SPIM print example

# This program takes input from the user and echoes it back

.data
# Constant strings to be output to the terminal
promptInt: .asciiz "Please input an integer: "
resultInt: .asciiz "Next integer is: "
linefeed: .asciiz "\n"
enterkey: .asciiz "Press any key to end program."
.text
main:
# prompt for an integer
li $v0,4 # code for print_string
la $a0,promptInt # point $a0 to prompt string
syscall # print the prompt
# get an integer from the user
li $v0,5 # code for read_int
syscall #get int from user --> returned in $v0
move $t0,$v0 # move the resulting int to $t0
# compute the next integer
addi $t0, $t0, 1 # t0 <-- t0 + 1
# print out text for the result
li $v0,4 #code for print_string
la $a0,resultInt # point $a0 to result string
syscall # print the result string
# print out the result
li $v0,1 # code for print_int
move $a0,$t0 # put result in $a0
syscall # print out the result
# print out a line feed
li $v0,4 # code for print_string
la $a0,linefeed # point $a0 to linefeed string
syscall # print linefeed
# wait for the enter key to be pressed to end program
li $v0,4 # code for print_string
la $a0,enterkey # point $a0 to enterkey string
syscall # print enterkey
# wait for input by getting an integer from the user (integer is ignored)
li $v0,5 # code for read_int
syscall #get int from user --> returned in $v0
# All done, thank you!
li $v0,10 # code for exit
syscall # exit program

Registers

By convention, many MIPS registers have special purpose uses. To help clarify this, SPIM defines aliases for each register that represent its purpose. The following table lists these aliases and the commonly accepted uses for the registers.

Register	Number	Usage
zero	0	Constant 0
at	1	Reserved for assembler
v0	2	Used for return values  from function calls.
v1	3
a0	4	Used to pass arguments to  procedures and functions.
a1	5
a2	6
a3	7
t0	8	Temporary (Caller-saved, need  not be saved by called procedure)
t1	9
t2	10
t3	11
t4	12
t5	13
t6	14
t7	15
s0	16	Saved temporary (Callee-saved,  called procedure must save and restore)
s1	17
s2	18
s3	19
s4	20
s5	21
s6	22
s7	23
t8	24	Temporary (Caller-saved, need not  be saved by called procedure)
t9	25
k0	26	Reserved for OS kernel
k1	27
gp	28	Pointer to global area
sp	29	Stack pointer
fp	30	Frame pointer
ra	31	Return address for function calls.

SPIM Instruction Set

This document gives an overview of the more common instructions used in the SPIM simulator. See Appendix A of Computer Organization and Design by Hennessy and Patterson for more details.

Overview

The SPIM simulator implements the full MIPS instruction set, as well as a large number of pseudoinstructions that correspond to one or more equivalent MIPS instructions. There are also a small number of system call commands used to interface with the console window of the SPIM simulator. Finally, SPIM renames registers according to commonly used conventions in order to facilitate the readability of programs.

Instructions and PseudoInstructions

The following is an abbreviated list of MIPS instructions and SPIM pseudoinstructions. This list is not complete. Notably missing are all Floating Point and coprocessor instructions.

• - Indicates an actual MIPS instruction. Others are SPIM pseudoinstructions.

Instruction Function

• add Rd, Rs, Rt Rd = Rs + Rt (signed)

• addu Rd, Rs, Rt Rd = Rs + Rt (unsigned)

• addi Rd, Rs, Imm Rd = Rs + Imm (signed)

• sub Rd, Rs, Rt Rd = Rs - Rt (signed)

• subu Rd, Rs, Rt Rd = Rs - Rt (unsigned)

• div Rs, Rt lo = Rs/Rt, hi = Rs mod Rt (integer division, signed)

• divu Rs, Rt lo = Rs/Rt, hi = Rs mod Rt (integer division, unsigned)

div Rd, Rs, Rt Rd = Rs/Rt (integer division, signed)

divu Rd, Rs, Rt Rd = Rs/Rt (integer division, unsigned)

rem Rd, Rs, Rt Rd = Rs mod Rt (signed)

remu Rd, Rs, Rt Rd = Rs mod Rt (unsigned)

mul Rd, Rs, Rt Rd = Rs * Rt (signed)

• mult Rs, Rt hi, lo = Rs * Rt (signed, hi = high 32 bits, lo = low 32 bits)

• multu Rd, Rs hi, lo = Rs * Rt (unsigned, hi = high 32 bits, lo = low 32 bits)

• and Rd, Rs, Rt Rd = Rs • Rt

• andi Rd, Rs, Imm Rd = Rs • Imm

neg Rd, Rs Rd = -(Rs)

• nor Rd, Rs, Rt Rd = (Rs + Rt)’

not Rd, Rs Rd = (Rs)’

• or Rd, Rs, Rt Rd = Rs + Rt

• ori Rd, Rs, Imm Rd = Rs + Imm

• xor Rd, Rs, Rt Rd = Rs ⊕ Rt

• xori Rd, Rs, Imm Rd = Rs ⊕ Imm

• sll Rd, Rt, Sa Rd = Rt left shifted by Sa bits

• sllv Rd, Rs, Rt Rd = Rt left shifted by Rs bits

• srl Rd, Rs, Sa Rd = Rt right shifted by Sa bits

• srlv Rd, Rs, Rt Rd = Rt right shifted by Rs bits

move Rd, Rs Rd = Rs

• mfhi Rd Rd = hi

• mflo Rd Rd = lo

li Rd, Imm Rd = Imm

• lui Rt, Imm Rt[31:16] = Imm, Rt[15:0] = 0

• lb Rt, Address(Rs) Rt = byte at M[Address + Rs] (sign extended)

• sb Rt, Address(Rs) Byte at M[Address + Rs] = Rt (sign extended)

• lw Rt, Address(Rs) Rt = word at M[Address + Rs]

• sw Rt, Address(Rs) Word at M[Address + Rs] = Rt

• slt Rd, Rs, Rt Rd = 1 if Rs < Rt, Rd = 0 if Rs ≥ Rt (signed)

• slti Rd, Rs, Imm Rd = 1 if Rs < Imm, Rd = 0 if Rs ≥ Imm (signed)

• sltu Rd, Rs, Rt Rd = 1 if Rs < Rt, Rd = 0 if Rs ≥ Rt (unsigned)

• beq Rs, Rt, Label Branch to Label if Rs == Rt

beqz Rs, Label Branch to Label if Rs == 0

bge Rs, Rt, Label Branch to Label if Rs ≥ Rt (signed)

• bgez Rs, Label Branch to Label if Rs ≥ 0 (signed)

• bgezal Rs, Label Branch to Label and Link if Rs ≥ Rt (signed)

bgt Rs, Rt, Label Branch to Label if Rs > Rt (signed)

bgtu Rs, Rt, Label Branch to Label if Rs > Rt (unsigned)

• bgtz Rs, Label Branch to Label if Rs > 0 (signed)

ble Rs, Rt, Label Branch to Label if Rs ≤ Rt (signed)

bleu Rs, Rt, Label Branch to Label if Rs ≤ Rt (unsigned)

• blez Rs, Label Branch to Label if Rs ≤ 0 (signed)

• bgezal Rs, Label Branch to Label and Link if Rs ≥ 0 (signed)

• bltzal Rs, Label Branch to Label and Link if Rs < 0 (signed)

blt Rs, Rt, Label Branch to Label if Rs < Rt (signed)

bltu Rs, Rt, Label Branch to Label if Rs < Rt (unsigned)

• bltz Rs, Label Branch to Label if Rs < 0 (signed)

• bne Rs, Rt, Label Branch to Label if Rs ≠ Rt

bnez Rs, Label Branch to Label if Rs ≠ 0

• j Label Jump to Label unconditionally

• jal Label Jump to Label and link unconditionally

• jr Rs Jump to location in Rs unconditionally

• jalr Label Jump to location in Rs and link unconditionally 

System Calls

In order to perform I/O with the console, SPIM provides a small library of system calls. In general, system calls are set up by placing a system call in register $v0, and any arguments in register $a0 and $a1. Returned values are placed in register $v0. See the table and the example program below for usage.

Service	System Call Code	Arguments	Result
Print_int	1	$a0 = integer
Print_float	2	$f12 = float
Print_double	3	$f12 = double
Print_string	4	$a0 = string
Read_int	5		Integer (in $v0)
Read_float	6		Float (in $f0)
Read_double	7		Double (in $f0)
Read_string	8	$a0 = buffer, $a1 = length
Sbrk	9	$a0 = amount	Address (in $v0)
exit	10

# This program takes input from the user and echoes it back

.data

# Constant strings to be output to the terminal

promptInt: .asciiz "Please input an integer: "

resultInt: .asciiz "Next integer is: "

linefeed: .asciiz "\n"

enterkey: .asciiz "Press any key to end program."

.text

main:

# prompt for an integer

li $v0,4 # code for print_string

la $a0,promptInt # point $a0 to prompt string

syscall #print the prompt

# get an integer from the user

li $v0,5 # code for read_int

syscall #get int from user --> returned in $v0

move $t0,$v0 # move the resulting int to $t0

# compute the next integer

addi $t0, $t0, 1 # t0 <-- t0 + 1

# print out text for the result

li $v0,4 #code for print_string

la $a0,resultInt # point $a0 to result string

syscall # print the result string

# print out the result

li $v0,1 # code for print_int

move $a0,$t0 # put result in $a0

syscall # print out the result

# print out a line feed

li $v0,4 # code for print_string

la $a0,linefeed # point $a0 to linefeed string

syscall # print linefeed

# wait for the enter key to be pressed to end program

li $v0,4 # code for print_string

la $a0,enterkey # point $a0 to enterkey string

syscall # print enterkey

# wait for input by getting an integer from the user (integer is ignored)

li $v0,5 # code for read_int

syscall #get int from user --> returned in $v0

# All done, thank you!

li $v0,10 # code for exit

syscall # exit program