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how write large number of bytes?

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reverser69
Expert Cheater

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Joined: 03 Sep 2014
Posts: 112

Posted: Thu Feb 04, 2021 5:42 pm Post subject: how write large number of bytes?

hi all i want to write 200 bytes in [rsi+rax+XXX] location. any help?[/code]

ParkourPenguin
I post too much

Reputation: 152

Joined: 06 Jul 2014
Posts: 4711

Posted: Thu Feb 04, 2021 6:54 pm Post subject:

Example C++ code:

Code:

#include <cstring>

void foo(char *a, char *b) {
std::memcpy(a, b, 200);
}

gcc unrolls it to this:

Code:

vmovdqu xmm0,[rsi]
vmovdqu [rdi],xmm0
vmovdqu xmm1,[rsi+16]
vmovdqu [rdi+16],xmm1
vmovdqu xmm2,[rsi+32]
vmovdqu [rdi+32],xmm2
vmovdqu xmm3,[rsi+48]
vmovdqu [rdi+48],xmm3
vmovdqu xmm4,[rsi+64]
vmovdqu [rdi+64],xmm4
vmovdqu xmm5,[rsi+80]
vmovdqu [rdi+80],xmm5
vmovdqu xmm6,[rsi+96]
vmovdqu [rdi+96],xmm6
vmovdqu xmm7,[rsi+112]
vmovdqu [rdi+112],xmm7
vmovdqu xmm0,[rsi+128]
vmovdqu [rdi+128],xmm0
vmovdqu xmm1,[rsi+144]
vmovdqu [rdi+144],xmm1
vmovdqu xmm2,[rsi+160]
vmovdqu [rdi+160],xmm2
vmovdqu xmm3,[rsi+176]
vmovdqu [rdi+176],xmm3
mov rax,[rsi+192]
mov [rdi+192],rax
ret

(sysv abi) (edit: offsets are in decimal here; note CE uses hexadecimal)
Use lea to make addressing modes easier.

If you don't want to unroll it, you could make a for loop.
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Last edited by ParkourPenguin on Thu Feb 04, 2021 8:29 pm; edited 1 time in total

Dark Byte
Site Admin

Reputation: 471

Joined: 09 May 2003
Posts: 25820
Location: The netherlands

Posted: Thu Feb 04, 2021 7:51 pm Post subject:

Code:

push rsi
push rdi
push rcx

lea rdi,[rsi+rax+XXX]
lea rsi,sourcebytes
mov rcx,#25 //25*8=200
db f3 48 a5 //rep movsq (apparently CE doesn't know how to assemble this, fixed in next version)

pop rcx
pop rdi
pop rsi

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reverser69
Expert Cheater

Reputation: 0

Joined: 03 Sep 2014
Posts: 112

Posted: Fri Feb 05, 2021 8:47 am Post subject:

thanks. both of you.

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