Produces opcode for "pop cs"

flat assembler
Message board for the users of flat assembler.

Index > Compiler Internals > Produces opcode for "pop cs"

Author

Thread

Dasn

Joined: 22 Nov 2004
Posts: 11

Dasn 22 Aug 2009, 13:43

Hi,
According to Intel's manual, I think "pop cs" is an illegal instruction.
But in some circumstance, fasm will generate bytes for "pop cs" other than report an error.

The following 3 test codes show this problem:

Code:

format binary
use16
pop cs

Code:

format MZ
use16
pop cs

Code:

format MZ
use32

segment main
pop cs

22 Aug 2009, 13:43

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20469
Location: In your JS exploiting you and your system

revolution 22 Aug 2009, 13:49

This has been discussed before, I can't find right now, but basically there are some old processors that do have a pop cs instruction (0x0f).

Although your example of use32/pop cs is a little troubling, any 32bit capable CPU won't have pop cs.

[edit]
I found one topic discussing it.
http://board.flatassembler.net/topic.php?t=5132
[edit2]
And another.
http://board.flatassembler.net/topic.php?t=4754

22 Aug 2009, 13:49

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 22 Aug 2009, 15:35

http://ref.x86asm.net/geek32.html#gen_note_opcd_POP_CS_0F

22 Aug 2009, 15:35

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum