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alkap



Joined: 18 Feb 2015
Posts: 44
Location: Dnipro, Ukraine
alkap
I have noticed that the 'display_string' function, defined in 'system.inc' does not use the CLD instruction. The said instruction does not seem to be used in 'fasm.asm' either.
Is that because fasm clears the direction flag implicitly in some way, or is there some other explanation?
Thanks.
Post 05 Feb 2016, 18:36
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7796
Location: Kraków, Poland
Tomasz Grysztar
fasm relies on the convention that DF flag remains cleared by default. So every time fasm changes the direction flag to 1 for some reason (with STD), it clears it back with CLD immediately after.

In DOS interface (SOURCE/DOS/FASM.ASM) you may notice that there is a single CLD during the startup and then DF remains 0 for the entire life of program. In modern systems the DF is already by convention cleared as default, so Windows/Linux versions do not even have that initial CLD. You may find this mention in the Anger Fog's document on calling conventions:
Anger Fog wrote:
The rules for the direction flag is the same in all systems. The direction flag is cleared by default. If the direction flag is set, then it must be cleared again before any call or return.
Post 05 Feb 2016, 19:57
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alkap



Joined: 18 Feb 2015
Posts: 44
Location: Dnipro, Ukraine
alkap
Thanks very much, Tomasz, for a prompt reply. I use fasm on Linux. The explanation you have given makes perfect sense.
Post 05 Feb 2016, 20:04
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