fasmg bug for 8086, 80186, 80286 imul dx, cx

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Shia

Joined: 02 Aug 2025
Posts: 2

Shia 14 Nov 2025, 12:21

Assembling this code in fasmg

Code:

INCLUDE=$PATH_TO_FASMG/fasmg/packages/x86/include/cpu fasmg hello.asm

Code:

include '8086.inc'
imul dx, cx

Results in this error:

on 8086.inc

Code:

hello.asm [2]:
        imul dx, cx
imul? [0]
Error: extra characters on line.

on 80186 80286

Code:

flat assembler  version g.kp60
hello.asm [2]:
        imul dx, cx
imul? [63]
Custom error: invalid combination of operands.

Looking on https://www.felixcloutier.com/x86/imul it seems that IMUL r16, r/m16 is a valid instruction.

14 Nov 2025, 12:21

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8468
Location: Kraków, Poland

Tomasz Grysztar 14 Nov 2025, 13:28

The modern instruction set references do not have the precise information on the pre-CPUID era instruction sets. I would recommend to use a classic reference like Ralph Brown's opcodes list that has comments about the CPU generation when a specific instruction variant was introduced:

OPCODES.LST wrote:

68 PUSH imm ; NECs & 186+
69 IMUL reg,imm,mem ; NECs & 186+
6A PUSH imm8 ; NECs & 186+
6B IMUL reg,imm8,mem ; NECs & 186+

OPCODES.LST wrote:

AF IMUL reg,mem ; 386+

Or, if you were really determined, you could dig through the archives of Intel documentation for specific CPU generations and see for yourself which opcodes were introduced at each stage.

14 Nov 2025, 13:28

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8468
Location: Kraków, Poland

Tomasz Grysztar 14 Nov 2025, 13:40

BTW, with fasm2 you're going to get more informative error messages. Try this:

Code:

use i8086
imul dx, cx

I uploaded a small correction to fasm2 encoder, as I discovered a bug while testing before replying.

14 Nov 2025, 13:40

Shia

Joined: 02 Aug 2025
Posts: 2

Shia 14 Nov 2025, 13:54

Thanks

14 Nov 2025, 13:54

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