Pointers in assembly language

Question

I am trying to understand how to use pointer in assembly. By reading some tutorials around internel,I think had undertantood some concepts. But when I'II go to try it,it did work. Below some attempts to translate C to ASM.

C

const char *s = "foo";
unsigned z = *(unsigned*)s;
if(!(z & 0xFF))
do_something();
if(!(z & 0xFFFF))
 do_b_something();

(here's not full code,but it's a word-check,thefore,there is more two stmts which checks 0xFF0000,0xF000000 respectivily.

ASM:

mov ebp,str
mov eax,ebp

mov eax,[eax]
and eax,0xFF
cmp eax,0
je etc

mov eax,[eax]
and eax,0xFFFF
cmp eax,0
je etc

It returns a seg fault.

And the try:

mov eax,dword ptr [eax]

that's generated by gcc compiler and you can see it in some other assemblies code,returns

invalid symbol

on FASM assembler. It isn't really supported by the FASM or am I missing something?

Answer 1

I think this is what you are attempting to do:

    mov  ebp,str
    mov  eax,ebp

    mov  ebx,[eax]
    test ebx,0xFF
    jz   low_byte_empty

    do_something:
         ; some code here...

low_byte_empty:
    test ebx,0xFFFF
    jz   low_word_empty

    do_b_something:
         ; some code here.

low_word_empty:

Explanation:

First, as JasonD already mentions in his answer, you are loading a pointer to eax, then doing a logical and to it, then you are using the result still in eax to address memory (some memory offset in the range 0x0 ... 0xFF).

So what goes wrong in your code: you can't keep in the same register both a pointer to a memory address and a value at the same time. So I chose to load the value from [eax] to ebx, you can also use some other 32-bit general register (ecx, edx, esi, edi) according to your needs.

Then, you don't need to use cmp to check if a register is empty, because all cmp does is that it does the subtraction and sets the flags. But ZF (zero flag) is already set by and, so cmp is absolutely unnecessary here. Then, as cmp is not needed here and we do not need the result either, we only want to update the flags, it's better to use test. test does exactly the same logical AND as and does, the only difference being that test does not store the result, it only updates the flags.

Answer 2

It's not at all clear what you're trying to do in the original code - doesn't look right.

However this:

mov eax,[eax]
and eax,0xFF
cmp eax,0
je etc

mov eax,[eax]

Isn't going to work. You're overwriting the contents of EAX with the value stored at the address in EAX, manipulating that value, and then trying to reload it after the branch without restoring the original pointer.

Answer 3

Following variant is simpler, smaller, faster and uses only one register.

    mov  eax, str
    mov  eax,[eax]

    test al, al
    jz   low_byte_empty

do_something_byte:
     ; some code here...

low_byte_empty:
     test ah, ah
     jz   low_word_empty         

do_something_word:
     ; some code here

low_word_empty: