汇编-复制字节数组-MIPS

Question

First of all, I have already checked related questions to this one, yet I am still not able to overcome the problem I have with this program.

What I am trying to do is, basically, take a byte[] input and duplicate it to another byte[], and print the duplicate array. My code is as above:

.data
hello: .asciiz "hello"
inp: .byte 5
dup: .byte 5

.text

main:
    la $a0, inp #get input
    li $v0, 8
    syscall

    la $s0, dup #load arrays on s0 and s1
    la $s1, inp     

    li $t0, 0   #instantiate offsets as 0
    li $t2, 0
Load:
    lb $t1, 0($s1)      #load first byte
    sub $t1, $t1, 48    #test if it is <0   
    bltz, $t1, exit     #if so go to exit
    add $t1, $t1, 48

    sb $t1, 0($s0)      #else save the byte
    add $s1, $s1, 1     #increment offsets
    add $s0, $s0, 1

    j Load

    la $a0, hello
    li $v0, 4
    syscall

exit:
    li $t1, 0
    add $s0, $s0, 1
    sb $t1, 0($s0)  #add null to the end of dup
    la $a0, dup
    li $v0, 4
    syscall

    jr $ra

I am new to MIPS and, I am not able to recognize what the problem is.

By the way, I am passing 123 as an input and I am getting countless of 1s as output, which tells me that I am stuck in the loop and never getting any further in $s1 (inp).

Answer 1

There are a couple of problems with your code:

First, .byte 5 doesn't reserve space for 5 bytes, it declares a single byte with the value 5. If you want 5 bytes you should say .space 5 (the bytes will be initialized with the value 0 IIRC).

Second, syscall 8 takes one more argument; $a1 = maximum number of characters to read , which you haven't specified. If you have room for 5 bytes in your buffer you should set $a1 to 5. Note that "maximum number of characters to read" actually means "maximum number of characters to read including the terminating null-character" .