From:	Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Memory writting in assembley
Date:	Mon, 11 May 1998 20:06:46 +0100
Organization:	None
Message-ID:	<GjW74CAGx0V1Ewmq@talula.demon.co.uk>
References:	<01bd7ccf$d4b5ca40$a6ef41c2 AT default>
NNTP-Posting-Host:	talula.demon.co.uk
MIME-Version:	1.0
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Dark Angel writes:
>In djgpp i used _farpokeb to write to the video memory, because protected
>mode don't let me do a direct write.
>
>How can it be done in assembley?

For any questions of this type, gcc itself is your best possible
teacher. Take your current C function, compile it with the -S switch,
and look at the .s file that gcc produces. Once you've seen exactly how
gcc does this, you will be in a good position to write something better
of your own (or perhaps decide that you can't improve on the C version
so you may as well not bother: this does often happen because the gcc
optimiser is pretty good :-)

>When i wan't to call a dos interrupt in djgpp i use dpmi_int, no problem.
>But if i wnat to call the same interrupt but in assembley, can use the
>function INT?

If the function only take integer parameters this is safe, but if it
uses any pointers to memory buffers you must copy this data to a
conventional memory buffer and then use a DPMI call. You could code this
directly in asm (using int 0x31 with ax=0x300, which is the DPMI
simulate real mode interrupt function that is called by __dpmi_int()),
or more easily you could just call the libc __dpmi_int() function from
your asm code.

I think this isn't really a good idea, though. It is only worth using
asm for really time-critical bits of code, and real mode interrupt calls
are by their very nature extremely slow. If you need to call such
interrupts, I think there is no point coding it in asm, and if speed is
so important, you should try to find ways of avoiding the interrupt
altogether...


--
Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
"Miracles are nothing if you've got the wrong intentions" - Mike Keneally

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