FIX:EXTRN ABS Doesn't Work As Software Interrupt Number

MS-DOS     | OS/2

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 5.1 and 5.1a
• Microsoft Macro Assembler for OS/2, versions 5.1 and 5.1a

SYMPTOMS

In the Microsoft Macro Assembler (MASM) versions 5.1 and 5.1a or QuickAssembler versions 2.01 and 2.51, using an absolute external symbol as a software interrupt number fails to generate the correct number at link time. The module accessing this PUBLIC symbol does not generate the proper number; instead, it gets set to 0. This is a problem only when the symbol is used with the INT instruction. In MASM version 5.1, using an absolute external symbol causes the linker to generate:

   error L2029: Unresolved externals.

RESOLUTION

You can work around this problem by using an equate declared in an include file rather than relying on an absolute external. The include file that contains this equate can be included in all the source files that need to access the equate. This allows the assembler to determine the value of the symbol, instead of relying on the linker to provide the value.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 5.1 and 5.1a and QuickAssembler versions 2.01 and 2.51 (buglist2.01 and buglist2.51). This problem was corrected in MASM version 6.0.

MORE INFORMATION

The following code demonstrates the incorrect result. To reproduce this error, assemble both HELLO.ASM and DISPLAY.ASM with MASM 5.1a, then link together using LINK version 5.01.20.

Sample Code

; HELLO.ASM ; Assemble options needed: none

.MODEL small, c DOSSEG

PUBLIC mydata mydata EQU 21h

.STACK 256

.CODE EXTRN display:proc start:

    call display

    mov ax, 04C00h
    int 21h

; DISPLAY.ASM ; Assemble options needed: none

.MODEL small, c DOSSEG

EXTRN mydata:ABS

.CODE display PROC

    mov ah,mydata ; Gets set to 21h by the linker
    int mydata    ; Doesn't get set to 21h by the linker
                  ; Results in Divide overflow
                  ; Results in Illegal Instruction under CodeView
    ret