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JohnnyReb



Joined: 25 Jun 2006
Posts: 8
JohnnyReb
What I would like to do is define a constant "if and only if" it has not been defined previously. I tried:

define TIMERS 1
IF ~ defined TIMERS
define TIMERS 2
end if

but the value of 2 is used.
Any help would be appreciated.

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JohnnyReb
Win if you can, Lose if you must, but always cheat!
Post 12 Aug 2006, 14:06
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UCM



Joined: 25 Feb 2005
Posts: 285
Location: Canada
UCM
This is because "if" is processed AFTER "define" and "equ" (which are processed in the preprocessing stage), so if your original code looked like
Code:
define TIMERS 1
if ~ defined TIMERS
define TIMERS 2
end if
mov eax,TIMERS
    
then it is preprocessed to
Code:
if ~ defined TIMERS
end if
mov eax,2
    
.
To make this work, then you will have to use '=' instead of 'define', because '=' is processed in the assembling stage and is affected by 'if'. However, because FASM does multiple passes, you cannot use this simple code:
Code:
TIMERS = 1
if ~ defined TIMERS
TIMERS = 2
end if
mov eax,TIMERS
    
This would work in this scenario, but if you were to not define TIMERS before the 'if' block, then it would cause an infinite loop and FASM would be unable to generate the code. (See the Design Principles article for more information.) Therefore, the solution is to use this:
Code:
TIMERS = 1
if ~ defined TIMERS | defined TIMERS_DEFINED_HERE
TIMERS = 2
TIMERS_DEFINED_HERE = 1
end if
mov eax,TIMERS
    
so that if you do not define TIMERS before the 'if'-block, it will be defined as 2.
Post 12 Aug 2006, 15:03
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7709
Location: Kraków, Poland
Tomasz Grysztar
Also see the FAQ (fourth question), and this article (for a bit longer reading).
Post 12 Aug 2006, 17:45
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