Instruction Set Architecture and Microcode

Architecture vs. Microarchitecture

  • “Architecture”/Instruction Set Architecture:
    • Programmer visible state (Memory & Register)
    • Operations (Instructions and how they work)
    • Execution Semantics (Interrupts)
    • Input/Output
    • Data Types/Sizes
  • Microarchitecture/Organization:
    • Tradeoffs on how to implement ISA for some metric (Speed, Energy, Cost)
    • Examples: Pipeline depth, number of pipelines, cache size, silicon area, peak power, execution ordering, bus widths, ALU widths

Software Developments

  • Up to 1955
    • Libraries of numerical routines
      • Floating point operations
      • Transcendental functions
      • Matrix manipulation, equation solvers
  • 1955-60
    • High Level Languages - Fortran 1956
    • Operating Systems -
      • Assemblers, Loaders, Linkers, Compilers
      • Accounting programs to keep track of usage and charges
    • Machines required experienced operators

Compatibility Problem at IBM

  • By early 1960’s IBM had 4 incompatible lines of computers!
    • 701: Science $\rightarrow$ 7094
    • 650 $\rightarrow$ 7074
    • 702 $\rightarrow$ 7080
    • 1401: Business $\rightarrow$ 7010
  • Each system had its own
    • Instruction set
    • I/O system and secondary storage: magnetic tapes, drums and disks
    • assemblers, compilers, libraries
    • market niche business, scientific, real time
  • IBM 360:
    • Probably, the first true instruction set architecture (ISA) that was implemented to be ISA.
    • Unify different lines of computers to IBM 360
    • Implement different versions specialized for different market

IBM 360: A General-Purpose Register (GPR) Machine

  • Processor state
    • 16 General purpose 32-bit registers
      • may be used as index and base register
      • register0 has some special properties
    • 4 floating point 64-bit registers
    • A program status word (PSW)
      • PC, condition codes, control flags
  • A 32-bit machine with 24-bit addresses
    • But no instruction contains a 24-bit address!
  • Data Formats
    • 8-bit bytes, 16-bit half-words, 32-bit words, 64-bit double-words
    • IBM 360 is why bytes are 8-bits long today!

IBM 360: Initial Implementations

  • IBM 360 ISA completely hid the underlying technological differences between various models
  • Milestones: The first ISA designed as portable hardware-software interface!
    • With minor modifications it still survives today!
  • IBM 360 $\rightarrow$ IBM 370 $\rightarrow$ IBM 370EX $\rightarrow$ zSeries

Same Architecture, Different Microarchitecture

Where Do Operands Come from And Where Do Results Go?

  • More explicitly named operands vs More instruction encoding space

  • Why 2 explicitly named operands? Some
    • ALU instruction does not even name the destination of the results
  • How Stack Computer Works and Why Other Types are Better?
    • Stack is part of the processor state
    • Conceptually stack is unbounded
    • If stack size stored in registers is too small, need frequent fetch/store from/to memory: expensive
    • Problem? : Bad locality
      • a variable A is used in multiple terms: A should be fetched multiple times

Classes of Instructions

  • Data transfer: LD, ST, MFC1, MTC1, MFC0, MTC0
  • Control Flow: BEQZ, JR, JAL, TRAP, ERET
  • Floating Point: ADD.D, SUB.S, MUL.D, C.LT.D, CVT.S.W
  • Multimedia (SIMD): ADD.PS, SUM.PS, MUL.PS, CLT.PS
  • String: REP MOVSB (x86)

Addressing Modes: How to Get Operands from Memory

Data Types and Sizes

  • Types
    • Binary Integer
    • Binary Coded Decimal (BCD)
    • Floating Point
      • IEEE 754
      • Cray Floating Point
      • Intel Extended Precision (80-bit)
    • Packed Vector Data
    • Addresses
  • Width
    • Binary Integer (8-bit, 16-bit, 32-bit, 64-bit)
    • Floating Point (32-bit, 40-bit, 64-bit, 80-bit)
    • Addresses (16-bit, 24-bit, 32-bit, 48-bit, 64-bit)

ISA Encoding

  • Fixed Width: Every Instruction has same width
    • Easy to decode
    • RISC Architectures: MIPS, PowerPC, SPARC, ARM…
    • Ex: MIPS, every instruction 4-bytes
  • Variable Length: Instructions can vary in width
    • Takes less space in memory and caches
    • CISC Architectures: IBM 360, x86, Motorola 68k, VAX…
    • Ex: x86, instructions 1-byte up to 17-bytes
  • Mostly Fixed or Compressed:
    • Ex: MIPS16, THUMB (only two formats 2 and 4 bytes)
    • PowerPC and some VLIWs
      • Store instructions compressed, decompress into Instruction Cache
  • (Very) Long Instruction Word (VLIW):
    • Multiple instructions in a fixed width bundle
    • Ex: Multiflow, HP/ST Lx, TI C6000
  • x86 (IA-32) Instruction Encoding

  • MIPS Instruction Encoding

Real World Instruction Sets

Why Diversity in ISAs?

  • Technology Influenced ISA
    • Storage is expensive, tight encoding important
    • Reduced Instruction Set Computer
      • Remove instructions until whole computer fits on die
    • Multicore/Manycore
      • Transistors not turning into sequential performance Application Influenced ISA
    • Instructions for Applications
      • DSP instructions
    • Compiler Technology has improved
      • SPARC Register Windows no longer needed
      • Compiler can register allocate effectively


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