1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
|
---
language: "MIPS Assembly"
filename: MIPS.asm
contributors:
- ["Stanley Lim", "https://github.com/Spiderpig86"]
---
The MIPS (Microprocessor without Interlocked Pipeline Stages) Assembly language
is designed to work with the MIPS microprocessor paradigm designed by J. L.
Hennessy in 1981. These RISC processors are used in embedded systems such as
gateways and routers.
[Read More](https://en.wikipedia.org/wiki/MIPS_architecture)
```assembly
# 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 firsts
# Let's print "hello world"
la $a0, hello_world # Load address of string stored in
# memory
li $v0, 4 # Load the syscall value (indicating
# type of functionality)
syscall # Perform the specified syscall with
# the given argument ($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 versons 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 backwards, 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 # Instruction that sends a signal when
# $t0 < $t1 with reuslt 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
# 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, $s1 # 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
```
|
