Q72708: FIX: MASM 6.0 May Generate Incorrect Code for IMUL Instruction

Article: Q72708
Product(s): Microsoft Macro Assembler
Version(s): 6.0
Operating System(s): 
Keyword(s): 
Last Modified: 04-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), version 6.0 
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SYMPTOMS
========

The Microsoft Macro Assembler (MASM) version 6.0 will generate incorrect code
when the IMUL instruction contains an operand that is a constant value from 128
to 255. No errors or warnings are displayed while assembling.

CAUSE
=====

The assembler incorrectly treats the constant as an 8-bit signed value rather
than a signed word. In converting from a signed byte to a signed word, it
extends the sign bit through the upper byte. Thus, 255 (0ffh) becomes -1
(0ffffh).

RESOLUTION
==========

To work around this problem, qualify the constant as a word. For example:

     IMUL di,WORD PTR 255          ; This works.

STATUS
======

Microsoft has confirmed this to be a problem in MASM version 6.0. This problem
was corrected in MASM version 6.0a.

MORE INFORMATION
================

The sample code below demonstrates this problem. The result of the IMUL
instruction is -1.

Sample Code
-----------

  ; Assemble options needed: none

  .MODEL small
  .286

  .CODE
  main PROC
      mov di, 1

      imul di, 255
  ;   imul di, WORD PTR 255 ; <--- This works.

      mov ax, 4C00h
      int 21h
  main ENDP
  END main

Additional query words: 6.00 buglist6.00 fixlist6.00a

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600
Version           : :6.0
Solution Type     : kbfix

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