READ CROSSBAR ELIMINATION IN A VLIW PROCESSOR

Abstract: A VLIW processor comprising: a plurality of functional units, an instruction issue register with several issue slots, a multi port register file having a plurality of read ports for supplying operands to the functional units and a coupling circuit between said read ports and functional units. The feature of the invention is that the coupling circuit is arranged to provide only a restricted set of connections, whereby a functional unit may receive operands only from a restricted number of read ports.

Description: READ CROSSBAR ELIMINATION IN A VLIW PROCESSOR The invention relates to the field of architecture of Very Long Instrucuon Word (VLIW) processors. More in particular, the invention relates to a VLIW processor as set forth in the precharacteπzing part of Claim 1. The mvenuon is an improvement on US Application Ser. No. 08/445,963 filed May 22, 1995 (PHA 21 ,777a) which is incorporated herein by reference. Pnor art VLIW processors can be as shown in Fig. 1 They are characteπzed by having an instruction issue register (TIR) 104 having a plurality of issue slots Each issue slot is for containing a respective operation All of the operations which are held simultaneously in the instruction issue register are to be begun in a same machine cycle. Each operation is to be executed on an appropπate one of the funcuonal units FUl , FU2, . , FUN The operations are RISC-type operations, in that an operation can be begun in each machine cycle for each slot of the IIR. The VLIW processor includes a multiport register file 101 This multiport register file 101 stores operands destined for the functional units FUl , FU2, . , FUN and results produced by the functional units. A read crossbar 102 directs operands from the multiport register file 101 to appropπate ones of the functional units FUl , FU2, . , FUN under control of signals from the instruction issue register 104 A wπte control unit 103 governs timing of wπting of results from the functional units FUl , FU2, .. , FUN to the multiport register file 101. It is a disadvantage of the pnor art that the read crossbar 102 takes up a lot of real estate on the processor chip. It is an object of the invention to make the VLIW processor fit in a smaller space. The invention provides for a processor according to the characteπzing part of Claim 1. Functional units are assigned to particular issue slots in the instruction register. By associating a respective limited subset of the read ports with each issue slot the read cross bar can be simplified or eliminated. The processor of the invention differs from pnor art processors in which there was one issue slot per functional unit, see e.g. US Pat no. 5, 103,311 (PHN 12,376a), in that the groupings of the functional units in the invention does not result in each group of functional units becoming a super unit. The units still function separately. They execute distinct operation types. They have distinct instruction latencies and distinction connections to the write control unit. BRIEF DESCRIPTION OF THE DRAWING The invention will now be described by way of non-limitative example with reference to the following drawings. Fig. 1 shows a prior art VLIW processor. Fig. 2 shows a VLIW processor according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 shows a VLIW processor according to the invention. The processor has an IIR 201 with m issue slots, S 0 , Si , ..., S m , where m is an integer greater than one. Each slot is for holding an operation to be performed on the functional units. As before, all the operations which are held simultaneously in the IIR are to be begun in a same machine cycle and all of the operations are RISC-like. As before, there is a multiport register file 202. This file has the same number, m, of read ports, RP 0 , RP,, ... RP m , as there are issue slots. This file has k write ports where k may or may not be the same as m. More information about multiport register files can be found in US Pat. App. Ser. No. 08/366,958 filed December 30, 1994, which is incorporated herein by reference. Each read port has a respective group of at least one functional unit which reads from that port. For instance, the drawing illustrates functional units FUQ O , FU m , ... FU 0No that read from RP 0 ; functional units FU 01 , FU U , ... FU 1N1 that read from RP,; and function units FU m0 , FU ml , ... FU nιNm that read from RP m . Write buses, 205, 206, .. , 207, 208, 209, .., 210, .. , 211 , 212, .. , 213, convey result data from the functional units to write control unit 203. Write control unit 203, as in US Pat. App. Ser. No. 08/445,963, determines when to route result data from the write buses to the multiport register file 202 and onto which write ports to route result data. In a sample VLIW processor with m=5 and 19 total functional units, a full read crossbar requires 95 connections. Each connection requires a set of two 32 bit read buses, i.e. a 64 bit connection plus some switching circuitry, as shown in US 08/445,963. It was found, as further explained below, that a limited crossbar requires 35 connections and no scheduling restrictions. In a limited crossbar only some inputs of functional units are connected to a particular output of the multiport register file, or, alternatively, only some outputs of the multiport register file are connected to some functional units. Limited crossbars would be constructed in accordance with the crossbar principles illustrated in US Pat. App. Ser No. 08/445,963 and US Pat. No. 5,313,551, which is incorporated herein by reference. With the crossbar eliminated entirely there were 19 connections to functional units, i.e. the same number of connections as there were functional units. Some modest scheduling restrictions were required from the compiler as a result of eliminating the crossbar. Reduced complexity of the crossbar leads to reduction of the critical timing path, chip area, design complexity and dissipation. In the tables at the end of the description some examples are given of assignment of functional units to issue slots in a VLIW processor. In the tables at the end of the description the following abbreviations are used: const: This is a unit which passes a constant to the write bus. alu: arithmetic and logic unit shift: shifter mult: multiplier falu: floating point alu fcomp: floating point compare fmult: floating point multiplier fdivid: floating point divider fsqroot: floating point square root mem: memory branch unit: for executing branches + indicates that a connection is possible between the functional unit in the left column and the issue slot that has the + in its column. A blank indicates that there is no connection. Table I gives a first example are of an assignment of functional units to issue slots in a VLIW processor with partial connections to connections. The partial crossbar embodiment would require 55 connections and not require any scheduling restrictions, beyond what is inherent from the number of functional units. It is not necessary to be able to connect every functional unit to every issue slot if there are multiple units of the same type. For instance, if we want to schedule a "const" operation in issue slot 0 we can always connect it with Cl . A connection with C2 is superfluous. Table II gives a second embodiment. This embodiment reduces the number of connections by recognizing that there are only five issue slots and that therefore certain instruction combinations are not necessary. In this embodiment, only 35 connections are required, and still no scheduling restrictions, beyond those inherent in the number of functional units. In other words, clearly if there is only one floating point multiply unit, only one floating point multiply operation can appear in any instruction. The term "scheduling restrictions" herein means the number of a particular class of operations which can appear in a same instruction. A restriction on which issue slot can contain an operation is not considered a scheduling restriction. Thus, it is possible to schedule operations in such a way that, for example, an "falu" operation is always in slot 1 and a "mem" operation is always in slot 0, without limiting which operations can occur simultaneously in one instruction. Therefore "falu" only needs a connection to issue slot 1. Table III gives another embodiment this embodiment would reduce the number of connections to 31 , but an additional scheduling restriction would need to be imposed. A floating point square root operation would not be able to appear in the same instruction with a floating point divide. The embodiment of Table IV would reduce the number of connections to 25, but impose the following additional scheduling restrictions over the embodiment of Table III. In the follow- ing examples, as more and more connections are eliminated, cumulative scheduling restrictions must be imposed. a) allow 2 operations out of SI , S2, falu, fcomp; and b) allow 2 operations out of Ml , M2, fmult, fdivid, fsqroot. The embodiment of table V reduces the number of connections to 22 and imposes the following additional schedule limitation compared with the previous table: allow 2 operations out of Cl , C2, Ml , M2, fmult, fdivid, fsqroot. The configuration of table VI has the minimum number of connections for 19 functional units, namely 19 connections. Moreover every functional unit is connected to only one issue slot. This configuration requires the following additional constraint over the previous configuration: allow 2 operations out of Bl , B2, SI , S2, falu, fcomp. Which and how many functional units should be assigned to which slot of the instruction issue register must be determined empirically based upon processor cost and experience of the optimality of processor function running sample programs. For instance, the number of floating point units should be minimized as these are expensive. In the preferred embodiment of the invention, where m=5 and k=5, there are 28 functional units. It was determined that an ALU and a constant unit had to be attached to each issue slot in order to avoid an unacceptable number of scheduling restrictions. The functional unit types in the preferred embodiment are defined in table VII. These functional units are connected to the issue slots according to table VIII. With this machine configuration, a great number of minor scheduling restrictions apply, in addition to those obviously imposed by the number of functional units. These restrictions lead to almost no performance loss on real applications. Examples of scheduling restrictions are: TABLE IX numerical limitation of operations that can set of functional units to which the be in a single instruction from a specified numerical limitation in the previous column set of functional units applies 2 IF1 , IF2, FC, FT 4 Dl , D2, Bl , B2, B3 3 Dl , D2, FA1 , FA2 3 SI , S2, PI , P2 3 PI , P2, Ml, M2 4 PI, P2, Bl , B2, B3 4 PI, P2, Ml , M2, Bl , B2, B3 Each VLIW instruction on this processor must satisfy all of the concurrent scheduling restrictions which result from the elimination of the crossbar. However, the above list is not a complete list of such restrictions. Processing an exhaustive list of scheduling restrictions is not the most efficient way to compile instructions for a processor according to the invention. Those of ordinary skill in the art can readily satisfy the scheduling constraints by designing a compiler to check each proposed compiled instruction against TABLE VIII to verify that that instruction can run. TABLES TABLE I (partial crossbar embodiment) Functional unit ft & type which issue slot can use this FU 0 1 2 3 4 Cl onst + + + C2:const + + Al .ALU 4- A2:ALU + A3: ALU + A4:ALU + A5:ALU + SI .shift + + S2: shift + 4- + Ml: mult + + M2:mult + + + falu + + + + + FC: fcomp + + + + + FM: fmult + + + + + FD: fdivid + + + + + FS: fsqroot + + + + + DM: mem + + + + + Bl:branch + + + B2: branch + + —-—_———, TABLE II (PARTIAL CROSSBAR IMPLEMENTATION) Functional unit # & type which issue slot can use this FU 0 1 2 3 4 Cl:const + + + C2: const 4- + Al:alu 4- A2:alu 4- A3:alu + A4:alu + A5:alu + Sl:shift 4- + S2: shift 4- + + Ml:mult + + 4- M2:mult 4- + FA: falu + FC: fcomp + FM: fmult + FD: fdivid + FS: fsqroot 4- + + + + DM: mem 4- Bl:branch 4- + 4- B2:branch + + TABLE III (PARTIAL CROSSBAR) Functional unit # & type which issue slot can use this FU 0 1 2 3 4 Ckconst 4- 4- 4- C2: const 4- 4- Al:alu 4- A2:alu 4- A3:alu 4- A4:alu 4- A5:alu 4- S lrshift + + S2:shift 4- 4- 4- Ml : mult 4- 4- 4- M2:mult 4- 4- FA: falu + FC: fcomp 4- FM: fmult 4- FD: fdivid 4- FS: fsqroot 4- DM: mem 4- Bl :branch 4- 4- 4- B2: branch 4- 4- TABLE IV (PARTIAL CROSSBAR) Functional unit ft & type which issue slot can use this FU 0 1 2 3 4 Cl onst 4- 4- 4- C2: const + 4- Al:alu 4- A2:alu 4- A3:alu 4- A4:alu 4- A5:alu 4- S kshift 4- S2: shift 4- Ml :mult 4- M2:mult 4- FA: falu + FC: fcomp 4- FM: fmult 4- FD: fdivid 4- FS: fsqroot 4- DM: mem 4- Bl :branch 4- + 4- B2: branch 4- 4- TABLE V (PARTIAL CROSSBAR) Functional unit ff & type which issue slot can use this FU 0 1 2 3 4 Cl onst 4- 4- 4- C2: const 4- 4- Al :alu 4- A2:alu 4- A3: alu 4- A4:alu 4- A5:alu 4- Slrshift + S2: shift 4- M mult 4- M2:mult 4- FA: falu 4- FC: fcomp 4- FM: fmult 4- FD: fdivid 4- FS: fsqroot + DM: mem 4- Bl :branch 4- 4- 4- B2:branch 4- + TABLE VI (CROSSBAR ELIMINATION) Functional unit ft & type which issue slot can use this FU 0 1 2 3 4 Cl xonst 4- C2: const 4- Alralu 4- A2:alu 4- A3: alu 4- A4:alu 4- A5:alu 4- Slrshift 4- S2: shift 4- Mlrmult 4- M2:mult 4- FA: falu 4- FC: fcomp 4- FM: fmult 4- FD: fdivid 4- FS: fsqroot 4- DM: mem 4- Bhbranch 4- B2r branch 4- TABLE VII (functional unit types) Type abbrevilateny typical operations ation constant const 1 transfers an opcode modifier to a register alu alu 1 integer add and subtract, integer and unary comparisons, boolean, half word extension, borrow and carry, merge and pack, byte selection, immediate operations conditioned on operands digital signal dspalu 2 clipping including clipped sums and differences processing and absolute values, averages, twos complements alu conditioned on operands, operations choosing the larger or smaller of two operands; plural parallel operations of the foregoing types on pairs of subsets of operand words, digital signal dspmul 3 plural parallel multiplies on pairs of subsets of processing operand words and summations of same multiplier data memory dmem 3 loads and stores including whole words and parts of words data memory dmemspec 3 data cache operations such as copies, invalidation special of blocks, identification of address tags, reading status bits, prefetches, and allocations shifter shift 1 arithmetic and logical shifts and rotations, including shifts of concatenations of operands branch branch 3 all sorts of jumps, invalidation of instruction cache blocks floating falu 3 floating point addition and subtraction and point alu absolute values; conversions between floating point and integer values and vice versa, predetermination of exceptions that would result from calculations and conversions performable in this unit integer and ifmul 3 floating point, integer, and unary multiplies floating including with truncation of results; point predetermination of exceptions that would result multiply from multiplies floating fcomp 1 floating point comparisons, determination of signs point of floating point numbers; predetermination of compare exceptions that would result from performing floating point compares and determination of signs; read or write the program status word, destination program counter or source program counter; samples of the hardware clock cycle counter tough ftough 17 floating point divides and square roots; floating predetermination of flags that would result from point multisame plies TABLE VIII Functional unit ft and type which issue slot can use this FU 0 1 2 3 4 Cl: const 4- C2: const 4- C3: const 4- C4: const 4- C5: const 4- Al : alu 4- A2: alu 4- A3: alu 4- A4: alu 4- A5: alu 4- Dl : dmem 4- D2: dmem 4- dmemspec 4- SI : shift 4- S2: shift 4- PI : dspalu 4- P2: dspalu 4- Ml: dspmul 4- M2: dspmul 4- Bl: branch 4- B2: branch + B3: branch + FA1: falu 4- FA2: falu 4- IFl: ifmul 4- IF2: ifmul + FC: fcomp 4- FT: ftough 4-

Claims: CLAIMS: 1. A VLIW processor comprising a plurality of functional units; an instruction issue register having a plurality of issue slots, each issue slot being for containing a respective operation, all operations which are held simultaneously in the instruction issue register being for commencement on the functional units in a same machine cycle, the instruction issue register being suitable for filling with a new set of operations with each machine cycle, the number of issue slots being less than the number of functional units; and a multiport register file having a plurality of read ports for supplying operands to the functional units, the number of read ports being the same as the number of issue slots in the instruction issue register; a coupling circuit between the read ports and the functional units for providing all connections to supply the operands, characterized in that - the coupling circuit is arranged to provide only a restricted set of connections, whereby at least one of the functional units may only receive operands from less than all of the read ports. 2. The processor of Claim 1 wherein each issue slot is arranged to supply operations to a respective subset of at least one of the functional units, each subset being associated with a respective read port, the coupling circuit providing connections to the functional units of each subset only from the read port associated with the subset or subsets to which the funtional unit belongs. 3. The processor of claim 1 or 2 wherein the coupling circuit provides connections to each functional unit from one and only one respective one of the read ports. 4. The processor of claim 3 wherein the functional units include a number of constant units which is the same as the number of issue slots. 5. The processor of claim 3 wherein the functional units include a number of alu's which is the same as the number of issue slots, each alu being associated with a respective one of the read ports, the coupling circuit providing only a respective connection to each alu from the read port associated to that alu.

Inventors: ANG MICHAEL, JACOBS EINO, ANG, MICHAEL, JACOBS, EINO
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: G06F9
IPC codes: G06F9, H04L7