A Minimalistic Introduction to MIPS Instruction

General Format

               31     26    21    16    11    6      0
                ______ _____ _____ _____ _____ ______
               |______|_____|_____|_____|_____|______|
                  OP     RS    RT    RD  SHAMT  FUNC

op	rs	rt	rd	shamt	funct
6 bits	5 bits	5 bits	5 bits	5 bits	6 bits

op

Operation code

rs

First source register operand

rt

Second source register operand

rd

Destination register operand

shamt

Shift amount - used in shift instructions

funct

Select the variant of the operation in the op code field

Specific Instruction Formats

Format	6 bits	5 bits	5 bits	5 bits	5 bits	6 bits	Comments
R	op	rs	rt	rd	shamt	funct	Arithmetic
I	op	rs	rt	address/immediate			Transfer, branch,immediate
J	op	target address					Jump

MIPS Instruction Set

The MIPS instruction set illustrates four underlying principles of hardware design:

Simplicity favors regularity.
Smaller is faster.
Good design demands compromise.
Make the common case fast.

Simplicity favors regularity

Consider the following example:

Category	Instruction	Example	Meaning	Comments
Arithmetic	add	add a,b,c	a=b+c	Always 3 operands
Arithmetic	subtract	sub a,b,c	a=b-c	Always 3 operands

Note that each operand has exactly three operands.

Smaller is faster.

MIPS has 32 32-bit registers,$v0,...$v31, a very large number would increase the clock cycle time.

Good design demands compromise.

The compromise represented by the MIPS design, was to make all the instructions the same length, thereby requiring different instruction formats.

Make the common case fast.

The MIPS instruction set addresses this principal by making constants part of arithmetic instructions. Furthermore, by loading small constants into the upper 16-bits of a register.

MIPS Instruction Set Summary

Arithmetic Instructions

Instruction	Example	Meaning	Comments
add	add $1,$2,$3	$1=$2+$3	Always 3 operands
subtract	sub $1,$2,$3	$1=$2-$3	Always 3 operands
add immediate	addi $1,$2,10	$1=$2+10	add constant
add unsigned	addu $1,$2,$3	$1=$2+$3	Always 3 operations
subtract unsigned	subu $1,$2,$3	$1=$2-$3	Always 3 operations
add immed.unsigned	addiu $1,$2,10	$1=$2+10	Always 3 operations

Logical

Instruction	Example	Meaning	Comments
and	and $1,$2,$3	$1=$2&$3	3 register operands
or	or $1,$2,$3	$1=$2\|$3	3 register operands
and immediate	andi $1,$2,10	$1=$2&10	AND constant
or immediate	or $1,$2,10	$1=$2\|10	OR constant
shift left logical	sll $1,$2,10	$1=$2<<10	Shift left by constant
shift right logical	srl $1,$2,10	$1=$2>>10	Shift right by constant

Data Transfer

Instruction	Example	Meaning	Comments
load word	lw $1,10($2)	$1=Memory[$2+10]	memory to register
store word	sw $1,10($2)	Memory[$2+10]=$1	register to memory
load upper immed.	lui $1,10	$1=10x2^16	load constant into upper 16 bits

Conditional Branch

Instruction	Example	Meaning	Comments
branch on equal	beq $1,$2,10	if($1==$2)go to PC+4+10	Equal test
branch on not equal	bne $1,$2,10	if($1!=$2)go to PC+4+10	Not equal test
set on less then	slt $1,$2,$3	if($2<$3)$1=1;else $1=0	Less than compare

Unconditional Jump

Instruction	Example	Meaning	Comments
jump	j 1000	go to 1000	Jump to target address
jump register	jr $31	go to $31	For switch, procedure return
jump and link	jal 1000	$31=PC+4;go to 1000	For procedure call

Assembler Syntax

Program includes .data and .text
Comments begin with #. Rest of line is ignored.
Identifier names are sequence of letters, numbers, underbars (_) and dots (.).
Labels are declared by putting them at beginning of line followed by colon. Use labels for variables and code locations.
Instruction format: op field followed by one or more operands: addi $t0, $t0, 1
Operands may be literal values or registers.
Register is hardware primitive, can stored 32-bit value: $s0
Numbers are base 10 by default. 0x prefix indicates hexadecimal.
Strings are enclosed in quotes. May include \n=newline or \t=tab. Used for prompts.

System Services in MIPS

To print an integer to the screen:

Put the integer into $a0
Set $v0 to 1
syscall

To print a string to the screen:

Put the address of the string into $a0
Set $v0 to 4
syscall

To read an integer from the keyboard

Set $v0 to 5
syscall
The integer entered will be in $v0

To exit

Set $v0 to 10
syscall