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From: knnngt@ukma.UUCP (Alan Kennington)
Newsgroups: net.micro.atari
Subject: Re: ST timer tick?
Message-ID: <2469@ukma.UUCP>
Date: Thu, 16-Jan-86 20:36:04 EST
Article-I.D.: ukma.2469
Posted: Thu Jan 16 20:36:04 1986
Date-Received: Sat, 18-Jan-86 00:31:06 EST
References: <1241@gitpyr.UUCP> <1368@cwruecmp.UUCP>
Reply-To: knnngt@ukma.UUCP (Alan Kennington)
Organization: Univ. of KY Mathematical Sciences
Lines: 18

**** "They eat lions, don't they?" - film title ****
	On the subject of timer A in the 78901 chip, I checked out the entire
bios code that is executed upon entry to xbtimer from start to finish. And it
all seemed to be beautifully in accord with the specifications for the
timer chip. I was just about to give up when I read in the bios manual that
some tricky business with the Hsync interrupt could be done by faking RTEs.
Then I realised that timer interrupts put a short stack frame onto the
supervisor stack. Which means that an RTE in Super mode is required. A C
program return can presumably only ever be an RTS. Hence the program
can not possibly work by diverting the timer interrupt to a C function
as an interrupt handler.
	First, can anybody out there confirm my brief analysis of the problem.
Second, has anybody discovered the means of RTEing at the end of a C program?
The bios manual defines xbtimer as a C program. But it doesn't mention
anything about having to write an assembly language program to handle
timer interrupts. Many of you must have long ago written programs to
handle such interrupts. So please tell me if I am incorrect in all of this.
Thanks.............................ak.