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Embedded Computer Architecture

Embedded Computer Architecture. ASIP Application Specific Instruction-set Processor 5KK73 Bart Mesman and Henk Corporaal. Application domain specific processors (ADSP or ASIP). DSP. Programmable CPU. Programmable DSP. Application domain specific.

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Embedded Computer Architecture

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  1. Embedded Computer Architecture ASIP Application Specific Instruction-set Processor 5KK73 Bart Mesman and Henk Corporaal

  2. Application domain specific processors (ADSP or ASIP) DSP Programmable CPU Programmable DSP Application domain specific Application specific processor flexibility efficiency Embedded Computer Archtiecture H.Corporaal and B. Mesman

  3. implementation Appl. domain GP ADSP Appl. domain implementation Application domain specific processors (ADSP or ASIP) • takes a well defined application domain as a starting point • exploits characteristics of the domain (computation kernels) • still programmable within the domain • e.g. MPEG2 coding uses 8*8 DCT transform, DECT, GSM etc ... performance: clock speed + ILP ILP,DLP, tuning to domain flexible dev. (new apps.) cost effective (high volume) problems - specification manual design, - design time and effort large effort => synthesized cores Platform Design H.Corporaal and B. Mesman

  4. www.adelantetech.com Platform Design H.Corporaal and B. Mesman

  5. OK? more appl.? Design process processor- model e.g. VLIW with shared RFs application(s) instance parameters 3 phases 1. exploration 2. hw design (layout) + processing 3. design appl. sw SW (code generation) HW design Estimations nsec/cycle, area, power/instr Estimations cycles/alg occupation Fast, accurate and early feedback no yes yes no go to phase 2 Platform Design H.Corporaal and B. Mesman

  6. * + * * + * * * * + 0 0 1 2 3 1 2 3 1 3 1 2 * * * * * * 1 1 5 3 4 3 4 4 * + * + + 2 2 3 6 3 6 6 + * + * * * * + * 3 3 7 5 7 8 5 8 8 7 8 * + * * * * * 4 4 9 10 5 9 5 9 5 * + * + 9 10 9 10 ASIP/VLIW architectures: list scheduling Candidate Conflict & Scheduled IPB LIST Priority Comp. Operation * 4 OPB MULT ALU IPB OPB 5 Platform Design H.Corporaal and B. Mesman

  7. Problem statement A compiler is retargetable if it can generate code for a ‘new’ processor architecture specified in a machine description file. A guarded register transfer pattern (GRTP) is a register transfer pattern (RTP) together with the control bits of the instruction word that control the RTP. a: = b + c | instr = xxxx0101 GRTPs contain all inter-RT-conflict information. Instruction set extraction (ISE) is the process of generating all possible GRTPs for a specific processor. Platform Design H.Corporaal and B. Mesman

  8. Problem statement Algorithm spec Processor spec (instance) in ch 4 this is part of the code generator FE ISE CDFG GRTP Code Generation Machinecode Platform Design H.Corporaal and B. Mesman

  9. Example: Simple processor [Leupers] I.(12:5) Inp RAM I.(20:13) I.(4) +1 PC I.(3:2) IM I.(1:0) I.(20:0) REG outp Platform Design H.Corporaal and B. Mesman

  10. Example: Simple processor [Leupers] Platform Design H.Corporaal and B. Mesman

  11. ASIP/VLIW architectures A|RT designer template as an example (= set of rules, a model) • Differences with GP VLIW processors • 1. // FUs • ASUs = complex appl. Spec. FUs (beyond subword //) • e.g. biquad, median, DCT etc … • larger grainsize, more heterogeneous, more pipelines • 2. Rfiles • many Rfiles (>5 vs 1 or 2) • limited # ports (3 vs 15) • limited size (<16 vs. 128) • 3. Issue slots • all in parallel vs. 5 Platform Design H.Corporaal and B. Mesman

  12. RF5 RF7 RF6 RF8 RF1 RF3 RF2 RF4 FU3 FU4 FU1 FU2 flags IR3 IR4 IR1 IR2 Instruction memory Con- trol Platform Design H.Corporaal and B. Mesman

  13. read address RF 1 control FU mux 1 write address RF 1 read address RF 2 mux 2 write address RF 2 output drivers ASIP/VLIW architectures • Additional characteristics of the A|RT designer template • interconnect network: busses + input multiplexers • mux control is part of the instruction • control can change every clock cycle • network can be incomplete • busses can be merged • memories are modeled as FUs • separate data in and data out • 2 inputs (data in and address) and 1 output • Each FU can generate one or more flags • instruction format (per issue slot) Platform Design H.Corporaal and B. Mesman

  14. 19 10 0 9 mux 2 mux 3 read RF1 write RF1 read RF2 write RF2 ALU instr. read RF3 write RF3 read RF4 write RF4 MAC instr. ASIP/VLIW architectures: example RF1 RF2 RF3 RF4 ALU MAC bus1 bus2 Platform Design H.Corporaal and B. Mesman

  15. ASIP/VLIW architectures : example Platform Design H.Corporaal and B. Mesman

  16. OK? assign ( a+b, ALU, fu_alu1) assign ( a+_, ALU, fu_alu2) assign ( _+_, ALU, fu_alu3) ASIP/VLIW architectures: design flow Algorithm spec Datapath synthesis RF1 : x = RF2 : y, RF3 : z | ALU = ADD Inmux = bus2 Change pragmas RTs Controller synthesis Estimations area, power, timing no VLIW makes relatively simple code selection possible yes Platform Design H.Corporaal and B. Mesman

  17. x4 x3 x2 x1 x0 Z-1 Z-1 Z-1 Z-1 c4 c3 c2 c1 c0 * * * * * + y Application examples (1)

  18. Application examples (1) 19 instructions per tap!! Processor Architectures and Program Mapping H. Corporaal, J. van Meerbergen, and B. Mesman

  19. Application examples (2) Bit level operations: finite field arithmetic 10 instructions!! Very simple in hardware

  20. source register ($2) 27 26 25 23 22 20 srl $13, $2, 20 andi $25, $13, 1 srl $14, $2, 21 andi $24, $14, 6 or $15, $25, $24 srl $13, $2, 22 andi $14, $13, 56 or $25, $15, $14 sll $24, $25, 2 7 6 5 4 3 2 destination register ($24) Application examples (2) Bit level operations : DES example Processor Architectures and Program Mapping H. Corporaal, J. van Meerbergen, and B. Mesman

  21. 18 17 16 13 $5 srl $24, $5, 18 srl $25, $5, 17 xor $8, $24, $25 srl $9, $5, 16 xor xor $10, $8, $9 srl $11, $5, 13 xor $12, $10, $11 andi $13, $12, 1 … 0 ... 1 $13 Application examples (2) Bit level operations : A5 example (GSM encryption)

  22. ASIP/VLIW architectures: feedback Platform Design H.Corporaal and B. Mesman

  23. Implementation Independent Design Database Low power aspects • Estimation area + speed power Mistral2 Estimation Database Architecture Platform Design H.Corporaal and B. Mesman

  24. GSM viterbi decoder : default solution EXU ACTIV AREA POWER alu_1 96% 3469 46196 romctrl_1 48% 39 259 acu_1 26% 327 1209 ipb_1 5% 131 105 opb_1 23% 1804 5801 ctrl 9821 135035 total 15591 188605 • controller responsible for 70% of power consumption • maximum resource-sharing • heavy decision-making : “main” loop with 16 metrics-computations per iteration • EXU-numbers include Registers for local storage 13750 Platform Design H.Corporaal and B. Mesman

  25. GSM viterbi decoder : no loop-folding EXU ACTIV AREA POWER alu_1 92% 3411 45073 romctrl_1 45% 39 255 acu_1 25% 294 1087 ipb_1 5% 107 86 opb_1 22% 1661 5340 ctrl 4919 70087 total 10431 121928 • area down by 33% • power down by 35% • next step: reduce # of program-steps with second ALU 14247 Platform Design H.Corporaal and B. Mesman

  26. GSM viterbi decoder : 2 ALU’s EXU ACTIV AREA POWER alu_1 69% 1797 12248 alu_2 65% 1393 8916 romctrl_1 67% 39 255 acu_1 37% 294 1087 ipb_1 8% 149 119 opb_1 33% 2136 6871 ctrl 8957 87235 total 14766 116731 9739 • cycle count down 30% • area up 42% • power down by 5% • next step: introduce ASU to reduce ALU-load Platform Design H.Corporaal and B. Mesman

  27. GSM viterbi decoder : 1 x ACS-ASU func ACS ( M1, M2, d ) MS, MS8 = begin MS = if ( M1+d > M2-d ) -> ( M1+d) || ( M2-d) fi; MS8 = if ( M1- d > M2+d) -> ( M1- d) || ( M2+d) fi; end; = EXU ACTIV AREA POWER alu_1 20% 261 105 acs_asu_1 83% 2382 3816 or_asu_1 10% 611 122 romctrl_1 16% 65 21 acu_1 36% 294 205 ipb_1 20% 107 43 opb_1 11% 163 35 ctrl 1864 3597 total 5747 7944 1930 • cycle count down 5X • power down20X! Platform Design H.Corporaal and B. Mesman

  28. GSM viterbi decoder : 4 x ACS-ASU EXU ACTIV AREA POWER alu_1 94% 243 97 acs_asu_1 95% 1041 420 acs_asu_2 95% 1041 420 acs_asu_3 95% 1041 420 acs_asu_4 95% 1041 420 split_asu_1 47% 90 18 or_asu_1 47% 592 118 romctrl_1 28% 48 6 acu_1 98% 212 85 ipb_1 23% 60 6 opb_1 50% 369 80 ctrl 1306 555 total 7084 2645 425 • cycle count down another 5X • area up 23% • power downanother 3X! Platform Design H.Corporaal and B. Mesman

  29. Implementation Independent Design Database GSM viterbi example : summary Mistral2 72x ! Platform Design H.Corporaal and B. Mesman

  30. OK? OK? more appl.? Discussion: phase 3 processor- model application(s) application(s) SW (code generation) HW design SW (code generation) Freeze processor model no no no yes yes no yes Application software development: constraint driven compilation Exploration phase Platform Design H.Corporaal and B. Mesman

  31. RF2 RF1 RF3 RF4 FU3 FU4 FU1 FU2 flags IR3 IR4 IR1 IR2 Instruction memory Con- trol Platform Design H.Corporaal and B. Mesman

  32. Discussion: problems with VLIWs code size and instruction bandwidth • code compaction = reduce code size after scheduling • possible compaction ratio ? • e.g. p0 = 0.9 and p1 = 0.1 • information content (entropy) = - pi log2 pi = 0.47 • maximum compression factor  2 • control parallelism during scheduling = switch between • different processor models (10% of code = 90% runtime) • architecture • reduce number of control bits for operand addresses • e.g. 128 reg (TM) -> 28 bits/issue slot for addresses only • => use stacks and fifos Platform Design H.Corporaal and B. Mesman

  33. Fully serial Classical encoding: fetching many nops n n A n n n n n n B n n n n n n n n n n n C n n Mixed serial/parallel n n n n n D n n n n n E n n n n n B A n n C n n F n n n n n n n n n n E n D n n Fully parallel n n n n n n G n F n n n n n n n n n n n n n n H n n n n n n G H A B C D E F G H A B C D E F G H A B C D E F G H A B C D E F G H 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 1 1 1 1 1 1 1 0 Velocity encoding

  34. Conclusions • ASIPs provide efficient solutions for well-defined application domains (2 orders of magnitude higher efficiency). • The methodology is interesting for IP creation. • The key problem is retargetable compilation. • A (distributed) VLIW model is a good compromise between HW and SW. • Although an automatic process can generate a default solution, the process usually is interactive and iterative for efficiency reasons. The key is fast and accurate feedback. Platform Design H.Corporaal and B. Mesman

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