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