase the program is read from the disk 46 and loaded Into a main memory 48 of a partTcular computer system configuration 50 where the Instructions are executed by appropriate Instructlon processlng units 52, 54, 56. Compoundlng could take place anywhere along thls path. In general as the compounding facility is located closer to an instruction processing unit (IPU), the time constraints become more strlngent. As the compounding facility is located further from the IPU, more instructions can be examined In a large sized instruction stream window to determine the best grouping for compounding for increasing execution performance.

Compoundillg an Assembly-language Program The flow diagram of FIGURE 6 shows the generation of a compound instructlon set program from an assembly language program in accordance with a set of cust~mized compounding rules 58 which reflect both the system and hardware architecture. The assembly language program is provlded as an input to a soHware compounding facility 59 which parses the program In ~N9-90-()40 - IS - 2 3 5 3 9 4 ~

ml-, m2-, ... mn-instruction ~loc~;s (m = 1, 2, ...) and produces the compound instructlon program. Suc'c`'esslve blocks of Instructlons are analyzed by the compounding facility 59. Blocks are numbered from 1 to n. Member instructions of block n are indicated by In, where m ranges from 1 up to the number of instructions in th~ block. The number of instructions in each block 60, 62, 64 in the byte stream which contains the group of instructions considered together for compounding depends on the design of the compounding facility and may vary from block to block.

As shown in FIGURE 6, this particular compounding facility is designed to consider two-way compounding for "m" number of instructions in each block.
The primary first step is to consider if the first and second instructions constitute a compoundable pair, and then if the second and third constitute a compoundable pair, and then if the third and fourth constitute a compoundable pair, all the way to the end of the block. Once the various possible compoundable pairs C1-C~ have been identified, the compounding facility can select the preferred of compounded instructions and use flags or identifier bits to identify the optimum grouping of compound instructions.

If there is no optimum ~rouping, all of the compoundable adjacent scalar instructions can be identified so that a branch to a target located amongst various compoun~ instructions can exploit any of the compounded pair which are encountered. Where multiple compounding units are available, multiple successive blocks in the instruction stream could be compounded at the same time.

Executing a Compounded Program The flow diagram of FIGURE 7 shows the execution of a compound instruction set program which has been generated by a hardware preprocessor 66 or a soffware preprocessor 67. A byte stream having compound instructions flows into a compound instruction (Cl) cache 68 that serves as a storage buffer providing fast access to compound Instructlons.

I N9-9()-()4() - I(i - 2 ~ ~ 3 9 41 Cl issue logic 69 fetches cornpound instructions from the Cl Cache and issues their individual compounded instructions to the appropriate functional units for parallel execution.

It is to be emphasized that instruction execution units (Cl EU) 71 such as ALU's in a compnund instruction computer system are capable of executing either scalar instructions one at a time by themselves or alternatively compounded scalar instructions in parallel with other compounded scalar instructions. Also, such parallel execution can be done in different types of execution units such as ALU's, floating point (FP) units 73, storage address-generation units (AU) 7~ or in a plurality of the same type of units (FP1, FP2, etc) in accordance with the computer architecture and the specific computer system configuration.

Control Information In Compounding . .

In the first tag format discussed in the aforementioned e)tample, compounding information is added to the instruction stream as one bit for every two bytes of text (instructions and data~.

In general, a tag containing controi information can be added to each instruction in the comi~ounded byte stream -- that is, to each non-compounded scalar instruction as well as to each compounded scalar instruction included in a pair, triplet, or larger compounded group. As used herein, identifier bits refers to that part of the tag used specifically to identify and differentiate those compounded scalar instructions forming a compounded group from the r~malning non-compounded scalar Instructions whlch remain in the compound instruction prograrn and that when fetched are executed slngly.

Where more than two sc:alar instructions can be grouped together to form a compound instruction, additional identifier bits may be advantageous. The minimum number of identifier bits needed to indicate the specific number of 20~39~1 l~Ns-9o-n4n - 17-scalar instructions actually compounded is the logarithm to the base 2 (rounded up to the nearest whole number) of the maximum number of scalar instructions that can be grouped to form a compound Instruction. For example, if the maximum is two, then one Identifier bit is needed for each compound instruction. If the maximum is three or four, then two identifier bits are needed for each compound instruction. If thz maximum is five, six, seven or eight, then three identifier bits are needed for each compound instruction.

A second tag format, implementing this encoding scheme, is shown below in Table 1:

~: N9-~()-040 1~ - 2 3 ~ 3 9 ~ 1 `~;
Table 1 identifier Total #
Bits Encoded meaning Compounded 00 This instruction is not none compounded with its following instruction 01 This instruction is two compounded with its one following instruction This instruction i5 three compounded with its two following instructions 11 This Instruction Is four compounded with its three following instructions Assuming instruction ali~nment is such that each halfword of text needs a tas, it can be appreciated that the IPU ignores all but the tag for the first instruction when an instruction stream is fetched for execution. In other words, a half word of fetched text is examined to determine if it begins a compound instruction by checking its identifier bits. In accordance with Table 1, if it is not the beginning of a compound Instructlon, its Identifler blts are zero. If the half word is the beginnln~ of a compound instruction containin~ two scalar instructions, the identifier bits are "1" for the first instruction and "0" for the second instruction. If the half word is the beginning of a compound instruction containing three scalar instructions, the Identifier bits are "2" for the first instructlon and "1" for the second Instructlon 1 N9-9()-o~() - 11 2~3941 and O for the third instrul.:tion. In other words, the identifier bits for each half word identify whether or not this particular half word is the beginning of a compound instruction while at the same time indicating the number of instructions which make up the compounded group.

Thi; method of encoding compound instructions assumes that if three instructions are compounded to form a triplet group, the second and third instructions are also compounded to form a pair group. In other words, if a branch to the second instruction in a triplet group occurs, the identifier bit 1for the second instruction hldicates that the second and third instruction will execute as a compounded pair in parallel, even though the first instruction in the triplet group was not executed.

It will be apparent to those skilled in the art that the present invention requlres an instruction stream to be compounded only once for a particular computer system configuration, and thereafter any fetch of compounded instructions will also cause a fetch of the identifier bits associated therewith.
This avoids the need for the ineflicient last-minute determination and selection of certain scalar instructions for parallel execution that repeatedly occurs every time the same or different instructions are fetched for execution in the so-called super scalar machlne.

Compounding Without Reference Points Between Instructions It is straightforward to create compound instructions from an instruction stream if reference points are known that indicate where instructions begin.
As used herein, a reference point means knowledge of which byte of text, which may contain instructions and/or data, is the first byte of an Instruction.This knowledge couid be obtained by some marking fleld or other Indicator whlch provides information about the location of instruction boundarles. In many computer systems such a reference point is expressly known only by the compiler at compile time and only by the CPU when instructlons are fetched. When compounding is done after compile time, a compiler could n~g-~()-n4n -2n 2~39~1 indicate with ~ags which bytes contain the first byte of an instruction and which contain data. Thi~` extra information results in a more efficient compounding facility since exact instruction locations are known. Of course, the compiler would differentiate between instructions and data in other ways in order to provide the cornpounding facility with specific information indicating instruction boundaries.

In a system with all 4-byte instructions aligned on a four byte boundary, one tag is associated with each four bytes of text. Similarly, if instructions can be aligned arbitrarily, a tag is needed for every byte of text. However, certain computer architectures allow instructions to be of variable length, and may further allow data anct instructions to be intermixed, thus complicating the compounding process. Of course, at execution time, instruction boundaries must be known to allow proper execution. But since compounding is preferably done a sufficient time prior to instruction execution, a technique is needed to compound instructions without knowledge of where instructions start and without knowledge of which bytes are data. This technique needs to be appllcable to all of the accepted types of architectures, including the RISC (Reduced Instruction Set Computers) architectures in which instructions are usually a constant length and are not intermixed with data. In connection with our invention, accomplishment of these tasks, which are genf!~rally described with reference to FIGURE 4-A, is achieved with the assistance of a compound instruction execution engine in an alternative environment.

Organization of Preferred System With Compounding Facility Generally the preferred operating envlronment may be represented by the operational environment shown in FIGURE 4-B. While the compounding facllity may be a software entity, the preferred embodiment of the compounding facility may ~e implemented by an instruction compounding unit. Referring to FIGURE 4-B of the drawings, there is shown a representative embodiment of a portion of a digital computer system or 1 ~19-9()-()4() -21 - 2~3~41 digital data processing sys~em constructed in accordance with the present invention with a cache m~naç~ement unit 144. This computer system is capable of processing two or more instructions in parallel. It includes a first storage mechanism for storing instructions and data to be processed in the form of a series of base instructions for a scalar machine. This storage mechanism is identified as higher-level storage 136. This storaye (also main memory ) is a large capacity lower speed storage mechanism and may be, for example, a large capacity system storage unit or the lower portion of a comprehensive hierarchical storage system.

The computer system of FIGURE 4-B also includes an Instruction compounding facility or mechanism for receiving instructions from the higher level storage 1.36 and associating with these instructions compound information in the form of tags which indicate which of these instructions may be processed in paralle with one another. A suitable instruction compounding unit is represented by the instruction compounding unit 137.
This instruction compounding unit 137 analyzes the incoming instructions for determining which ones may be processed in parallel. Furthermore, instruction compounding unit 137 produces for these analyzed instructions tag bits which indicate which irlstructions may be processed in parallel with one another and which o~es may not be processed in parallel with one another but must be processed singly.

The FIGURE 4-B system further includes a second storage mechanism coupled to the instruction cnmpounding unit 137 for receiving and storing the analyzed instructions and their associated tag fields so that these stored compounded instructions rs7ay be fetched. This second or further storage mechanism is represented by compound instruction cache 138. The cache 138 is a smaller capacity higher speed storage mechanism of the kind commonly used for improving the performance rate of a computer system by reducing the frequency of having to access the lower speed storage mechanism 136.

I,N9-9()-04() - 22 - 2 0 ~ 3 9 ~1 The FIGURE 4-B system further includes a plurali~y of functional instruction processing unit~î 139, 140, 141, et cetera. These functional units 139-141 operate in parallel with one another in a concurrent manner and each, on its own, is capable of processing one or more types of machine-ievel instructions. Examples of functional units which may be used are: a general purpose arithmetic and logic unit (ALU), an address generation type ALU, a data dependency collapsing ALU, a branch instruction processlng unit, a data shifter unit, a floating point processing unit, and so forth. A given computer system may include two or more or some of the possible functional units.
For exampie, a given computer system may include two or more general purpose ALUs. Also, no given computer system need include each and every one of these different types of functional units. The particular confi~uration will depend on the nature of the particular computer system being considered.

The computer system of FIGURE 4-B also includes an instruction fetch and issue mechanism coupled to compound instruction cache 138 for supplying adjacent instructions stored therein to different ones of the functional instruction processing units 139-141 when the instruction tag bits indicate that they may be processed in parallel. This mechanism also provides single instructions to individual functional units when their tag bits indicate parallel execution is not possible and they must be processed singly.
This mechanism is represented by instruction fetch and issue unit 142. Fetch and issue unit 142 fetches instructions form the cache 138 and examines the tag bits and instruction operation code (opcode) fields, performing a decode function, and based upon such examinations and other such pipeline control signals as may be relevant, sends the instruction under consideration to the appropriate ones of the functional units 138- 141.

In the context of SCISM architecture, in-cache instruction compounding is provided by the instructlon compounding unit 137 so that compoundlng of each cache line can take place at the input to the compound instruction cache 138. Thus, as each cache line Is fetched from the main memory 136 into the ~ N9-9()-041) - 2~ - ~ 0 5 3 ~ ~ ~

cache 138, the line is analyzed for compounding in the unit 137 and passed, with compounding inforr~ation tag bits, for storage in the compound instruction cache 138.

Prior to caching, the line is compounded in the instruction compounding unlt 137 which generates a set of tag bits. These tag bits may be appended directly to the instructions with which they are associated. Or they may be provided in parallel with the instructions themselves. In any case, the bits are provided for storage together with their line of instructions in the cache 138. As needed, the compounded instruction in the cache 138 is fetched together with its tag bit information by the instruction and issue unit 142.
Instruction fetch and issue unit 142 and compound instruction cache 138 are designed such that a maximum length compound instruction (as defined by the system) may be fetched from compound instruction cache 138 and issued to the functional units 139~ 140, 141, and so on. As instructions are received by the fetch and issue unit 142, their tag bits are examined to determine by decoding examination whether they may be processed in parailel and the opcode fields are examined to determine which of the available functlonal unlts is most appropriate for their processing. If the tag bits indicate that two or more of the instructions are suitable for processing in parallel, then they are sent to the appropriate ones in the functional units in accordance with the codings of their opcode Fields. Such instructions are then processed concurrently with one another by their respective functional units.

When an instruction is encountered that is not suitable for parallel processing, it is sent to the appropriate functional unit as determined by its opcode and it is thereupon processed alone and singly by itself in the selected functional unit.

In the Ideal case, where plural instructions are always bein0 processed in parallel, the instruction execution rate of the computer system would be N
times as great as for the case where instructions are executed one at a time, 20~3941 rl~N9-s()-o4() - 2~-with N being the numbel of instructions in the groups which are being processed in parallel.

Further Control Information (Tag) in Compounding As stated previously, the control field or tag contains information delimiting compound instruction boundarles, but may contain as much addltional information as IS deemed eflicacious for a particular implementation.

For example, in a third tag format, a control field (tag) might be defined as an 8-bit field, to t, t2 t3 t4 tS t6 t7 with the bits defined as follows Bit Function to If 1, this instruction marks the beginning of a compound instruction.
t, If 1, ther. execute two compound instructions in parallel t2 If 1, then this compound instruction has more than one execution cycle.
t3 If 1, then suspend pipelining.
f4 If instruction is a branch, then if this bit is 1, the branch is predicted to be taken.
f5 If 1, then this instructlon has a storage interlock from a previous compound instructlon.
t6 If 1, then enable dynamic instruction issue.
t7 If 1, then this instruction uses an ALU.

~Ns-sn-040 - 2~ - 2 ~ 5 3 9 ~ ~

Classifyin~ Instructions for Con~pounding In general, the compounding facility will look for classes of instructions that may be executed in parallel, and ensure that no interlocks between members of a compound instruction exist that cannot be handled by the hardware. When compatible sequences of instructions are found, a compound instruction is created. For example, the System/370 architecture might be partitioned into the following classes:

1. RR-format loads, logicals, arithmetics, compares o LCR -- Load Complement o LPR -- Load Positive o LNR -- Load Negative o LR -- Load Register o LTR -- Load and Test o NR -- AND
o OR -- OR
o XR -- Exclusive OR
o AR -- Add o SR -- Subtract o ALR -- Add Logical o SLR -- Subtract Logical o CLR -- Compare Logical o CR -- Compare 1 ~9-9~) ~wn 2h - 2 V 5 3 9 4 1 2. RS-format shifts (no storage access) .;
o SRL -- Shift Right Logicai o SLL -- Shift Left Logicai o SRA -- Shift Right Arithmetic o SLA -- Shift Leff Arithmetic o SRDL -- Shift Right Logical o SLDL -- Shift Left Logical o SRDA -- Shift Right Arithmetic o SLDA -- Shift Left Arithmetic 3. Branches -- on count and index o BCT-- Branch on Count (RX-format~
o BCTR -- Branch on Count (RR-format) o BXH -- Branch on Index High (RS-format) o BXLE -- Branch on Index Low (RS-format) 4. Branches -- on condition o BC -- Branch on Condition (RX-format) o BCR -- Branch on Condition (RR-format) 5. Branches -- and link o BAL -- Branch and Link (RX-format) o BALR -- Branch and l ink (RR-format) o BAS -- Brar,ch and Save (RX-format) o BASR -- Branch and Save (RR-format) 2~53~41 1 ~9-9()-()4() - 27 -6. Stores o STCM -- Store Characters Under Mask (0-4-b~/te store, RS-format) o M1VI -- Move Immediate (1 byte, Sl-format) o ST -- Store (4 bytes) o STC -- Store Character (1 byte) o STH -- Store Half (2 bytes) 7. Loads o LH -- Load Half (2 bytes) o L -- Load (4 bytes) 8. LA-- Load Address 9. RX-format arithmetics, I,:~gicals, inserts, compares o A -- Add o AH -- Add Half o AL -- Add Logical o N--AND
o O -- OR
o S -- Subtract o SH -- Subtract Half o SL -- Subtract Logical o X-- Exclusive OR
o iC -- Insert Character o ICM -- Insert Characters Under Mask (0- to 4-byte fetch) o C -- Compare o CH -- Compare Half o CL -- Compare Loglcal I,N9-s()-04() - 2x - 2 ~ 5 3 ~ 41 o CLI -- Compare Logical Ir,lmediate o CLM -- Compare Logi.^al Character Under Mask 10. TM -- Test Under Mask The rest of the System/370 instructions are not considered to be compounded for execution in ~his invention. This does not preclude them from being compounded on a future compound instruction execution engine.

One of the most common sequences in programs is to execute an instruction of the TM or RX-forrnat compare class, the result of which Is used to control the execution of a BRANC~i-on-condition type instruction which immediately follows. Performance can be improved by executing the COMPARE and the BRANC:H instructions in parallel, and this is sometimes done dynamically in high performance instruction processors. Some difficulty lies in quickly identifying all the various members of the COMPARE
class of instructions and all the members of the BRANCH class of instructions in a typical architecture during the instruction decoding process. This difficulty is avoided by the invention, because the analysis of all the members of the classes are accompl!shed ahead of time and a compound instruction which is 0uaranteed to work. is c,reated.

Many classes of instructions may be execu1ed in parallel, depending on how the hardware is designed. In addition to the COMPARE and BRANCH
compound instruction desctibed above, other compound instructions can be envisioned, such as LOADS and RR-~ormat instructions, BRANCH and LOAD
ADDRESS, etc. A compound instruction may even include multiple instructions from the same class, for example, RR-format arithmetlc, if the processor has the necessary execution units.

Optimizing the Compounding of Instructions 2~3941 I N9-90-()4() ~() In any realizable instruction processor, there will be an upper limit to the number of instructions that can comprise a compound instruction. This upper limit, m, must be specified to the compounding facility which is creating the executable instructions by generating compound instructions, so that it can generate compound instructions no longer than the maximum capability o the underlying hardware. Note that m is strictly a consequence of the hardware Implementation; i-l does not limit the scope of instructions that may be analyzed fGr compounding in a given code sequence by the software. In general, the broader the scope of the analysis, the greater the parallelism achieved will be, as more advantageous compoundings are recognized by the compounding facility. To illustrate, consider the sequence:

X1 ;any compoundable instruction X2 ;any co~poundable instruction LOAD R1,(X) ;load Rl from memory location X
ADD R3,R1 ;R3 = R3 ~ R1 SUB Rl,R2 ;R1 = R1 - R2 COMP R1,R3 ;compare R1 with R3 X3 ;any compoundable instruction X4 ;any compoundable instruction.

If the hardware-imposed upper limit on compoundlng is m=2, then there are a number of ways to compound this sequence of instructions depending on the scope of the compuunding facility. If the scope were equal to four, then the compounding fac31ity would produce the pairings <- X1 > <X2 LOAD> <ADD SUB> <COMP X3> cx4 ->, relieving completely the hazards between the LOAD and the ADD and the SUB and the COMP. On the other hand, a superscalar machine with m=2, which pairs instructions in its instruction issue logic on strictly a first-in-first-ou1 basis, would produce the pairlngs <X1 X2~ <LOAD ADD> <SUB COMP> <X3 X4>, incurring the full penalty of the interlocking instructlons. Unfortunately, the LOAD and ADD cannot be executed In parallel because ADD requires the result of the load. Similarly, the SUB and COMP cannot execute In parallel. Thus no performance improvement is gained.

I Ns-s()-()4() ~(~ 2 ~ ~ 3 9 41 Branching and C~mpound Instructions The systems describe~ provide a method of generating a compound instruction program and furthe, suggest a variety of methods for maintaining this association of compounding information (tags) with instruction text in a relatlvely permanent manner. But since there can be no guarantee that a branch into the middle of a compound instruction wlll not occur, what is not thus far apparent is how this compounding information can be maintained, and further, and more importantly, how correct program behavior can be maintained, in the presence of branches to arbitrary locations. This problem is in fact solved by instruction fetch and issue unit 142 of FIGURE 4-B, as willbe explained. First, assume a fourth definition of the compounding tag, T, such that T= 1 if the associated instruction either begins a compound instruction or is an instruction to be executed singly. Further, T= O for member instructions of a compound instruction other than the first. Then, assuming that instruction fetch and issue unit 142 can fetch a maximum-length compouncl instruction (as stated above), when fetching instructions in response to a taken branch, it always fetches a block of text equai to the maximum lenyth compound instruction beginning at exactly the branch target address, and then executes all Instructions fetched with T=O as a compound instruction, up to the point in the fetched text where an instruction is encountered having T=1, indicating the beginning of the next compound instruction. If the T bit of the branch target is 1, then it is the beginning of a compound instruction and may be executed directly. FIGURE
8 illustrates this situation. The notation used in FIGURE 8 is Im~ where n is the compound instruction number, and m is the number of the instruction within compound instruction n. The allowable range for m is from 1 to the maximum length of a compound instruction, which, in this example, is assumed to be three. Instruction 2 of compound Instructlon 1 is a branch Instruction, that, ill this example, will have two posslble target paths, "a" and "b". The "a" path branches to the middle (12) of compound Instruction j, while the "b" path branches to the first instruction of compound instruction j. If thebranch were to follow path "a", the hardware fetches the maximum length 2~3941 I N9-90-()40 - ~1 -compound instruction, that ïs, three instructions, then executes 1~ and /~ as a comps~und instruction Th~ remainder of the fetch, namely /,, is recogni~ed to be the beg,nning of either a new compound or single instruction (from its having T=1) and is held in reserve while the next block of text is is fetched for subsequent execution. If the branch instruction took the "b" path to the first instruction of compound instruction j, the hardware would again fetch a block of text equal in length to the maximum length compound instruction, yielding, in this case, a complete compound instruction, namely 1~ and /~.
Execution of that compound instruction proceeds directly.

This technique is equally applicable to other tag formats, such as the second tag format discussed above.

Note in FIGURE 8 that branch instruction B~ is compounded with the instructions following it, but since the branch is taken, the prefetched text followlng the branch, whether it is comprised of member units of 1he compound instruction containing the branch or subsequent single or compound instructions or in fact data ~in systems that allow data and instructions to intermix), cannot be allowed to update the machine state if correct program behavior is to be maintained. Further, fetching the target of a branch Instruction may result in the fetching of a compound instruction containing a second branch that too can be followed by text that can not be allowed to update the machine state should the second branch be taken.
This branch-to-branch sequence may continue ad infinitum. In both cases, the solution for maintaining correct program behavior is to prevent instructions that should not have been executed from updating the machine state. That is, the execution results of prefetched text sequentially following the taken branch must be nullified. Algorithmically, this may be stated as follows:
1. Begin the execution of the compound instruction, including any branch instruction within the cornpound instruction.

2as3s4l r ~-9()-()4() - .~2 -2. If the compound instruction does in fact contain a branch instruction, do not permit any instruct~ons following the branch, be they in the same compound instruction as the branch, or in subsequent compound or single instructions, to update the machine state.
3. If the branch is taken, nullify the execution results of any instructions sequentially following the taken branch. Do not, however, nullify the execution of the branch target.

While we have described our 7nventions in their preferred embodiment and alternative embodiments, various modifications may be made, both now and in the future, as those skilled in the art will appreciate upon the understanding of our disclosed inventions. Such modifications and future improvements are to be understood to be intended to be within the scope of the appended claims which are to be construed to protect the rights of the inventors who first conceived of these inventions.

Claims

1. A system for processing instructions with an instruction processor, comprising compounding decoding means for decoding a series of base instructions of a scalar machine and for generating a series of compound instructions with an instruction format text having appended control bits in the instruction format text enabling the execution of the compound instruction format text in said instruction processor;
fetch means for fetching said compound instructions;
decoding means for decoding fetched compound and single instructions:
execution means for executing compounding and single instructions;
and wherein said system provides a compound instruction program to said execution means which preserves intact the scalar execution of said base instructions of a scalar machine.

2. A system for processing instructions with an instruction processor according to claim 1, wherein said base instructions are program instructions originally constructed for a scalar machine which differs from said instruction processor, but which may be executed by said instruction processor by compilation or assembly of said compound instruction program.

3. A system for processing instructions with an instruction processor according to claim 1, wherein an original base instruction may be of varying length, and there is provided a compounding facility having a plurality of n compoundable units which enables execution by said instruction processor of compound instructions of n member instruction unit length, and wherein in each compound instruction is found a control field relevant to execution of said compound instruction.

4. A system for processing instructions with an instruction processor according to claim 3, wherein said control field contains function information with function bit information indicating the beginning of a compound instruction or the beginning of a scalar instruction.

5. A system for processing instructions with an instruction processor according to claim 3, a compound instruction includes delimiter bit information which set in one condition delimits a compound instruction and wherein the compound instruction contains field information which links the initial and other member units of the compound instruction.

6. A system for processing instructions with an instruction processor according to claim 5 wherein compound instructions and scalar instructions are intermixed in said compound instruction program, and said delimiter bit information indicates whether a member instruction unit of the compounded format text belongs to a previously initiated compound instruction or is the start initiator member unit of another instruction of the compounded format text which may be scalar or compounded in execution of the compounded instruction program.

7. A system for processing instructions with an instruction processor according to claim 1 including a pre-execution time compounding facility in which a program having instructions of a compare or branch class the result of which is related to another instruction member unit of the compounded instruction program which follows, and pre-execution time analysis of the various members of the compare class and all members of the branch class are identified during a pre-execution time instruction decoding by said compounding decoding means.

8. A system for processing instructions with an instruction processor according to claim 1 wherein said system is provided with architecture compounding rule means based on the architectural information of the instruction processor identified for execution of said compound instruction program, said compounding rule means providing a check that ensures that no interlocks between member instruction units of a compound instruction exist which cannot be handled by the instruction processor.

9. A system for processing instructions with an instruction processor according to claim 1 wherein said compound instruction format text includes control field information for a compound instruction, and wherein said control field information is selected from a group consisting of field information with the function that:
a) marks the beginning of a compound instruction, b) causes execution of two compound instructions in parallel c) indications that the compound instruction has more than one execution cycle d) suspends pipelining e) causes a branch predicted to be taken if the instruction is a branch and the field is set for taking a branch f) indicates whether of not the instruction has a storage interlock from a previous compound instruction g) enables dynamic instruction issue h) indicates whether or not the instruction uses an arithmetic logic unit (ALU).

10. A system for processing instructions with an instruction processor according to claim 9 wherein the control field which marks the beginning of a compound instruction functions as a delimiter for instructions, including instructions which are not compounded but remain scalar for execution.

11. A system for processing instructions with an instruction processor according to claim 1, wherein said compound instruction program is executed by a process which includes fetching up to the maximum length compound instruction permitted by the pre-compounding of base instructions and by beginning execution at a branch target address and executing all instructions where a beginning control field indicates said instructions are members of a compounded instruction.

12. A system for processing instructions with an instruction processor according to claim 1 further including means for checking compounding rules for the instruction processor based upon architectural information of the instruction processor to execute the instruction format text as a compound instruction program that insures that no interlocks between member instruction units of a compound instruction exist which cannot be executed by the instruction processor which is to execute the compound instruction program.

13. A system for processing instructions with an instruction processor according to claim 1 wherein said compound instruction program originates from a series of base instructions which is either originally in source or assembly language applicable to a machine which is different from said instruction processor which is to execute said compound instruction program, and said compound instruction program is recompiled or compounded for said instruction program with static optimization applied to the machine code to enhance performance of the compound Instruction program when executed by said instruction processor.

14. A system mechanism for processing instructions in a machine with branching in the middle of a compound instruction, comprising:
decoding means for decoding a compound instruction contained in fetched text and composed of member units for examining appended control bits in an instruction format of said compound instruction which are relevant to the execution of the compound instruction, and issuing means, responsive at least in part to the decoding means, for issuing the member units from a target address to the end of the compound instruction and maintaining the remainder of the text for potential execution after the execution of the compound instruction has been initiated.

15. A system mechanism for processing instructions in a machine with branching in the middle of a compound instruction, comprising:
decoding means for decoding a compound instruction composed of member instruction units for examining appended control bits in an instruction format of said compound instruction which are relevant to the execution of the compound instruction, and nullifying means for nullifying any execution of a member unit of a compound instruction upon occurrence of possible conditions which would affect the correctness of results of execution of the said member unit of the compound instruction or the correctness of a series of compound instructions coupled as a compound instruction program, said correctness being based upon the relevant interrelationship of member units of the compound instruction with other instructions of the series; and means for continuing the execution of the series of instructions as if the nullified execution had not occurred.

16. A system mechanism for processing instructions with branching in the middle of a compound instruction according to claim 15, wherein the compound instruction is a multimember compound instruction and one of said member units of the compound instruction is a branch followed by at least a second member unit, and wherein during execution if a branch is taken the results of the second member unit are nullified.

17. A system mechanism For processing instructions with branching in the middle of a compound instruction according to claim 14, wherein said compound instruction includes a member unit that is a branch instruction which is able to be executed in parallel with another member unit of the compound instruction, then said nullifying means are provided, if a branch is taken, to nullify the member unit(s) following the branch taken so that the effect of the subsequent instruction is as if said nullified member unit(s) werenot executed.

18. A system mechanism for processing instructions with branching in the middle of a compound instruction according to claim 15, wherein if a second member unit of a compound instruction after a branch appears to be an instruction, and such said second member unit could begin to be executed as if it were an instruction, the nullifying means nullifies the execution of the second member unit.

19. A system mechanism for processing instructions with branching in the middle of a compound instruction according to claim 14, where fetching means have been provided to fetch a maximal compound instruction and wherein said decoding means causes execution of component member units of a compound instruction to start at an arbitrary point in the compound instruction and execute to the end of the compound instruction.

20. A system mechanism for processing instructions with branching in the middle of a compound instruction according to claim 14, wherein said decoding means detects the end of a compound instruction and causes the issuing means to maintain the remainder of the instruction text for later execution.