Csci 136 Computer Architecture IICsci 136 Computer Architecture II– Summary of MIPS ISA– Summary of MIPS ISA

Xiuzhen Chengcheng@gwu.edu

Announcement

Homework assignments #2:Readings: Sections 3.6, 3.7, 3.10, 3.11, 3.13, 3.14

Problems 3.5, 3.10, 3.12, 3.22, 3.23, 3.24, 3.25

Project #1 is due on 11:59PM, Feb 13, 2003.

What is an ISA?

A very important abstractionProvide the interface between the low-level software and hardware

May have multiple hardware implementations

Needs to answer the following questionsWhat is the minimum instruction set to be supported?

Use general purpose register or not?

CIRS or RISC design?

Instruction format?

Addressing mode?

… …

Basic ISA Classes

Accumulator ArchitectureStack ArchitectureLoad-Store (General Purpose Register) Architecture

Register – RegisterRegister – MemoryMemory – Memory

Code sequences for (C=A+B) for 4 classes of instruction set

Stack Accumulator

Register

(load-store)

Push A Load A Load R1,A

Push B Add B Load R2,B

Add Store C

Register

(register-memory)

Load R1,A

Add R1,B

Store C, R1 Add R3,R1,R2

Pop C Store C,R3

° Since 1975, all machines use general purpose registers

° Advantages of registers

• registers are faster than memory

• registers are easier for a compiler to use

- e.g., (A*B) – (C*D) – (E*F) can do multiplies in any order vs. stack

• registers can hold variables

- memory traffic is reduced, so program is sped up (since registers are faster than memory)

- code density improves (since register named with fewer bits than memory location)

General Purpose Register Dominates

Memory Addressing

° Since 1980 almost every machine uses addresses to level of 8-bits (byte)

° 2 questions for design of ISA:

• Since one could read a 32-bit word as four loads of bytes from sequential byte addresses or as one load word from a single byte address, How do byte addresses map onto words?

• Can a word be placed on any byte boundary?

What about MIPS?

ISA Operation Summary

Support these simple instructions, since they will dominate the number of instructions executed:

load, store, add, subtract, move register-register, and, shift, compare equal, compare not equal, branch, jump, call, return;

Summary: Salient features of MIPS

• 32-bit fixed format inst (3 formats)

• 32 32-bit GPR (R0 contains zero) and 32 FP registers (and HI LO)

– partitioned by software convention

• 3-address, reg-reg arithmetic instr.

• Single addressing mode for load/store: base+displacement– no indirection, scaled

• 16-bit immediate plus LUI

• Simple branch conditions

– compare against zero or two registers for =,– no integer condition codes

Summary: MIPS Instruction set design

Use general purpose registers with a load-store architecture: yes or no?

Provide 32 general purpose registers plus separate floating-point registers:

What’s the addressing mode supported by MIPS?

Use fixed instruction encoding if interested in performance and use variable instruction encoding if interested in code size :

Summary: MIPS Instruction set designSupport these data sizes and types: 8-bit, 16-bit, 32-bit

integers and 32-bit and 64-bit IEEE 754 floating point numbers:

Support these simple instructions, since they will dominate the number of instructions executed: load, store, add, subtract, move register-register, and, shift, compare equal, compare not equal, branch, jump, call, and return:

Aim for a minimalist instruction set:

MIPS Hardware Design Principles

Simplicity favors regularityKeeping the hardware simple!

R-Type instruction format

Smaller is faster32 general purpose registers, no more, no less.

Good design demands good compromisesR, I, J, 3 types of instruction formats

Make the common case fast!I-type instructions for constant numbers

In-Class Exercise: Reverse a String

Write a MIPS procedure to reverse a null-terminated character string. Assume the address of the string is in $a0 and the address of the reversed string is in $a1. Also assume the spaces needed by the reversed string have been pre-allocated.

In-Class Exercise: Reverse a StringWrite a MIPS procedure to reverse a null-terminated character string. Assume the address of the string is in $a0 and the address of the reversed string is in $a1. Also assume the spaces needed by the reversed string have been pre-allocated.

reverseStr:addiu $sp, $sp, -32sw $s0, 16($sp)sw $s1, 20($sp)

move $s0, $a0move $s1, $a1

addi $sp, $sp, -1sb $zero, 0($sp)

push: lbu $t0, 0($s0)beq $t0, $zero, popaddi $s0, $s0, 1addi $sp, $sp, -1sb $t0, 0($sp)j push

pop: lbu $t0, 0($sp)addi $sp, $sp, 1sb $t0, 0($s1)beq $t0, $zero, doneaddi $s1, $s1, 1j pop

done: lw $s0, 16($sp)lw $s1, 20($sp)addi $sp, $sp, 32jr $ra

Questions?