ID Number: Q81888
5.10 5.10a | 5.10 5.10a
MS-DOS | OS/2
buglist5.10 buglist5.10a fixlist6.00
Summary:
PROBLEM ID: MSM9203001
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.
RESOLUTION
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 reference words: 5.10 5.10a 6.00