DOCUMENT:Q72708  04-MAY-2001  [masm]
TITLE   :FIX: MASM 6.0 May Generate Incorrect Code for IMUL Instruction
PRODUCT :Microsoft Macro Assembler
PROD/VER::6.0
OPER/SYS:
KEYWORDS:

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

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

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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