Save 32bits from a 64bit register in x64 env

Question

I'm getting back to my previous topic, but with nothing more than a few thoughts.

I want to store a 32-bit integer in a matrix in assembler in x64. Let's say I have:

int** matrix = ...;
matrix[x][y] = 0;

Normally I'd simply calculate a position to which I would like to save by doing:

mov rax, [rdi + rsi * 8]
mov [rax + rdx * 4], rcx

My question is: how can I align rcx to save only 4-bytes? Or maybe I do save 4-bytes only and there is no chance to overwrite some other integer next to the one to overwrite?

score 3 · Answer 1 · answered Mar 25 '13 at 13:40

3

Use ecx instead of rcx.

That's all.

answered Mar 25 '13 at 13:40

Alexey Frunze

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But rcx is a parameter of the "function" I call to make the assignment. Does it mean ecx stands for the 32 bits of the register? – user767849 Mar 25 '13 at 13:43
1

OMG, you don't even know that ecx is part of rcx? – Alexey Frunze Mar 25 '13 at 13:44
Nop? Quite new to asm and x64. Sorry to say. – user767849 Mar 25 '13 at 13:45
3

@user767849 yes, ecx is the mnemonic for the 32 lower bits, like cx is for the 16 LSbits, like bl is for the 8 LSbits – Lorenzo Dematté Mar 25 '13 at 13:46
Thanks, dema80. That's how you answer without some weird nerdy rage. – user767849 Mar 25 '13 at 13:49
1

@user767849 There's an illustration of amd64 registers on this page: http://freecode4freedom.blogspot.de/2009/12/porting-32-bit-appications-to-64-bit-in.html The last picture on the page shows how parts of registers are addressed. – us2012 Mar 25 '13 at 14:43
Thanks. Will see into that! – user767849 Mar 25 '13 at 18:17

Save 32bits from a 64bit register in x64 env

1 Answers1