I am trying to get my head around some inline ASM but for some reason this isn't behaving as I would have expected. Why isn't this setting the value of x equal to 62?
#include <stdio.h>
int main()
{
int x = 525;
int* y = &x;
_asm
{
mov eax, 62
mov [y], eax
}
printf("%i", x);
getchar();
return 0;
}
The code results in 525 being output. I expected it to be 62.
There's a perfectly excusable misunderstanding here:
surely [y] would mean [0xCCCCCCCC] (assuming the address of x was 0xCCCCCCCC)
In high-level theory, yes. The trouble is, in actual assembly [0xCCCCCCCC] makes no sense - the CPU can't dereference a memory address directly - it can only load a value from that address into a register, then dereference that.
Similarly, since y is a variable, not a register, it's implicitly treated as an address1 i.e. y inside the asm block is the equivalent of &y in the C code outside2. As you can see by stepping through in a debugger, what happened is the assembler simply ignored the brackets that don't make sense (rather than throwing a helpful error) and assembled the equivalent of mov y, eax.
The way to get what you expect would be something like this:
asm {
mov eax, 62
mov edx, y
mov [edx], eax
}
[1] this clearly isn't GCC. GCC extended asm is a whole different ball game...
[2] somewhat of a simplification - it's an "address" from a C point of view, but in assembly context it's a memory operand, which is really more like the use of an address. When I compiled this, y came out as [ebp-20], which from the high level view is "an address on the stack".
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