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From: bjoern@drdhh.hanse.de (Bjoern Kriews)
Newsgroups: comp.sys.atari.st.tech
Subject: Re: Vertical Blank routines
Message-ID: <3386.06.91@drdhh.hanse.de>
Date: 11 Jun 91 18:51:30 GMT
References: <79358@eerie.acsu.Buffalo.EDU>
Reply-To: bjoern@drdhh.hanse.de
Organization: Digital Island
Lines: 26

chu@acsu.buffalo.edu (john c chu) writes:
>Can someone mail me the dos and don'ts of using routines that run
>every vertical blank? If the Abacus ST Internals is accurate in this
>case, I think I know how to install a routine. However, I'd like to
>know if it is ok to call an XBIOS routine during that time, what the
>maximum length a routine should be and so on...

Before calling BIOS or XBIOS routines from interrupts, you should
use BIOS_ON, afterwards BIOS_OFF, which are macros used to manipulate
the bios register stack.

#define	SAVPTR		(*(char **)0x4a2)
#define	BIOS_ON		(SAVPTR += 46)
#define	BIOS_OFF	(SAVPTR -= 46)

>(also, is it correct to add your routine to the VBL queue by altering
>the system variables? Their locations do not (and will not) change
>between versions of TOS?)

Yes, it's ok.



---
bjoern@drdhh.hanse.de = Bjoern Kriews / Stormsweg 6 / 2000 Hamburg 76 / FRG
"gaaga mahwe Bjoern urgl ufzae Turbo-C bnub"     (J. Willamowius)