Switching tasks and global protection fault

[mrjbom]

Code: Select all

  pop dword [esp + 48] ;pop cs
  pop word [esp + 32] ;pop eip

Functions clobber the arguments you push onto the stack. Use "add esp, 8" or "pop eax; pop eax" to clean up the stack after the function call.

Why should I use add esp, 8? Shouldn't I use add esp, 6? After all, eip and cs take up 6 bytes.

[nexos]

CS and EIP are 32 bits, just like every other register on IA32.

[mrjbom]

CS and EIP are 32 bits, just like every other register on IA32.

Here it is, thanks for the information, I did not expect.

[bzt]

Here it is, thanks for the information, I did not expect.

Plus, all values on the stack must be aligned. You could use a MOV with unaligned SP relative addresses, but PUSH/POP can't operate otherwise. This becomes problematic a bit when you start pushing SIMD registers which are larger than 4 bytes and require larger alignments as well. You have to be careful in your Assembly code (the C compiler aligns values properly for you, but in Assembly it's up to you).

Cheers,
bzt

[mrjbom]

Here it is, thanks for the information, I did not expect.

Plus, all values on the stack must be aligned. You could use a MOV with unaligned SP relative addresses, but PUSH/POP can't operate otherwise. This becomes problematic a bit when you start pushing SIMD registers which are larger than 4 bytes and require larger alignments as well. You have to be careful in your Assembly code (the C compiler aligns values properly for you, but in Assembly it's up to you).

Cheers,
bzt

Yes, I understand that it is better to use push/pop than to manage the stack manually.