Ch 2. Instructions: Language of the Computer -(6)

3. MIPS Design Principles

MIPS ISA Key Idea

• Goals of instruction set design for MIPS
  • Maximize performance
  • Minimize cost
  • Reduce design time (of compiler and hardware)
  • RISC: By the simplicity of Hardware

MIPS Register Model

• 32 x 32bits General Purpose Registers(범용 레지스터) (Integer)
  • R0 always = 0
  • R0~R31 : 32Bits register
• 32 x 32 bits Floating Point Registers
  • 16 x 64 bits (double precision)
  • even reg: less-significant word
  • odd reg: more-significant word
• HI (32bits), LO(32bits)
  • 곱하기(총 64bits) 나누기(Q, R) 때 사용
• PC(Program Counter)

Register Naming Convention

• R0~R31
• R0: $zero constant 0
• R2~R7: function call
  • R2~R3: return
  • R4~R7: parameters, arguments
• R8~R25: user가 assembly language를 쓸 때 사용

Memory Structure

• Byte addressable
  • 주소를 갖고있는 최소 단위가 byte
  • 1 address = 8 bits
• Word addressable
  • 잘 보이지 않고 쓰이지 않음
  • 1 word가 갖는 bits는 컴퓨터마다 다름(MIPS: 32bits)
  • word마다 주소를 줌
• MIPS에서는 byte마다 주소를 가지고 있음 1word = 32bits = 4bytes
  • 0~3: word0
  • 4~7: word1

MIPS Design Principles

1. Simplicity favors Regularity

• Most of arithmetic/logic instructions have 3 operands
  • 순서는 고정(destination first)
• Ex)
  • MIPS: ADD C, A, B (C = A+B)
  • MIPS: SUB C, A, B (C = A-B)

2. Smaller is faster

• arithmetic/logic instructions 의 경우 register를 사용 (register addressing mode)
• Ex)
  • MIPS: add $s0, $s1, $s2
• MIPS: only 32 registers (limited)
  • register의 개수가 늘어남 -> hardware가 복잡 -> propagation delay ↑
    -> clock cycle time ↑

3. Make common case fast

• Constants는 꽤 자주 operands로 쓰인다.
• Ex)
  • MIPS: A = A+1, B = A - 5
• Sol1) 메모리에 할당 and load from memory
  • Memory access: 시간이 오래 걸림
  • 많이 안쓰이는 상수에 대해 고려
• Sol2) 자주 쓰는 register만 create hard-wired register
  • R0 = $zero = constant 0
• Sol3) instruction안에 넣는 방법 (Immediate addressing mode)
  • MIPS: ADDI $3, $3, 1 (A= A+1)
  • 작은 상수를 instruction 안에 두면 메모리에서 프로세서로 실행하기 위해 load
  • 메모리에 두는 것보다 더 빨리 access
  • register의 낭비 감소

4. Good design demands a compromise

• Machine language structure:
  • Length: 32bits (Design Principle 1)
  • R-type format
    
  • ADD $t0, $s1, $s2
    • $t0 -> rd = R8
    • $t1 -> rs = R17
    • $t2 -> rt = R18
    • ADD -> op, funct(0,32)
  • 문제점:
    • LW, SW, ADDI
    • 1000 -> 2^10 -> 10bits 필요 (표현불가)
  • I-type format: for Immediate value
    
  • ADDI $s0, $s1, 4
    • $s0 -> rt = R16
    • $s1 -> rs = R17
    • ADDI -> op = R8
    • 4 -> constant