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Q81888: FIX: MASM Incorrectly Allows Relative Values as Constant

Article: Q81888
Product(s): Microsoft Macro Assembler
Version(s): 5.1,5.1a
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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

- Microsoft Macro Assembler (MASM), versions 5.1, 5.1a 
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SYMPTOMS
========

The Microsoft Macro Assembler (MASM) versions 5.1 and 5.1a incorrectly allow
relative expressions to be used as constant expressions if the relative
expression is used inside an IF1 directive. Instead of generating an error, the
assembler uses the current value of the relative expression as a constant
expression.

STATUS
======

Microsoft has confirmed this to be a problem in MASM versions 5.1 and 5.1a. This
problem was corrected in MASM version 6.0.

MORE INFORMATION
================

A constant expression is a numeric expression that can be resolved at assembly
time. A relative expression is a expression that cannot be resolved at assembly
time and must be resolved at link time. Relative expressions include all data
and code labels, including the current location counter, $. Anything that
generates a fixup record is a relative expression.

MASM 6.0 correctly generates the following error when a relative expression is
used in a place where only a constant is allowed:

  error A2026: constant expected

Converting MASM 5.1 programs to MASM 6.0 programs that rely on this problem in
MASM 5.1 may be difficult. The difference between two relative expressions is a
constant expression, and this may be used to replace the single relative
expression. The sample code below demonstrates this workaround. This solution
may not be valid in all instances. Since MASM 6.0 evaluates these types of
expressions on the first pass only, the constant value may not be what was
expected if subsequent passes change the values of the relative expressions.

In MASM 6.0, the IF1 directive is always considered true. The IF2 directive is no
longer supported in MASM 6.0.

Sample Code
-----------

  ; Assemble options needed: none

  .MODEL small

  .STACK

  .CODE
  start: mov     ax, @data
         mov     ds, ax

         mov     ax, 4C00h
         int     21h

  IF1

  ;The following line causes an A2026 error in MASM 6.0.
  ProgSize       CATSTR   <Program Size is >, %$

  ;The following line assembles error free in MASM 5.1 and MASM 6.0.
  ;ProgSize       CATSTR    <Program Size is >, %($-start)

  ENDIF

  %      %OUT    ProgSize

  END    start

Additional query words: 5.10 5.10a 6.00 buglist5.10 buglist5.10a fixlist6.00

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM510a
Version           : :5.1,5.1a
Solution Type     : kbfix

=============================================================================

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