MIPS Architecture and Assembly Language Overview

Adapted from: http://edge.mcs.dre.g.el.edu/GICL/people/sevy/architecture/MIPSRef(SPIM).html

[Register Description] [I/O Description]


Data Types and Literals

Data types:

Literals:

Registers

This is from Figure 9.9 in the Goodman&Miller text
Register
Number
Alternative
Name
Description
0
zero
the value 0
1
$at
(assembler temporary) reserved by the assembler
2-3
$v0 - $v1
(values) from expression evaluation and function results
4-7
$a0 - $a3
(arguments) First four parameters for subroutine.
Not preserved across procedure calls
8-15
$t0 - $t7
(temporaries) Caller saved if needed. Subroutines can use w/out saving.
Not preserved across procedure calls
16-23
$s0 - $s7
(saved values) - Callee saved.
A subroutine using one of these must save original and restore it before exiting.
Preserved across procedure calls
24-25
$t8 - $t9
(temporaries) Caller saved if needed. Subroutines can use w/out saving.
These are in addition to $t0 - $t7 above.
Not preserved across procedure calls.
26-27
$k0 - $k1
reserved for use by the interrupt/trap handler
28
$gp
global pointer.
Points to the middle of the 64K block of memory in the static data segment.
29
$sp
stack pointer
Points to last location on the stack.
30
$s8/$fp
saved value / frame pointer
Preserved across procedure calls
31
$ra
return address

See also Britton section 1.9, Sweetman section 2.21, Larus Appendix section A.6


Program Structure

Data Declarations

Code

Comments


Data Declarations

format for declarations:

Note: labels always followed by colon ( : )

example
	
var1:		.word	3	# create a single integer variable with initial value 3
array1:		.byte	'a','b'	# create a 2-element character array with elements initialized
				#   to  a  and  b
array2:		.space	40	# allocate 40 consecutive bytes, with storage uninitialized
				#   could be used as a 40-element character array, or a
				#   10-element integer array; a comment should indicate which!	


Load / Store Instructions

load:

	lw	register_destination, RAM_source

store word:

	sw	register_source, RAM_destination

#store byte (low-order) in source register into RAM destination

load immediate:

	li	register_destination, value

 

example:
	.data
var1:	.word	23		# declare storage for var1; initial value is 23

	.text
__start:
	lw	$t0, var1		# load contents of RAM location into register $t0:  $t0 = var1
	li	$t1, 5		#  $t1 = 5   ("load immediate")
	sw	$t1, var1		# store contents of register $t1 into RAM:  var1 = $t1
	done



Indirect and Based Addressing

load address:

	la	$t0, var1

indirect addressing:

	lw	$t2, ($t0)
	sw	$t2, ($t0)

based or indexed addressing:

	lw	$t2, 4($t0)
	sw	$t2, -12($t0)

Note: based addressing is especially useful for:

 

example

		.data
array1:		.space	12		#  declare 12 bytes of storage to hold array of 3 integers
		.text
__start:	la	$t0, array1		#  load base address of array into register $t0
		li	$t1, 5		#  $t1 = 5   ("load immediate")
		sw $t1, ($t0)		#  first array element set to 5; indirect addressing
		li $t1, 13		#   $t1 = 13
		sw $t1, 4($t0)		#  second array element set to 13
		li $t1, -7		#   $t1 = -7
		sw $t1, 8($t0)		#  third array element set to -7
		done


Arithmetic Instructions

		add	$t0,$t1,$t2	#  $t0 = $t1 + $t2;   add as signed (2's complement) integers
		sub	$t2,$t3,$t4	#  $t2 = $t3  $t4
		addi	$t2,$t3, 5	#  $t2 = $t3 + 5;   "add immediate" (no sub immediate)
		addu	$t1,$t6,$t7	#  $t1 = $t6 + $t7;   add as unsigned integers
		subu	$t1,$t6,$t7	#  $t1 = $t6 + $t7;   subtract as unsigned integers

		mult	$t3,$t4		#  multiply 32-bit quantities in $t3 and $t4, and store 64-bit
					#  result in special registers Lo and Hi:  (Hi,Lo) = $t3 * $t4
		div	$t5,$t6		#  Lo = $t5 / $t6   (integer quotient)
					#  Hi = $t5 mod $t6   (remainder)
		mfhi	$t0		#  move quantity in special register Hi to $t0:   $t0 = Hi
		mflo	$t1		#  move quantity in special register Lo to $t1:   $t1 = Lo
					#  used to get at result of product or quotient

		move	$t2,$t3	#  $t2 = $t3



Control Structures

Branches

		b	target		#  unconditional branch to program label target
		beq	$t0,$t1,target	#  branch to target if  $t0 = $t1
		blt	$t0,$t1,target	#  branch to target if  $t0 < $t1
		ble	$t0,$t1,target	#  branch to target if  $t0 <= $t1
		bgt	$t0,$t1,target	#  branch to target if  $t0 > $t1
		bge	$t0,$t1,target	#  branch to target if  $t0 >= $t1
		bne	$t0,$t1,target	#  branch to target if  $t0 <> $t1

Jumps

		j	target	#  unconditional jump to program label target
jr $t3 # jump to address contained in $t3 ("jump register")

Subroutine Calls

subroutine call: "jump and link" instruction

	jal	sub_label	#  "jump and link"

subroutine return: "jump register" instruction

	jr	$ra	#  "jump register"

Note: return address stored in register $ra; if subroutine will call other subroutines, or is recursive, return address should be copied from $ra onto stack to preserve it, since jal always places return address in this register and hence will overwrite previous value


System Calls and I/O (SPIM Simulator)

The following table lists the possible syscall services.

Service Code
in $v0
Arguments Results
print_int
1
$a0 = integer to be printed
print_float
2
$f12 = float to be printed
print_double
3
$f12 = double to be printed
print_string
4
$a0 = address of string in memory
read_int
5
integer returned in $v0
read_float
6
float returned in $v0
read_double
7
double returned in $v0
read_string
8
$a0 = memory address of string input buffer
$a1 = length of string buffer (n)
sbrk
9
$a0 = amount address in $v0
exit
10

e.g.   Print out integer value contained in register $t2
li $v0, 1 # load appropriate system call code into register $v0; # code for printing integer is 1 move $a0, $t2 # move integer to be printed into $a0: $a0 = $t2 syscall # call operating system to perform operation
e.g. Read integer value, store in RAM location with label int_value (presumably declared in data section)
li $v0, 5 # load appropriate system call code into register $v0; # code for reading integer is 5 syscall # call operating system to perform operation sw $v0, int_value # value read from keyboard returned in register $v0; # store this in desired location e.g. Print out string (useful for prompts) .data string1 .asciiz "Print this.\n" # declaration for string variable, # .asciiz directive makes string null terminated .text main: li $v0, 4 # load appropriate system call code into register $v0; # code for printing string is 4 la $a0, string1 # load address of string to be printed into $a0 syscall # call operating system to perform print operation
e.g. To indicate end of program, use exit system call; thus last lines of program should be: li $v0, 10 # system call code for exit = 10 syscall # call operating sys