Assembly Language

This page describes how gcc passes arguments, which is required when accessing those arguments from inside a piece of assembly.

Here's an example of how gcc compiled a function

00c06950 <zobbo>:

int __attribute__ ((noinline)) zobbo(uint8_t drive_num, uint32_t block_num, uint8_t *buffer) {
    drive_num++;

    if (drive_num == 42) {
  c06950:	0c2f 0029 0007 	cmpib #41,%sp@(7)
  c06956:	671e           	beqs c06976 <zobbo+0x26>
        return NOT_OK;
    }

    block_num--;

    if (block_num == 99) {
  c06958:	7064           	moveq #100,%d0
  c0695a:	b0af 0008      	cmpl %sp@(8),%d0
  c0695e:	670e           	beqs c0696e <zobbo+0x1e>
        errno = 99;
        return NOT_OK;
    }

    if (*buffer == '!') {
  c06960:	206f 000c      	moveal %sp@(12),%a0
  c06964:	0c10 0021      	cmpib #33,%a0@
  c06968:	57c0           	seq %d0
  c0696a:	49c0           	extbl %d0
        return NOT_OK;
    }

    return OK;
}
  c0696c:	4e75           	rts
        errno = 99;
  c0696e:	7063           	moveq #99,%d0
  c06970:	23c0 0000 1160 	movel %d0,1160 <errno>
        return NOT_OK;
  c06976:	70ff           	moveq #-1,%d0
}
  c06978:	4e75           	rts