I am writing a simple MIPS assembly code that iterates through characters of a string
and finds an instance of a given substring..
First, the c code:
for (i=0; i<length(text)-length(sub_string); i++) {
match = TRUE
for (j=0; j<length(sub_string); j++)
if (text[i+j] != sub_string[j]) {
match = False;
break;
}
}
And this is part of my MIPS code.
.text
# a0: text
# a1: substring
# v0: count
myfunction:
move $s0, $ra
move $a2, $a0
jal strlen
move $t1, $v1 # text_len
move $a2, $a1
jal strlen
move $t2, $v1 # substring_len
sub $t1, $t1, $t2 # text_len - substring_len
li $t0, 0 # i = 0
j i_test
i_body:
li $t3, 1 # match = TRUE
li $t4, 0 # j = 0
j j_test
i_test:
blt $t0, $t1, i_body
move $ra, $s0
jr $ra
j_body:
add $t6, $a0, $t0 # &text[i]
add $t6, $t6, $t4 # &text[i+j]
lb $t6, 0($t6) # text[i+j]
add $t7, $a1, $t4 # &sub_string[j]
lb $t7, 0($t7) # sub_string[j]
beq $t6, $t7, j_skip # if (text[i+j] == sub_string[j])
# if (text[i+j] != sub_string[j])
li $t3, 0 # match = FALSE
j j_break # break
j_test:
blt $t4, $t2, j_body
j_skip:
addi $t4, $t4, 1 # j++
j j_body
j_break:
addi $t0, $t0, 1 # i++
j i_body
strlen:
li $v1, 0 # i = 0
b strlen_test
strlen_body:
addi $v1, $v1, 1 # len++
strlen_test:
add $t0, $a2, $v1 # &str[i]
lb $t0, 0($t0) # str[i]
bne $t0, $0, strlen_body # loop
jr $ra
.globl main
main:
la $a0, text1
la $a1, substring1
jal myfunction
la $a0, text1
jal print_string
li $v0, 10 # Exit
syscall
print_string:
li $v0, 4
syscall
la $a0, newline
li $v0, 4
syscall
jr $ra
.data
text1: .asciiz "Hello, world!"
substring1: .asciiz "lo"
newline: .asciiz "\n"
However, when I run this code, I am getting this infinite loop of bad
address errors:
Exception occurred at PC=0x00400070
Bad address in data/stack read: 0x10021778
Exception 7 [Bad address in data/stack read] occurred and ignored
Exception occurred at PC=0x00400078
Bad address in data/stack read: 0x10021786
Exception 7 [Bad address in data/stack read] occurred and ignored
Exception occurred at PC=0x00400070
Bad address in data/stack read: 0x10021779
Exception 7 [Bad address in data/stack read] occurred and ignored
Exception occurred at PC=0x00400078
Bad address in data/stack read: 0x10021787
Exception 7 [Bad address in data/stack read] occurred and ignored
Exception occurred at PC=0x00400070
Bad address in data/stack read: 0x1002177a
Exception 7 [Bad address in data/stack read] occurred and ignored
Exception occurred at PC=0x00400078
Bad address in data/stack read: 0x10021788
Exception 7 [Bad address in data/stack read] occurred and ignored
Exception occurred at PC=0x00400070
Bad address in data/stack read: 0x1002177b
Exception 7 [Bad address in data/stack read] occurred and ignored
Exception occurred at PC=0x00400078
Bad address in data/stack read: 0x10021789
Exception 7 [Bad address in data/stack read] occurred and ignored
I realized the bad address error is coming from the 3rd and 5th lines of
j_body (the load byte instructions). I've initialized the string address, so I am not sure why this is
happening...
Here are my questions:
Why am I getting this infinite loop? (I thought I've implemented the for loops for i and j correctly)
Why am I getting a bad address error??
Thanks :)
j_test:
blt $t4, $t2, j_body
j_skip:
addi $t4, $t4, 1 # j++
j j_body
j_break:
addi $t0, $t0, 1 # i++
j i_body
Shouldn't you be jumping to j_break if the condition for j_test fails? The way this code is written right now you proceed with executing the body of the "j loop" even if j >= strlen(substring).
Also, the j i_body after j_break: should probably be j i_test (you've increased i, so you'll want to check if condition for the "i loop" still is true).