[solved] imul ebx,2

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ouadji

Joined: 24 Dec 2008
Posts: 1081
Location: Belgium

ouadji 08 Sep 2011, 17:55

the fasm compiler allows "imul ebx,2" Shocked

(result after compiling: "imul ebx,ebx,2")

the two operand form of imul does not allow an immediate value as second operand.
that said, there is an error about this in the intel doc.

Quote:

Two-operand form — With this form the destination operand (the first
operand) is multiplied by the source operand (second operand). The destination
operand is a general-purpose register and the source operand is an immediate
value, a general-purpose register, or a memory location. The product is then
stored in the destination operand location.

... and the source operand is an immediate value <--- false,
a general-purpose register, or a memory location.

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Last edited by ouadji on 08 Sep 2011, 21:57; edited 1 time in total

08 Sep 2011, 17:55

AsmGuru62

Joined: 28 Jan 2004
Posts: 1624
Location: Toronto, Canada

AsmGuru62 08 Sep 2011, 19:51

Intel Manual lists these instructions:

IMUL r32,r32,imm8
IMUL r32,imm8

And both of these have exact same encoding.
Load OLLYDBG (or other debugger) and try these two.
You will see the same instruction bytes for both.

08 Sep 2011, 19:51

ouadji

Joined: 24 Dec 2008
Posts: 1081
Location: Belgium

ouadji 08 Sep 2011, 20:28

Quote:

Intel Manual lists these instructions:
IMUL r32,r32,imm8
IMUL r32,imm8 <--- ?

i don't see anything about "IMUL r32,immediate value"

Volume 2A - Instruction Set Reference, A-M
253666-038US - April 2011

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08 Sep 2011, 20:28

AsmGuru62

Joined: 28 Jan 2004
Posts: 1624
Location: Toronto, Canada

AsmGuru62 08 Sep 2011, 21:46

It is been removed from the latest manual, but my old manual from 2007 still has it.

I am thinking that Intel removed it, because it is the same code as IMUL r32,r32,imm8. They removed redundant information.

It properly compiles into longer version - there is no bug(s) here. Not in manuals and not in FASM itself.

08 Sep 2011, 21:46

ouadji

Joined: 24 Dec 2008
Posts: 1081
Location: Belgium

ouadji 08 Sep 2011, 21:56

ok, understood, thank you AsmGuru62

edit: IMUL r32, imm32 works too.

Code:

;http://www.ousob.com/ng/iapx86/ngce71.php

    69 /r iw    IMUL r16,imm16          ; 186+
    69 /r iw    IMUL r16,r/m16,imm16    ; 186+
    69 /r id    IMUL r32,imm32          ; 386+ <-----
    69 /r id    IMUL r32,r/m32,imm32    ; 386+

    6B /r ib    IMUL r16,imm8           ; 186+
    6B /r ib    IMUL r16,r/m16,imm8     ; 186+
    6B /r ib    IMUL r32,imm8           ; 386+ <-----
    6B /r ib    IMUL r32,r/m32,imm8     ; 386+

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08 Sep 2011, 21:56

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