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flat assembler > Linux > Trying to pass a structure to registry via stack; linux, x64

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jorido



Joined: 23 Jan 2017
Posts: 53
Trying to pass a structure to registry via stack; linux, x64
There's a function which accepts a struct and its length as its arguments. I'm trying to pass a struct to it via registers, without using "struc", but manually:


Code:

  ;....................

  push 3; 3rd field of a struct
  push 2; 2nd field of a struct
  push 1; 1st field of a struct
  mov rdirsp; load a struct ---(dw, dw, dw) --- into rdi
  mov rsi6 ; its size dw + dw + dw = 6

  ;......................
  syscall




The app isn't working as expected. Is there a mistrake in the way of how I'm passing a struct via stack?
Post 13 Oct 2017, 05:04
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ProMiNick



Joined: 24 Mar 2012
Posts: 88
Location: Russian Federation, Sochi
In AMD64 mode there is no 16 bit push.

Why not just: mov rdi,1+2 shl 16 + 3 shl 32?
with pushes it will be:
push 3
push 1+2 shl 16
mov rdi,rsp
Post 13 Oct 2017, 13:23
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6633
Location: Kraków, Poland

ProMiNick wrote:
In AMD64 mode there is no 16 bit push.

The original AMD64 architecture had 16-bit PUSH in long mode (as a side effect of the prefix-opcode symmetries) and it was a documented behavior*. Later, when Intel adopted the same architecture, it removed these atypical encoding variants (I don't blame them).

You can still get fasm to generate the 16-bit variants in long mode with the PUSHW mnemonic. That said, I would not recommend even trying this route. Probably only the earliest CPUs implementing x86-64 architecture really supported this.

___
* I just opened the original x86-64 manuals to look for this just to make sure that I have no false memory of it. I'm attaching the excerpt I found - the PUSH instruction is listed earlier in the table, but the note below the table is what I wanted to show. This may be one of the first time I opened these paper manuals in ages, the last time I was really reading them was back in 2004 when working on x86-64 support for fasm.


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amd64_16bit.jpg


Post 13 Oct 2017, 14:01
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