MISP
# Comments are denoted with a '#'
# Everything that occurs after a '#' will be ignored by the assembler's lexer.
# Programs typically contain a .data and .text sections
.data # Section where data is stored in memory (allocated in RAM), similar to
# variables in higher-level languages
# Declarations follow a ( label: .type value(s) ) form of declaration
hello_world: .asciiz "Hello World\n" # Declare a null terminated string
num1: .word 42 # Integers are referred to as words
# (32-bit value)
arr1: .word 1, 2, 3, 4, 5 # Array of words
arr2: .byte 'a', 'b' # Array of chars (1 byte each)
buffer: .space 60 # Allocates space in the RAM
# (not cleared to 0)
# Datatype sizes
_byte: .byte 'a' # 1 byte
_halfword: .half 53 # 2 bytes
_word: .word 3 # 4 bytes
_float: .float 3.14 # 4 bytes
_double: .double 7.0 # 8 bytes
.align 2 # Memory alignment of data, where
# number indicates byte alignment
# in powers of 2. (.align 2
# represents word alignment since
# 2^2 = 4 bytes)
.text # Section that contains
# instructions and program logic
.globl _main # Declares an instruction label as
# global, making it accessible to
# other files
_main: # MIPS programs execute
# instructions sequentially, where
# the code under this label will be
# executed first
# Let's print "hello world"
la $a0, hello_world # Load address of string stored
# in memory
li $v0, 4 # Load the syscall value (number
# indicating which syscall to make)
syscall # Perform the specified syscall
# with the given argument ($a0)
# Registers (used to hold data during program execution)
# $t0 - $t9 # Temporary registers used for
# intermediate calculations inside
# subroutines (not saved across
# function calls)
# $s0 - $s7 # Saved registers where values are
# saved across subroutine calls.
# Typically saved in stack
# $a0 - $a3 # Argument registers for passing in
# arguments for subroutines
# $v0 - $v1 # Return registers for returning
# values to caller function
# Types of load/store instructions
la $t0, label # Copy the address of a value in
# memory specified by the label
# into register $t0
lw $t0, label # Copy a word value from memory
lw $t1, 4($s0) # Copy a word value from an address
# stored in a register with an
# offset of 4 bytes (addr + 4)
lb $t2, label # Copy a byte value to the
# lower order portion of
# the register $t2
lb $t2, 0($s0) # Copy a byte value from the source
# address in $s0 with offset 0
# Same idea with 'lh' for halfwords
sw $t0, label # Store word value into
# memory address mapped by label
sw $t0, 8($s0) # Store word value into address
# specified in $s0 and offset of
# 8 bytes
# Same idea using 'sb' and 'sh' for bytes and halfwords. 'sa' does not exist
### Math ###
_math:
# Remember to load your values into a register
lw $t0, num # From the data section
li $t0, 5 # Or from an immediate (constant)
li $t1, 6
add $t2, $t0, $t1 # $t2 = $t0 + $t1
sub $t2, $t0, $t1 # $t2 = $t0 - $t1
mul $t2, $t0, $t1 # $t2 = $t0 * $t1
div $t2, $t0, $t1 # $t2 = $t0 / $t1 (Might not be
# supported in some versions of MARS)
div $t0, $t1 # Performs $t0 / $t1. Get the
# quotient using 'mflo' and
# remainder using 'mfhi'
# Bitwise Shifting
sll $t0, $t0, 2 # Bitwise shift to the left with
# immediate (constant value) of 2
sllv $t0, $t1, $t2 # Shift left by a variable amount
# in register
srl $t0, $t0, 5 # Bitwise shift to the right (does
# not sign preserve, sign-extends
# with 0)
srlv $t0, $t1, $t2 # Shift right by a variable amount
# in a register
sra $t0, $t0, 7 # Bitwise arithmetic shift to
# the right (preserves sign)
srav $t0, $t1, $t2 # Shift right by a variable amount
# in a register
# Bitwise operators
and $t0, $t1, $t2 # Bitwise AND
andi $t0, $t1, 0xFFF # Bitwise AND with immediate
or $t0, $t1, $t2 # Bitwise OR
ori $t0, $t1, 0xFFF # Bitwise OR with immediate
xor $t0, $t1, $t2 # Bitwise XOR
xori $t0, $t1, 0xFFF # Bitwise XOR with immediate
nor $t0, $t1, $t2 # Bitwise NOR
## BRANCHING ##
_branching:
# The basic format of these branching instructions typically follow <instr>
# <reg1> <reg2> <label> where label is the label we want to jump to if the
# given conditional evaluates to true
# Sometimes it is easier to write the conditional logic backward, as seen
# in the simple if statement example below
beq $t0, $t1, reg_eq # Will branch to reg_eq if
# $t0 == $t1, otherwise
# execute the next line
bne $t0, $t1, reg_neq # Branches when $t0 != $t1
b branch_target # Unconditional branch, will
# always execute
beqz $t0, req_eq_zero # Branches when $t0 == 0
bnez $t0, req_neq_zero # Branches when $t0 != 0
bgt $t0, $t1, t0_gt_t1 # Branches when $t0 > $t1
bge $t0, $t1, t0_gte_t1 # Branches when $t0 >= $t1
bgtz $t0, t0_gt0 # Branches when $t0 > 0
blt $t0, $t1, t0_gt_t1 # Branches when $t0 < $t1
ble $t0, $t1, t0_gte_t1 # Branches when $t0 <= $t1
bltz $t0, t0_lt0 # Branches when $t0 < 0
slt $s0, $t0, $t1 # "Set on Less Than"
# when $t0 < $t1 with result in $s0
# (1 for true)
# Simple if statement
# if (i == j)
# f = g + h;
# f = f - i;
# Let $s0 = f, $s1 = g, $s2 = h, $s3 = i, $s4 = j
bne $s3, $s4, L1 # if (i !=j)
add $s0, $s1, $s2 # f = g + h
L1:
sub $s0, $s0, $s3 # f = f - i
# Below is an example of finding the max of 3 numbers
# A direct translation in Java from MIPS logic:
# if (a > b)
# if (a > c)
# max = a;
# else
# max = c;
# else
# if (b > c)
# max = b;
# else
# max = c;
# Let $s0 = a, $s1 = b, $s2 = c, $v0 = return register
ble $s0, $s1, a_LTE_b # if(a <= b) branch(a_LTE_b)
ble $s0, $s2, max_C # if(a > b && a <=c) branch(max_C)
move $v0, $s0 # else [a > b && a > c] max = a
j done # Jump to the end of the program
a_LTE_b: # Label for when a <= b
ble $s1, $s2, max_C # if(a <= b && b <= c) branch(max_C)
move $v0, $s1 # if(a <= b && b > c) max = b
j done # Jump to done
max_C:
move $v0, $s2 # max = c
done: # End of program
## LOOPS ##
_loops:
# The basic structure of loops is having an exit condition and a jump
# instruction to continue its execution
li $t0, 0
while:
bgt $t0, 9, end_while # While $t0 is less than 10,
# keep iterating
#actual loop content would go here
addi $t0, $t0, 1 # Increment the value
j while # Jump back to the beginning of
# the loop
end_while:
# 2D Matrix Traversal
# Assume that $a0 stores the address of an integer matrix which is 3 x 3
li $t0, 0 # Counter for i
li $t1, 0 # Counter for j
matrix_row:
bgt $t0, 3, matrix_row_end
matrix_col:
bgt $t1, 3, matrix_col_end
# Do stuff
addi $t1, $t1, 1 # Increment the col counter
matrix_col_end:
# Do stuff
addi $t0, $t0, 1
matrix_row_end:
## FUNCTIONS ##
_functions:
# Functions are callable procedures that can accept arguments and return
#Â values all denoted with labels, like above
main: # Programs begin with main func
jal return_1 # jal will store the current PC in $ra
# and then jump to return_1
# What if we want to pass in args?
# First we must pass in our parameters to the argument registers
li $a0, 1
li $a1, 2
jal sum # Now we can call the function
# How about recursion?
# This is a bit more work since we need to make sure we save and restore
# the previous PC in $ra since jal will automatically overwrite
# on each call
li $a0, 3
jal fact
li $v0, 10
syscall
# This function returns 1
return_1:
li $v0, 1 # Load val in return register $v0
jr $ra # Jump back to old PC to continue exec
# Function with 2 args
sum:
add $v0, $a0, $a1
jr $ra # Return
# Recursive function to find factorial
fact:
addi $sp, $sp, -8 # Allocate space in stack
sw $s0, ($sp) # Store reg that holds current num
sw $ra, 4($sp) # Store previous PC
li $v0, 1 # Init return value
beq $a0, 0, fact_done # Finish if param is 0
# Otherwise, continue recursion
move $s0, $a0 # Copy $a0 to $s0
sub $a0, $a0, 1
jal fact
mul $v0, $s0, $v0 # Multiplication is done
fact_done:
lw $s0, ($sp)
lw $ra, 4($sp) # Restore the PC
addi $sp, $sp, 8
jr $ra
## MACROS ##
_macros:
# Macros are extremely useful for substituting repeated code blocks with a
# single label for better readability
# These are in no means substitutes for functions
# These must be declared before it is used
# Macro for printing newlines (since these can be very repetitive)
.macro println()
la $a0, newline # New line string stored here
li $v0, 4
syscall
.end_macro
println() # Assembler will copy that block of
# code here before running
# Parameters can be passed in through macros.
# These are denoted by a '%' sign with any name you choose
.macro print_int(%num)
li $v0, 1
lw $a0, %num
syscall
.end_macro
li $t0, 1
print_int($t0)
# We can also pass in immediates for macros
.macro immediates(%a, %b)
add $t0, %a, %b
.end_macro
immediates(3, 5)
# Along with passing in labels
.macro print(%string)
la $a0, %string
li $v0, 4
syscall
.end_macro
print(hello_world)
## ARRAYS ##
.data
list: .word 3, 0, 1, 2, 6 # This is an array of words
char_arr: .asciiz "hello" # This is a char array
buffer: .space 128 # Allocates a block in memory, does
# not automatically clear
# These blocks of memory are aligned
# next to each other
.text
la $s0, list # Load address of list
li $t0, 0 # Counter
li $t1, 5 # Length of the list
loop:
bge $t0, $t1, end_loop
lw $a0, ($s0)
li $v0, 1
syscall # Print the number
addi $s0, $s0, 4 # Size of a word is 4 bytes
addi $t0, $t0, 1 # Increment
j loop
end_loop:
## INCLUDE ##
# You do this to import external files into your program (behind the scenes,
# it really just takes whatever code that is in that file and places it where
# the include statement is)
.include "somefile.asm"Last updated