Abstract:
A pipelined data processing unit includes an instruction sequencer and n functional units capable of executing n operations in parallel. The instruction sequencer includes a random access memory for storing very-long-instruction-words (VLIWs) used in operations involving the execution of two or more functional units in parallel. Each VLIW comprises a plurality of short-instruction-words (SIWs) where each SIW corresponds to a unique type of instruction associated with a unique functional unit. VLIWs are composed in the VLIW memory by loading and concatenating SIWs in each address, or entry. VLIWs are executed via the execute-VLIW (XV) instruction. The iVLIWs can be compressed at a VLIW memory address by use of a mask field contained within the XV 1  instruction which specifics which functional units are enabled, or disabled, during the execution of the VLIW. The mask can be changed each time the XV 1  instruction is executed, effectively modifying the VLIW every time it is executed. The VLIW memory (VIM) can be further partitioned into separate memories each associated with a function decode-and-execute unit. With a second execute VLIW instruction XV 2,  each functional unit&#39;s VIM can be independently addressed thereby removing duplicate SIWs within the functional unit&#39;s VIM. This provides a further optimization of the VLIW storage thereby allowing the use of smaller VLIW memories in cost sensitive applications.

Description:
RELATED APPLICATIONS 
     The present invention claims the benefit of U.S. Provisional Application Ser. No. 60/067,511 entitled “Method and Apparatus For Dynamically Modifying Instructions in a Very Long Instruction Word Processor” and filed Dec. 4, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to improvements in parallel processing, and more particularly to advantageous techniques for providing dynamic very long instruction word (VLIW) sub-instruction selection for execution time parallelism in an indirect VLIW processor. 
     BACKGROUND OF THE INVENTION 
     In a VLIW processor, a typical problem is that it is difficult to make effective use of the full capabilities of the fixed length VLIWs available in the hardware. In previous designs, this design problem led to a very porous VLIW memory containing many No Operation (NOP) instructions within the VLIWs. Some machines have attempted to encode the NOPs to more fully utilize the VLIW memory space. One motivation of such attempts was to make better use of the costly VLIW memory included in these earlier processors. The encoded NOPs were typically assigned to each specific VLIW with no reuse of the VLIW possible in different areas of the program. 
     There are other needs to be met by a VLIW parallel data processor. For example, it is desirable to pipeline operations in order to achieve a steady state flow of data for maximum throughput. Consider the case of matrix multiplication using a VLIW architecture with four short instruction words (SIWs) per VLIW. In the example of FIG. 1, a 4-element vector  2  and a 4×4 matrix  4  are multiplied. Given a processor with operands stored in a register file and VLIW execution units that operate on register file source data operands and deliver result data to the register file, it can be reasonably assumed that the vector elements are stored in data registers R 20 =a 0 , R 21 =a 1 , R 22 =a 2 , and R 23 =a 3 , and the 4×4 matrix  4  is stored in a processor accessible memory. FIG. 2 illustrates how the entire operation is handled in a typical prior art approach. Each row in table  10  represents a unique short instruction word (SIW) or VLIW instruction with the program flow beginning at the top of the table and proceeding time-wise down the page. The Load operation is an indexed load that incrementally addresses memory to fetch the data element listed and load it into the specified register R 0  or R 1 . The Add and Mpy instructions provide the function Rtarget=Rx Operation Ry, where Rtarget is the operand register closest to the function name and the source operands Rx and Ry arc the second and third register specified. Each unique VLIW memory address is identified with a number in the first column. The table  10  of FIG. 2 shows that a minimum of seven VLIWs, each stored in a unique VLIW memory address, and three unique SIWs, are required to achieve the desired results in the prior art. It is important to note that of the seven VLIWs, three VLIWs, namely numbers  1 ,  2 , and  7 , use only two SIWs per VLIW, the other four use three SIWs per VLIW. When a four instruction slot VLIW contains only two SIWs, the other two slots contain NOP instructions. When the four instruction slot VLIW contains three SIWs, the other slot contains a single NOP. With a five instruction slot VLIW as will be described in greater detail below, even poorer usage of the VLIW memory results using prior art techniques. In the vector matrix example, a five slot VLIW will use 7*5=35 VLIW memory locations with 17 NOPs assuming the fifth slot is not used for this matrix multiplication example. The prior art approach results in a very porous VLIW memory with numerous NOP instructions. 
     It is desirable to reduce the number of unique VLIW memory addresses to accomplish the same task since this makes more efficient use of the available hardware. It is also desirable to reduce duplicate instructions in the VLIW memory storage. This is an important consideration that allows a smaller VLIW memory to be designed into a processor thereby minimizing its cost. Further, if the same VLIW memory address could be shared by multiple sections of code and even multiple programs then the latency cost of loading the VLIW memories can be minimized, as compared to prior art approaches, and amortized over the multiple programs thereby improving overall performance. In addition, it is desirable to extend this concept into multiple Processing Elements (PEs) and to a controller Sequence Processor (SP) of a Single Instruction Multiple Data stream (SIMD) machine 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is preferably used in conjunction with the ManArray architecture various aspects of which are described in greater detail in U.S. patent application Ser. No. 08/885,310 filed Jun. 30, 1997, U.S. patent application Ser. No. 08/949,122 filed Oct. 10, 1997, U.S. patent application Ser. No. 09/169,255 filed Oct. 9, 1998, U.S. patent application Ser. No. 09/169,256 filed Oct. 9, 1998, U.S. patent application Ser. No. 09/169,072 filed Oct. 9, 1998, and U.S. patent application Ser. No. 09/187,539 filed Nov. 6, 1998, Provisional Application Ser. No. 60/068,021 entitled “Methods and Apparatus for Scalable Instruction Set Architecture” filed Dec. 18, 1997, Provisional Application Ser. No. 60/071,248 entitled “Methods and Apparatus to Dynamically Expand the Instruction Pipeline of a Very Long Instruction Word Processor” filed Jan. 12, 1998, Provisional Application Ser. No. 60/072,915 entitled “Methods and Apparatus to Support Conditional Execution in a VLIW-Based Array Processor with Subword Execution” filed Jan. 28, 1988, Provisional Application Ser. No. 60/077,766 entitled “Register File Indexing Methods and Apparatus for Providing Indirect Control of Register in a VLIW Processor” filed Mar. 12, 1998, Provisional Application Ser. No. 60/092,130 entitled “Methods and Apparatus for Instruction Addressing in Indirect VLIW Processors” filed Jul. 9, 1998, Provisional Application Ser. No. 60/103,712 entitled “Efficient Complex Multiplication and Fast Fourier Transform (FFT) Implementation on the ManArray” filed Oct. 9, 1998, and Provisional Application Ser. No. 60/106,867 entitled “Methods and Apparatus for Improved Motion Estimation for Video Encoding” filed Nov. 3, 1998, respectively, and incorporated herein in their entirety. 
     The present invention addresses the need to provide a compressed VLIW memory and the ability to reuse instruction components of VLIWs in a highly advantageous way. In one aspect, the present invention comprises a SIW fetch controller for reading instructions from the SIW memory (SIM), a VLIW memory (VIM) to store composed VLIWs at specified addresses, a VLIW controller for indirectly loading and reading instructions from the VIM, and instruction decode and execution units. VLIWs in the present invention are composed by loading and concatenating multiple SIWs in a VIM address prior to their execution. 
     In a SIMD machine, the SIW fetch controller resides in the SIMD array controller SP which dispatches the fetched 32-bit instructions to the array PEs. The SP and the PEs include a VIM, a VIM controller, and instruction and decode execution units. The concepts discussed in this disclosure apply to both the indirect VLIW (iVLIW) apparatus and mechanism located in the SP controller and each PE in a multiple PE array SIMD machine. 
     After at least one VLIW is loaded into VIM, it may be selected by an execute-VLIW (XV) instruction. There are two types of XV instructions described in this invention. The first one XV 1  provides sub-VLIW SIW selection across the slots at the same VIM address for execution time parallelism. The second XV 2  provides sub-VLIW SIW selection with independently selectable SIWs from the available SIWs within each of the slots VIM sections for execution time parallelism. The XV 1  instruction is described first with an example demonstrating the advantages of this approach. The XV 2  instruction description follows with an example demonstrating its inherent advantages. 
     The XV 1  instruction causes the stored VLIW to be read out indirectly based upon address information that is computed from a VIM base address register and an immediate Offset value that is present in the XV 1  instruction. The XV 1  instruction contains Mask-Enable-bits which select the instructions from the read-out VLIW that are to be scheduled for execution. In a preferred ManArray embodiment there are 8-bit Mask-Enable-bits, one bit per execution unit, supporting up to 8-SIWs in a single VLIW. For the first implementation, 5 SIWs are preferably used. 
     Due to the use of a VIM base register, Vb, unlimited VIM storage is possible. For each Vb base address, the XV 1  instruction preferably supports, in the first implementation, an 8-bit offset thereby allowing 256 VLIWs per Vb address. The preferred ManArray architecture specifies that up to 8 SIWs can be stored per VIM address and a minimum of eight Mask-Enable-bits, one per slot, are supported by the preferred embodiment. Also, because each VIM entry has a unique address, each VIM entry can be loaded, modified, executed, or disabled, independently. 
     With eight SIW slots available per VIM entry, up to 255 unique combinations of SIW types can be stored in each entry, where, for example, SIW instruction types can include Store, Load, Arithmetic Logic Unit (ALU), Multiply Accumulate Unit (MAU), and Data Select Unit (DSU) instruction types. Each combination represents a unique indirect VLIW (iVLIW) available for XV 1  execution. Furthermore, when invoking the execution of SIWs from a previously loaded VIM entry via the XV 1  containing the 8-bit mask, up to 255 unique iVLIW operations can be invoked from that VIM entry alone. 
     The XV 2  instruction provides the capability to remove duplicate instructions within groups of VLIWs within a slot specific section of the VIM. This capability provides optimum packing of instructions within the VIM thereby further optimizing its efficiency and minimizing its size for specific applications. 
     A more complete understanding of the present invention, as well as other features and advantages of the invention will be apparent from the following Detailed Description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the multiplication of a vector by a matrix; 
     FIG. 2 illustrates a prior art approach to the multiplication operation of FIG. 1; 
     FIG. 3 illustrates a ManArray 2×2 iVLIW processor showing the connections to a plurality of processing elements connected in an array topology for use in conjunction with the present invention; 
     FIG. 4A illustrates an iVLIW data path with a VIM in accordance with a presently preferred embodiment of the present invention; 
     FIG. 4B illustrates a presently preferred XV 1 , execute VLIW, instruction and syntax/operation details therefor; 
     FIG. 4C illustrates a presently preferred LV 2 , load/modify VLIW- 2 , instruction and syntax/operation details therefor; 
     FIG. 4D illustrates a presently preferred XV 2 , execute VLIW, instruction and syntax/operation details therefor; 
     FIG. 5 illustrates aspects of an iVLIW XV 1  pipeline with across slot compression utilized with an exemplary XV 1  instruction format in accordance with the present invention; 
     FIG. 6 illustrates the approach of the present invention applied to the multiplication operation of FIG. 1; 
     FIG. 7 illustrates dynamic cycle-by-cycle iVLIW instruction selection across instruction slots for execution time parallelism; 
     FIG. 7A illustrates program code using XV 1  instructions for the multiplication operation of FIG. 1; 
     FIG. 8 illustrates aspects of an iVLIW XV 2  pipeline with within slot compression utilizing an exemplary XV 2  instruction format in accordance with the present invention; 
     FIG. 9 illustrates dynamic cycle-by-cycle iVLIW instruction selection with within slot compression for execution time parallelism; and 
     FIG. 10 illustrates program code using XV 2  instructions for the multiplication operation of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     In a preferred embodiment of the present invention shown in FIG. 3, a ManArray 2×2 iVLIW Single Instruction Multiple Data stream (SIMD) processor  100  containing a controller sequence processor (SP) combined with a processing element- 0  (PE 0 ) SP/PE 0   101 , as covered in more detail in co-pending application Ser. No. 09/169,072 entitled “Methods and Apparatus for Dynamic Merging an Array Controller with an Array Processing Element”, and three additional PEs  151 ,  153 , and  155  are utilized to implement the dynamic iVLIW modification techniques of the present invention. The SP/PE 0   101  contains a fetch controller  103  to allow the fetching of SIWs from a 32-bit instruction memory  105 . The fetch controller  103  provides the typical functions needed in a programmable processor such as a program counter (PC), a branch capability, digital signal processing loop operations, support for interrupts, and also provides the instruction memory control which could include an instruction cache if needed by an application. In addition, the SIW I-Fetch controller  103  dispatches 32-bit SIWs to the other PEs in the system by way of a 32-bit instruction bus  102 . 
     In this exemplary system, common elements are used throughout to simplify the explanation. It will be recognized that further implementations are not limited to this restriction. For example, the execution units  131  in the combined SP/PE 0   101  can be separated into a set of execution units optimized for the control function, e.g. fixed point execution units, and the PE 0  as well as the other PEs can be optimized for a floating point application. For the purposes of this invention description, it is assumed that the execution units  131  are of the same type in the SP/PE 0  and the PEs. In a similar maimer SP/PE 0  and the other PEs use a five instruction slot iVLIW architecture which contains a VIM memory  109  and an instruction decode and VIM controller function unit  107  which receives instructions as dispatched from the SP/PE 0 &#39;s I-Fetch unit  103  and generates the VIM addresses-and-control signals  108  required to access the iVLIWs, identified by the letters SLAMD in block  109 , stored in the VIM. The loading of the iVLIWs is described in more detail in co-pending patent application Ser. No. 09/187,539 filed Nov. 6, 1998 and entitled “Methods and Apparatus for Efficient Synchronous MIMD Operations with iVLIW PE-to-PE Communications”. Also contained in the SP/PE 0  and the other PEs is a common PE configurable register file  127  which is described in more detail in co-pending patent application Ser. No. 09/169,255 filed Oct. 9, 1998 and entitled “Method and Apparatus for Dynamic Instruction Controlled Reconfiguration Register File with Extended Precision”. Due to the combined nature of the SP/PE 0   101 , the data memory interface controller  125  must handle the data processing needs of both the SP controller, with SP data in memory  121 , and PE 0 , with PE 0  data in memory  123 . The SP/PE 0  controller  125  also is the source of the data that is sent over the 32-bit broadcast data bus  126 . The other PEs,  151 ,  153 , and  155  contain common physical data memory units  123 ′,  123 ″, and  123 ′″ though the data stored in them is generally different as required by the local processing done on each PE. The interface to these PE data memories is also a common design in PEs  1 ,  2 , and  3  and indicated by PE local memory and data bus interface logic  157 ,  157 ′ and  157 ″. Interconnecting the PEs for data transfer communications is the cluster switch  171  more completely described in co-pending patent application Ser. No. 08/885,310 filed Jun. 30, 1997 and entitled “Manifold Array Processor”, U.S. Pat Ser. No. 08/949,122 filed Oct. 10, 1997 and entitled “Methods and Apparatus for Manifold Array Processing”, and U.S. Pat Ser. No. 09/169,256 filed Oct. 9, 1998 and entitled “Methods and Apparatus for ManArray PE-to-PE Switch Control”. The above noted applications are assigned to the assignee of the present invention and incorporated herein by reference in their entirety. The interface to a host processor, other peripheral devices, and/or external memory can be implemented in many ways. The primary mechanism shown for completeness is contained in the DMA control unit  181  that provides a scalable ManArray data bus  183  that connects to devices and interface units external to the ManArray core. The DMA control unit  181  provides the data flow and bus arbitration mechanisms needed for these external devices to interface to the ManArray core memories via bus  185 . 
     FIG. 4A shows an overall basic iVLIW data path  400  in which a fetched instruction is stored in an instruction register  401  which is connected to a VIM load and store control function unit  403 . The VIM load and store control function provides interface signals to a VIM  405 . The output of the VIM  405  is pipelined to an iVLIW register  407 . 
     One presently preferred XV 1  instruction  425  is shown in FIG.  4 B. XV 1  instruction  425  is for 32-bit encoding as seen in encoding block  430  and has a presently preferred syntax/operation shown in syntax/operation block  435  as described further below. The XV 1  instruction  425  is one of the control group of instructions defined by the Group field bits  30  and  31  and is used to select VLIWs from VLIW memory (VIM) and execute individual instruction slots of the specified SP or PE, selectable by the SP/PE bit  29 . The VIM address is computed as the sum of a base VIM address register Vb (V 0  or V 1 ) plus an unsigned 8-bit offset VIMOFFS shown in bits  0 - 7 , the block of bits  431 , of encoding block  430  of FIG.  4 B. The VIM address must be in the valid range for the hardware configuration otherwise the operation of this instruction is undefined. 
     Similarly, FIG. 4C shows a presently preferred LV 2  instruction  455  for a load/modify VLIW- 2  function. An encoding block  450  for 32-bit encoding and syntax/operation block  460  are shown. FIG. 4D shows an XV 2  instruction  475  having a 32-bit encoding block  470  and syntax/operation block  480 . 
     Dynamic SP and PE iVLIW Across Slot Compression Operations 
     The XV 1  instruction is advantageously used to modify, enable/disable sub-iVLIW instructions, and indirectly execute iVLIW instructions in the SP and the PEs. The iVLIWs have been loaded into VIM by use of the LV instruction, also referred to as LV 1  elsewhere in this application, which is described in more detail in previously mentioned co-pending application Ser. No. 09/187,539. As illustrated in FIG. 5, each VIM  516  entry preferably consists of five SIW slots (one per execution unit) and associated with each SIW slot are additional state bits, of which only 5 are shown (one d-bit per slot). Included among the five execution units are a store unit  540  associated with VIM  51   6  store instruction slot  520 , load unit  542  associated with load instruction slot  522 , an arithmetic-logical unit (ALU)  544  associated with ALU instruction slot  524 , a multiply-accumulate unit (MAU)  546  associated with an MAU instruction slot  526 , and a data-select unit (DSU)  548  associated with a DSU instruction slot  528 . 
     The five state d-bits  521 ,  523 ,  525 ,  527 , and  529  are LV-loaded disable bits for the instruction slots that indicate either: the SIW slot is available-for-execution or it is not-available for-execution. A binary value suffices to distinguish between the two states. An instruction slot with its d-bit set to the not-available-for-execution state is interpreted as an NOP (no-operation) instruction by the execution unit. In addition, the appropriate d-bit for that functional slot position is loaded into bit- 31  of that slot. Alternatively, the d-bits may be grouped and stored in a different location within a VIM address line while still maintaining the relationship between a d-bit and its associated execution unit and instruction slot within the iVLIW. 
     FIG. 5 illustrates an iVLIW XV 1  pipeline  500 , in which a received XV 1  instruction is loaded into an instruction register  1  (IR 1 )  510 . The output of IR 1  is pre-decoded  512  early in the pipeline cycle prior to loading the VLIW Instruction Register  2  values (IR 2 )  514 . Upon receipt of an XV 1  instruction in IR 1   510 , the VIM address  511  is calculated by use of the specified Vb registers,  501  or  502 , as selected by the Vb signal  509  sourced from IR 1   510  bit- 9 , and added by adder  504  to the offset value included in the XV 1  instruction via path  503 . It is noted that by using bit- 8  and bit- 9  together up to 4 Vb registers can be specified. The resulting VIM address  507  is passed through multiplexer  508  to address the VIM  516 . The iVLIW at the specified address is read out of the VIM  516  and passes through the multiplexers  530 ,  532 ,  534 ,  536 , and  538 , to the IR 2  registers  514 . As an alternative to minimize the read VIM  516  access timing critical path, the output of VIM  516  can be latched into a register whose output is passed through a multiplexer prior to the decode state logic. The enable mask bits, bits  10 - 17  of the XV 1  instruction stored in IR 1   510 , are distributed via path  537  to bit storage latches, S bit- 14  to S latch  550 , L bit- 13  to L latch  552 , A bit- 12  to A latch  554 , M bit- 11  to M latch  556 , and D bit- 10  to D latch  558 . 
     These enable-mask-bits override the d-bit available-for-execution setting for each instruction slot for the XV execution cycle. In more detail, the Load VLIW- 1  (LV 1 ) instruction causes the d-bit setting to be loaded into each VIM slot. The d-bit per execution unit slot represents the enabled or disabled status of the slot position. An inactive state of this bit, for example a “0”, represents the disabled state and the active state of this bit, for example a “1”, represents the enabled state. If the d-bit in a slot is enabled, it may be over-ridden by the XV 1  mask enable bit appropriate for that slot. If the d-bit in a slot is disabled, it can not be overridden by the XV 1  mask enable bit appropriate for that slot. In other words, a slot disabled by a LV 1  instruction can not be reenabled by an XV 1  instruction. Conversly, a slot enabled by a LV 1  instruction can be disabled or kept enabled by an XV 1  instruction. The simple logic to accomplish this is located in each functional unit. This capability is required for the functionality described herein and to efficiently support synchronous MIMD operations as described in more detail in co-pending application Ser. No. 09/187,539 filed Nov. 6, 1998. Alternatively, the latches  550 - 558  can be avoided if the timing path allows the override logic to be placed at the output of the VIM prior to IR 2  clocking. In either case, the functional unit&#39;s decode and execute logic  540 - 548  either executes all instructions received from VIM or executes an NOP based upon the setting of the d-bits and the mask enable bits. For the XV 1  execution, the IR 2 MUX 1  control signal  519  in conjunction with the pre-decode XVcl control signal  517  cause all the IR 2  multiplexers,  530 ,  532 ,  534 ,  536 , and  538 , to select the VIM output paths,  541 ,  543 ,  545 ,  547 , and  549 . The mask enable bits are present at their latch inputs. At the end of the pre-decode cycle, the VLIW IR 2   514  and the mask enable bit latches are clocked and their outputs become present at the inputs of the functional units. At this point, the five individual decode and execution stages of the pipeline,  540 ,  542 ,  544 ,  546 , and  548 , are completed, executing the instruction or an NOP, in synchrony providing the iVLIW parallel execution performance. To allow a non-XV single 32-bit functional instruction to execute by itself in the PE or SP, the bypass VIM path  535  is shown. For example, when a simplex ADD instruction is received into IR 1   510  for parallel array execution, the pre-decode function unit  512  generates the IR 2 MUX 1   519  control signal, which in conjunction with an ADD instruction pre-decode signal, causes the ALU multiplexer  534  to select the bypass path  535 . Since in this case there is no XV instruction in execution, the enable-mask-bits are ignored by the functional unit logic. 
     Any combination of individual instruction slots may be executed via the execute slot parameter ‘E={SLAMD}’, where S=Store Unit (SU), L=Load Unit (LU), A=Arithmetic Logic Unit (ALU), M=Multiply-Accumulate Unit (MAU), and D=Data Select Unit (DSU). A blank ‘E=’ parameter does not execute any slots. The Vx bit- 24  specifies if this XV 1  overrides the LV UAF setting. Vx=0 means do not override LV UAF setting and Vx=1 means override the LV UAF setting with the one specified in this XV 1 &#39;s UAF field bits  22  and  23 . The Unit Affecting Flags (UAF) parameter ‘F=[AMDN]’ overrides the UAF specified for the VLIW when it was loaded via the LV instruction. The override selects which arithmetic instruction slot (A=ALU, M=MAU, D=DSU) or none (N=NONE) is allowed to set condition flags for this execution of the VLIW. The override does not affect the UAF setting specified via the LV instruction. A blank ‘F=’ selects the UAF specified when the VLIW was loaded. 
     Condition Flags are set by the individual simplex instruction in the slot specified by the setting of ‘F=’ parameter from the original LV instruction or as overridden by a ‘F=[AMD]’ parameter in the XV 1  instruction. Condition flags are not affected when ‘F=N’. 
     The XV 1  operation takes one execute cycle to complete, though pipeline considerations must be taken into account based upon the individual simplex instructions in each of the slots that are executed. 
     Overall operation of the present invention may be better understood by examining its application to the exemplary problem presented in the discussion of the prior art. In the table  600  of FIG. 6, each row represents an iVLIW. Each unique VIM address  610  is identified with a number in the first column. The boxed table entries represent SIWs that are masked (i.e. disabled) by the XV 1  instruction during execution. In table  600  of FIG. 6, the shaded iVLIWs  612 ,  614 ,  616  and  618  at VIM address- 0  highlight four occasions in which the SIWs stored at VIM address- 0  are invoked by the XV 1  instruction, each time with a different mask. The first time the instructions are invoked, only the Load Unit is allowed to execute and the Multiply-Accumulate and the Store Units are masked out by the XV 1  instruction. The second time the VIM address- 0  iVLIW instructions are invoked, the Load and Multiply-Accumulate Units are allowed to execute and the Store Unit is masked out by the XV 1  instruction. The third time, all three units are allowed to execute. Finally, the fourth time the instructions stored in address- 0  iVLIW are invoked, only the Store Unit is allowed to execute and the Load and Multiply-Accumulate Units are masked out. In the 2×2 ManArray  100  of FIG. 3, four independent vector* matrix operations, on independent local PE data stored in each PEs local data memories, will be occurring in parallel and in synchronism while maintaining a single thread of control with the dispatching of the 32-bit XV 1  instructions to each PE. The iVLIWs identified in this example are the same in each PE with the operands accessed from each PE&#39;s local register file and local data memory. 
     FIG. 7 illustrates the FIG. 6 example&#39;s four iVLIWs as stored in VIM addresses  0 - 3   710  and the cycle-by-cycle dispatched instruction sequence  720  to the decode-and-execution units. The dispatched instruction sequence  720  illustrates the reuse and cycle-by-cycle redefinition of the iVLIWs that are executed in parallel at execution time. This redefinition is a selecting of the SIWs stored in an iVLIW at a specific VIM address. For example, iVLIW- 0  is shown used in instruction execution cycles  1 ,  3 , and  7  with different groupings of the iVLIW- 0 &#39;s SIWs. In cycle- 1 , only the Load unit instruction is used. In cycle- 3  the Load and MAU units are used and in cycle- 7 , the Store, Load, and MAU units are used. FIG. 7 illustrates how the present invention accomplishes a significant saving in the number of unique VLIW memory entries required by a program, by effectively “packing” several VLIW operations into a single VLIW memory entry. Thus, with the present invention, multiple programs can share a single VLIW memory entry, by storing multiple iVLIWs at the same VIM address. The example also demonstrates the use of the invention to build up and tear down software pipelines. Furthermore, the size of the iVLIW memory in a parallel processor can be kept from becoming prohibitively expensive to build. FIG. 7A illustrates exemplary program code  730  using the XV 1  instruction to accomplish the vector* matrix multiplication of FIG. 1 on the 2×2 ManArray  100  of FIG.  3 . 
     Dynamic SP and PE iVLIW Within Slot Compression Operations 
     For the within slot compression mechanism, the VIM is divided up into separate VIM sections each associated with the functional decode-and-execute units. Each of the VIMs&#39; address maps are divided into multiple 4-bit addressable sections as governed by the offset field included in a second version of the execute iVLIW instruction, XV 2 , with a separate offset that can be specified for each VIM slot section. This VIM configuration and XV 2  addressing option provide the ability to independently select instructions within each VIM slot 4-bit address range. By providing this ability, duplicate SIWs within the 16 addressable iVLIW range can be eliminated providing greater packing of SIWs within the composite VIM. Of course, many variations of addressing options can be envisioned utilizing the inventive techniques to allow the independent selection of SIWs within a partitioned VIM but one is described further below to illustrate various aspects of the within-slot-compression in accordance with the present invention. 
     The XV 2  instruction is similar to the XV 1  instruction in that it is used to modify, enable/disable sub-iVLIW instructions, and indirectly execute iVLIW instructions in the SP and PEs. It does so, however, in a different way than the XV 1  instruction. For the XV 2  instruction, it is still assumed that the iVLIWs have been loaded into this new partitioned VIM by use of a new version of the Load VLIW instruction, LV 2   455  shown in FIG.  4 C. The LV 2  encoding block  450  consists of a CtrlOp field, bits  25 - 28 , that represent the LV 2  instruction opcode, a load instruction bit- 23  that specifies if at least one instruction is to be loaded or if only the disable d-bit for the specified address is to be loaded. Bit- 22  is the disable d-bit that is loaded. Bits  18 - 21  specify that up to 16 instructions are to be loaded in the specified functional unit&#39;s VIM, bits  15 - 17 , beginning at the address specified by the Vb register address, bit- 9 , plus the VIMOFFS offset address, bits  0 - 7 . The syntax/operation details are shown in block  460 . The XV 2  instruction  475  is shown in FIG.  4 D. The encoding format is shown in encoding block  470  with new bit fields as follows. The UAF field bits  23  and  24  are not optional on XV 2  and must be specified with each XV 2  use. The VIM base register selection Vb is bit  20  and the five offset fields are Store VIM offset (SOFS) bits  16 - 19 , Load VIM offset (LOFS) bits  12 - 15 , ALU VIM offset (AOFS) bits  8 - 11 , MAU VIM offset (MOFS) bits  4 - 7 , and DSU VIM offset (DOFS) bits  0 - 3 . The syntax/operation is shown in block  480 . 
     Referring to FIG. 8 which illustrates aspects of an iVLIW XV 2  pipeline  800 , VIM  816  consists of multiple independent memory units each associated with their functional decode and execute units. Independent addressing logic is provided for each slot VIM. As illustrated in FIG. 8 each VIM entry preferably consists of five SIW slots (one per execution unit) and associated with each SIW slot are additional state bits, of which 5 are shown (one d-bit per slot). Included among the five execution units are a store unit  840  associated with store instruction VIM  820 , load unit  842  associated with load instruction VIM  822 , an arithmetic-logical unit (ALU)  844  associated with an ALU instruction VIM  824 , a multiply-accumulate unit (MAU)  846  associated with MAU instruction VIM  826 , and a data-select unit (DSU)  848  associated with DSU instruction VIM  828 . 
     The FIG. 8 VIM address adder functional blocks, as exemplified by ALU VIM address adder  804 , are different than the adder functional block  504  of FIG. 5 in order to support the VIM address increment capability required by the Load VLIW- 2  (LV 2 ) Instruction of FIG. 4C as described in Syntax/Operation block  460 . This capability allows the instructions following the LV 2  instruction to be loaded at: 
     (V[ 01 ]+VIMOFFS)[UnitVIM]←1 st  Instruction following LV 2   
     (V[ 01 ]+VIMOFFS+1)[UnitVIM]←2 nd  Instruction following LV 2   
     (V[ 01 ]+VIMOFFS)+InstrCnt)[UnitVIM]←1 st  (InstrCnt) th  Instruction following LV 2   
     The instruction count InstrCnt is a binary coded number, 0 thru F, that represents from 1 to 16 instructions that can be loaded into up to 16 consecutive UnitVIM locations. 
     The five state d-bits  821 ,  823 ,  825 ,  827 , and  829  are LV-loaded disable bits for the instruction slots that indicate either: the SIW slot is available-for-execution or it is not-available-for-execution. A binary value suffices to distinguish between the two states. An instruction slot with its d-bit set to the not-available-for-execution state is interpreted as an NOP (no-operation) instruction by the execution unit. In addition, the appropriate d-bit for that functional slot position is loaded into bit- 31  of that slot. 
     The operation of the iVLIW XV 2  pipeline  800  is as follows. A received XV 2  instruction is loaded into instruction register  1  (IR 1 )  810 . The output of IR 1  is pre-decoded by pre-decode function unit  812  early in the pipeline cycle prior to loading the VLIW instruction register  2  values in IR 2   814 . Upon receipt of an XV 2  instruction in IR 1   810 , multiple VIM addresses are calculated in parallel. The calculations differ for each VIM slot section due to each having its own offset value as set by the XV 2  instruction. Each Vim slot calculation is of the form Vb+ 0 extend{unitOFS[ 4 ]} where Vb represents one of two VIM address registers, and  0 extend aligns the 4-bit (unitOFS[ 4 ]) value with the extent of Vb. For example, the ALU VIM&#39;s address  811  is calculated by Vb+ 0 extend{AOFS[ 4 ]}, where the Vb value is sourced from either V 0  or V 1  as selected by the Vb field, bit- 20 , of IR 1 . Addition is accomplished by adder  804 . The AOFS[ 4 ]=IR 1  bits  8 - 11  are connected to adder  804  with the adder  804  output  807  being passed through multiplexer  808  to create the ALU VIM slot address  811 . The zero extend logic is not shown for clarity. The ability to mask an entry with XV 2  can be achieved without the use of the enable mask bits that were described in connection with the discussion of XV 1  operation. If a programmer desires the ability to mask an entry, he or she must plan for it in advance and ensure that within the group of  16  addresses, provided by the offset field, at least one of the SIWs is marked as disabled by use of the disable d-bit. For the particular IVLIW that is to be executed with a specific slot masked off, the unitOFS offset for that unit&#39;s VIM is used to select the disabled SIW previously stored. If no slots need to be disabled, full use of the 16 addresses are available for “enabled” SIWs. If only one slot is desired to be disabled, only that slot&#39;s VIM need contain the disabled instruction. Once the VIM addresses are selected, the appropriate SIWs are read out and sent to their decode and execution units. Since in XV 2  operation there is no need for the enable-mask-bits, there are no additional state bit latches required for input to the decode and execution units. Another difference between XV 1  and the XV 2  operation is that for XV 2 , the UAF field is always selected. The XV 2  operation takes one execute cycle to complete, though pipeline considerations must be taken into account based upon the individual simplex instructions in each of the slots that are executed. 
     The present invention may be better understood by examining the application of the XV 2  instruction to the exemplary problem in the discussion of the prior art. FIG. 9 illustrates the iVLIWs required using the XV 2  approach. Each decode and execution unit is associated with its own VIM each with different storage requirements for iVLIW usage. This is also depicted in FIG. 8 wherein a different address range per functional unit VIM is shown; ( 0 -(A- 1 )) store VIM  820 , ( 0 -(B- 1 ) load VIM  822 , ( 0 -(C- 1 )) ALU VIM  824 , ( 0 -(D- 1 )) MAU VIM  826 , and ( 0 -(E- 1 )) DSU VIM  828 . In FIG. 9, composite VIM  910  shows the five decode and execution units&#39; VIMs. The storage requirements for this composite VIM  910  are as follows: the Store VIM requires only 2 locations, the Load VIM uses 3 locations, the ALU VIM uses 4 locations, the MAU VIM uses 5 locations, and the DSU VIM only needs one location. Only one set of disabled locations are used in each VIM which is shown by convention to be in VIM address  0  in each of the unit&#39;s VIMs. Dispatched instruction sequence  920  illustrates the reuse and cycle-by-cycle redefinition of the iVLIWs that are executed in parallel at execution time. This redefinition is a selecting of the SIWs stored in the units VIMs, but not all at the same VIM address as is done with XV 1 . For example, in instruction execution cycle- 4  of sequence  920  the store SIW is accessed from address  0  in the Store VIM, a disabled d-bit instruction, the Load SIW is accessed from address  2  in the Load VIM, a Load R 1 , Data instruction, the ALU SIW is accessed from address  3  in the ALU VIM, an Add R 9 ,R 5 ,R 6  instruction, the MAU SIW is accessed from address  3  in the MAU VIM, a Mpy R 5 ,R 0 ,R 22  instruction, and the DSU SIW is accessed from address  0  in the DSU VIM, a disabled d-bit instruction. FIG. 10 illustrates program code  1000  for using XV 2  instructions to accomplish the vector* matrix multiplication of FIG. 1 on the 2×2 ManArray  100  of FIG.  3 . Specifically, in the 2×2 ManArray of FIG. 3, four independent vector* matrix operations, on independent local PE data stored in each PE&#39;s local data memories, will be occurring in parallel and in synchronism while maintaining a single thread of control with the dispatching of the 32-bit XV 2  instructions to each PE. The iVLIWs identified in this exemplary description are the same in each PE with the operands accessed from each PE&#39;s local register file and local data memory. 
     In comparing the XV 2  approach with the XV 1  approach, it is observed that only 15 locations are used in the composite VIM  910  of the XV 2  approach illustrated in FIG. 9 and 20 locations are used in the VIM  710  of the XV 2  approach illustrated in FIG.  7 . Both approaches are significantly better than the prior art in their utilization of VLIW memory storage. Typically, 35 locations would have been required in a five slot VLIW prior art system. It is also noted that for cost sensitive applications, the XV 2  approach allows each functional unit&#39;s VIM to be cost optimized to the application. 
     While the present invention has been disclosed in the context of presently preferred methods and apparatus for carrying out the invention, various alternative implementations and variations will be readily apparent to those of ordinary skill in the art.