FIX: MASM 6.0 May Generate Incorrect Code for IMUL Instruction (72708)

The information in this article applies to:

Microsoft Macro Assembler (MASM) 6.0

This article was previously published under Q72708

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

Modification Type:	Minor	Last Reviewed:	10/30/2003
Keywords:	kbfix KB72708