Xref:	news2.mv.net comp.os.msdos.djgpp:3186
From:	Shawn Hargreaves <slh100 AT york DOT ac DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: near pointers "reentrancy" ..
Date:	Fri, 26 Apr 1996 19:07:49 +0100
Organization:	The University of York, UK
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Reply-To:	Shawn Hargreaves <slh100 AT york DOT ac DOT uk>
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To:	djgpp AT delorie DOT com
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On 25 Apr 1996, HASAN VALDOVINO CARLOS wrote:

> I am writting a sound library for DJGPP and I have started to use near 
> pointers to access conventional memory (where the DMA buffer is allocated). 
> Evenrything works great, but I'd like to know if the near pointer routines 
> are reentrant, since I enable/disable them every time the application calls 
> the routine used to update the audio DMA buffers. The routine can be called 
> in background from the timer interrupt service routine or from "foreground".

General advice for any question of that kind: read the source. It's the
best documentation there is, and the libc sources are a useful thing to
have around.

In your case, I would imagine you will run into problems. I assume what
you want to do is just:

	enable nearpointers
	mix samples
	disable nearpointers

in your interrupt handler, right?

But if nearpointers are enabled in the foreground, this interrupt handler
will disable them when it shouldn't. You could add a semaphore flag to
count nested levels of enablement, but that is getting messy.

Another problem with near pointers is the call the a library function to
enable/disable them. You need to lock all the code used inside interrupt
handlers, and that is tricky to do with a libc function.

You'd probably be better off just enabling near pointers when your program
starts up and leaving them enabled. Or use far pointers: you only have to
load the segment register once to copy a big block of data, so they can be
pretty fast.


Shawn Hargreaves.        Why is 'phonetic' spelt with a ph?
Check out Allegro and FED on http://www.york.ac.uk/~slh100/

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