I have a function string_length that has the following assembly code
0x08048e90 <+0>: push %ebp
0x08048e91 <+1>: mov %esp,%ebp
0x08048e93 <+3>: mov 0x8(%ebp),%edx // assign whatever I declared into edx
0x08048e96 <+6>: mov $0x0,%eax // assign eax = 0
0x08048e9b <+11>: cmpb $0x0,(%edx) // compare edx to byte of 0 (null..?)
0x08048e9e <+14>: je 0x8048ea9 <string_length+25> // if equal, jump to +25
0x08048ea0 <+16>: add $0x1,%eax // else, add 1 to eax
0x08048ea3 <+19>: cmpb $0x0,(%edx,%eax,1) // compare byte 1*eax+edx with 0,
0x08048ea7 <+23>: jne 0x8048ea0 <string_length+16> // if not equal, back to +16
0x08048ea9 <+25>: pop %ebp // pop ebp
0x08048eaa <+26>: ret
Since the function name is string_length, I am assuming it will return how many characters is in the string.
what I am confused about is the
cmpb $0x0,(%edx)
is this comparing whatever is pointed to edx to the byte of 0, and 0 in ASCII is null..?
and
cmpb $0x0,(%edx,%eax,1)
is comparing, in bytes, 1*eax+edx. If edx is a string, does that mean edx will first be converted its ascii value and then perform the calculation?
This:
cmpb $0x0,(%edx)
takes a byte that EDX points to (i. e. contains the address of) and compares it to zero. This:
cmpb $0x0,(%edx,%eax,1)
takes a byte that EDX+EAX points to and compares it to zero. EDX serves as the string base pointer, EAX is the index. Scale is 1 because we're working with bytes. Think of the whole loop this way: for(eax=0; edx[eax] != 0; eax++)
.
The equivalent C code would be something like this:
int string_length(const char *edx)
{
int eax = 0;
while (edx[eax] != NULL) eax++;
return eax;
}
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