8086 undefined opcode?

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Artlav

Joined: 23 Dec 2004
Posts: 188
Location: Moscow, Russia

Artlav 23 Dec 2009, 16:34

I've been looking around for some time, but can't find the answer for less than 286 - what will a 8086 CPU do if it hits an undefined opcode?

23 Dec 2009, 16:34

DOS386

Joined: 08 Dec 2006
Posts: 1904

DOS386 23 Dec 2009, 16:56

Artlav wrote:

I've been looking around for some time, but can't find the answer for less than 286 - what will a 8086 CPU do if it hits an undefined opcode?

There is NO illegal instruction exception on 8086 - it will do strange stuff, most likely just hang Sad

SSSSSE5 on 8086 ? No problem, just decode to POPE CS and hang Sad

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Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

23 Dec 2009, 16:56

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4623
Location: Argentina

LocoDelAssembly 23 Dec 2009, 17:05

Quote:

No problem, just decode to POPE CS and hang

This pope thing again... (not everybody know that's your way of calling POP Smile

)

23 Dec 2009, 17:05

chaoscode

Joined: 21 Nov 2006
Posts: 64

chaoscode 27 Dec 2009, 03:20

what is POPE?

27 Dec 2009, 03:20

windwakr

Joined: 30 Jun 2004
Posts: 827

windwakr 27 Dec 2009, 03:26

chaoscode wrote:

what is POPE?

It's the way DOS386 defines pop. I think it's just so push and pop are the same length for him, lol.

EDIT: Loco says it right above you anyways.

LocoDelAssembly wrote:

(not everybody know that's your way of calling POP )

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27 Dec 2009, 03:26

Tyler

Joined: 19 Nov 2009
Posts: 1215
Location: NC, USA

Tyler 27 Dec 2009, 03:29

Chaoscode wrote:

what is POPE?

LocoDelAssembly wrote:

[DOS386's] way of calling POP

@DOS386: What do you mean by "hang"? Sounds destructive.

27 Dec 2009, 03:29

DOS386

Joined: 08 Dec 2006
Posts: 1904

DOS386 27 Dec 2009, 03:34

windwakr wrote:

so push and pop are the same length for

right

27 Dec 2009, 03:34

sinsi

Joined: 10 Aug 2007
Posts: 794
Location: Adelaide

sinsi 27 Dec 2009, 03:36

A lot of opcodes use 0Fh as the prefix byte - unfortunately to an 8088/8086 this is the 'pop cs' instruction.
Your computer will hang or reboot since CS has changed...
Maybe you jump into the middle of a DOS write-to-disk function and trash your C:

push/pope - why not use psh/pop ?

27 Dec 2009, 03:36

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 27 Dec 2009, 03:40

Why not just push/pop Question

27 Dec 2009, 03:40

Fanael

Joined: 03 Jul 2009
Posts: 168

Fanael 27 Dec 2009, 10:43

revolution wrote:

Why not just push/pop

And why not just stmdb/ldmia?

27 Dec 2009, 10:43

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 27 Dec 2009, 11:20

ldmfd/stmfd

27 Dec 2009, 11:20

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 28 Dec 2009, 09:08

Artlav wrote:

I've been looking around for some time, but can't find the answer for less than 286 - what will a 8086 CPU do if it hits an undefined opcode?

Accroding to the manual, 186/188 generates int 6 if it executes 0F, 63-67, F1, FE /7, or FF/7. (Don't know why it doesn't list FE /2-6 and few others.)

28 Dec 2009, 09:08

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