FIX: MASM 6.0 May Generate Incorrect Code for IMUL Instruction

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, version 6.0
• Microsoft Macro Assembler for OS/2, 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

    mov ax, 4C00h
    int 21h