Why when mul al is ah?

Index > Programming Language Design > Why when mul al is ah?

Author

Thread

xverizex

Joined: 24 Jul 2024
Posts: 3
Location: Russia

xverizex 24 Jul 2024, 22:05

I'm writing emulation of i386. I encounter with the problem. R/M byte of mul operation is not fit logically.

Code:

            0000:0000      31c0           xor ax, ax
            0000:0002      31c9           xor cx, cx
            0000:0004      0480           add al, 0x80
            0000:0006      80c104         add cl, 4
            0000:0009      f6e1           mul cl
            0000:000b      c3             ret

As you can see, in operation mul cl it bytes f6 e1.
f6 is mul byte operation.
e1 - 1110 0001
e1 is - mod 11 is the registers. 100 is ah, 001 is cl. Why mul operation al cl looks like mul operation ah cl?

24 Jul 2024, 22:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20430
Location: In your JS exploiting you and your system

revolution 24 Jul 2024, 23:12

The "middle" bits are not the register, they encode the instruction.

;...
011 = neg <-- 0xf6 0xd8 + reg
100 = mul <-- 0xf6 0xe0 + reg
101 = imul
110 = div
111 = idiv

24 Jul 2024, 23:12

xverizex

Joined: 24 Jul 2024
Posts: 3
Location: Russia

xverizex 24 Jul 2024, 23:15

revolution wrote:

The "middle" bits are not the register, they encode the instruction.

;...
011 = neg <-- 0xf6 0xd8 + reg
100 = mul <-- 0xf6 0xe0 + reg
101 = imul
110 = div
111 = idiv

Oh thanks. I don't find this information in documentation i386. Maybe I miss one. Can you take mind where this is described?

24 Jul 2024, 23:15

xverizex

Joined: 24 Jul 2024
Posts: 3
Location: Russia

xverizex 24 Jul 2024, 23:36

revolution wrote:

The "middle" bits are not the register, they encode the instruction.

;...
011 = neg <-- 0xf6 0xd8 + reg
100 = mul <-- 0xf6 0xe0 + reg
101 = imul
110 = div
111 = idiv

I figured out.
I attached screenshot. F6 /4 . /4 is a 100 bit. Thanks. Now, I figured out.

Description:
Filesize:	2.3 KB
Viewed:	1137 Time(s)

24 Jul 2024, 23:36

< 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