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Choices in Designing an ISA

Choices in Designing an ISA. Uniformity . Should each instruction Be the same length (in bits or bytes?) Take the same time to execute? Complexity . How many different instructions? How closely linked to High Level languages? Tradeoffs – Complex Instructions

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Choices in Designing an ISA

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  1. Choices in Designing an ISA • Uniformity. Should each instruction • Be the same length (in bits or bytes?) • Take the same time to execute? • Complexity. • How many different instructions? • How closely linked to High Level languages? • Tradeoffs – Complex Instructions • Take up less code memory to store them • Need a rather complex CPU design Comp 3014 The Nature of Computing

  2. Instruction Encoding Example destination All Sam’s instructions take up 32 bits. opcode Source regs Sam’s instructions start with the opcode then the destination reg- ister then the source register rd <- rs + rt e.g. add r3, r1, r2 means r3 = r1 + r2 First 6 bits for the opcode. 3 2 1 Comp 3014 The Nature of Computing

  3. Intel 80x86 ISA The most popular of all • 1971: Intel invents microprocessor 4004/8008, 8080, 8085 • 1975: Major design effort for new 16-bit ISA, (iAPX432) but … • 1978: 8086 dedicated registers, segmented address, 16-bit • 8088; 8-bit version of 8086 added as after thought • 1980: IBM selects 8088 as basis for IBM PC • 1980: Intel 432 finally ready but… • 1980: 8087 floating point coprocessor: • 1982: 80286 24-bit address, protection, memory mapping • 1985: 80386 32-bit address, 32-bit GP registers, paging • 1989: 80486 • 1992 Pentium • 1995 Pentium Pro • 1997 Pentium Pro with MMX multimedia acceleration Comp 3014 The Nature of Computing

  4. Some x86 instructions mov ax , [bx + c] mov [ax] , bx add ax , bx add [bx] , ax These look rather like Sam’s RISC ops Let’s compare the RR and RM ISA’s. Clearly RR needs more memory while the RM uses stronger operations But this is not. Here the contents of ax is being added straight into memory ! The x86 is a register – memory ISA and Sam is a register – register ISA ldi r1 , a ldi r2 , b add r3,r1,r2 st r3 , b mov ax, a add b,ax Sam Intel x86 Comp 3014 The Nature of Computing

  5. 10 9 8 7 6 5 4 3 2 1 0% 10% 20% 30% Variable Length Instructions All Sam’s instructions had the same length, 32 bits. This is also true for other RISC ISA’s such as SPARC and MIPS. Compare this with the x86 instruction vary from 1 to 17 bytes. Here’s some stats. Clearly long complex instructions are used infrequently Instruction Length (bytes) But the use does depend on the app. Frequency of use Comp 3014 The Nature of Computing

  6. T1 T2 T3 T4 T5 T1 T2 T3 T4 T5 Reg Write Reg Write Mem Access Mem Access ALU ALU Decode, Reg Op Decode, Reg Op Fetch Fetch Variable Time Instructions Here’s a timing diagram for an Intel add add ax , [bx + c] [bx + c] ax = ax + mem[..] … and the second to actually add memory to register ax We need two adds. The first to get the address summed up … Comp 3014 The Nature of Computing

  7. Potent x86 Instructions 1.Application Greenspan strcmp(str, Greenspan); 2.High-Level Language (‘C’) 3.Intel ISA code Comp 3014 The Nature of Computing

  8. Top 10 Intel x86 Instructions Rank Instruction Usage 1 load 22% 2 conditional branch 20% 3 arithmetic / logic 19% 4 compare 16% 5 store 12 % 6 move reg - reg 4% 7 call - return 2% We see that most instructions are Simple load, store, calculate, branch. None of Intel’s potent stuff figures here. So why did Intel design instructions no-one uses ? Comp 3014 The Nature of Computing

  9. Semantic Gap Twixt HLL and ML In the 1970’s Hardware costs decreased. So we got faster CPUs but Memory was expensive. ld r1,B ld r2,0 ld r3,[r1 + r2] add r4,r4,r3 addi r2,r2,1 str r4,[r2+5] … … … add r4,r3,r2 • Bigger programs means more expensive programs • Shortage of Good Programmers • Unreliable Software • Response : Reduce programming Costs • Develop powerful HLL easy to learn so no mistakes • But is Semantic Gap between HLL and ML • Software runs inefficiently - Poor Performance • Compilers become Complex • So Close the Semantic Gap • Machine executes HLL constructs in hardware • Lots of addressing Modes Add the column of sales figures But this potent stuff is not being used ! Comp 3014 The Nature of Computing

  10. - Easy to Decode Ops - Fast Issue Rate - Only load and Store references memory - Lots of registers Emerging Design Guidelines ISA R&D into the 80’s • Let’s downshift and make things simpler … • Use simple instructions, load, store, add • Many of these will do one x86 potent op • Need more memory, but memory is becoming cheap • More CPU cycles, but can still be faster 1980 Berkeley Patterson RISC (SPARC) 1981 Stanford Hennessy MIPS Comp 3014 The Nature of Computing

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