CODEWITHSUNDEEP
mips
lochnesscookie
lochnesscookie

Reputation: 197

In MIPS, how exactly are command line arguments stored in $a1?

Consider the following code, which reads the first command line argument and then prints it:

lw $a0, 0($a1)
li $v0, 4
syscall

This makes sense, because arguments are always stored in word-aligned offsets from $a1. However, supposes the first program argument is 'jklfjdsaklfjdsklfjsdklfjsklfjsklfjsdaklfjlsdkjfjfksalfjsadlkf'. There's no way this gigantic string fits into the four bytes between 0($a1) and 4($a1). Yet the code still runs and prints the string. Why?

Also how does the entire string get put into $a0? Because again it shouldn't really fit.

Upvotes: 0

Views: 886

Answers (1)

old_timer
old_timer

Reputation: 71546

char fun0 ( char *x )
{
    return(x[11]);
}
char fun1 ( char **x )
{
    return(x[1][1]);
}

mips

00000000 <fun0>:
   0:   8082000b    lb  $2,11($4)
   4:   03e00008    jr  $31
   8:   00000000    nop

0000000c <fun1>:
   c:   8c820004    lw  $2,4($4)
  10:   00000000    nop
  14:   80420001    lb  $2,1($2)
  18:   03e00008    jr  $31
  1c:   00000000    nop

arm

00000000 <fun0>:
   0:   e5d0000b    ldrb    r0, [r0, #11]
   4:   e12fff1e    bx  lr

00000008 <fun1>:
   8:   e5903004    ldr r3, [r0, #4]
   c:   e5d30001    ldrb    r0, [r3, #1]
  10:   e12fff1e    bx  lr

x86

0000000000000000 <fun0>:
   0:   0f b6 47 0b             movzbl 0xb(%rdi),%eax
   4:   c3                      retq   
   5:   90                      nop
   6:   66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   d:   00 00 00 

0000000000000010 <fun1>:
  10:   48 8b 47 08             mov    0x8(%rdi),%rax
  14:   0f b6 40 01             movzbl 0x1(%rax),%eax
  18:   c3                      retq

so when you have int main ( int argc, char *argv[]) argv is an address and the compiler passes that address. If you are working at a different level, then the operating system/environment defines that the command line string is passed that way and that would cover the "why" question. You didnt provide enough information to answer the question.

And it basically has nothing to do with mips, it has to do with an implementation of some system that is targeted to mips. The same types of implementations can be used on non-mips targets as well.

Upvotes: 1

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