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From: dmb@TIS.COM (David M. Baggett)
Newsgroups: comp.sys.atari.st
Subject: How to call the bios from within process termination handlers
Message-ID: <9001060449.AA23484@TIS.COM>
Date: 6 Jan 90 04:49:44 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 105


The original thread was about changing resolution from within a
process termination handler (i.e., a function installed at the GEM 
process termination vector 0x102).

Atari's Allan Pratt claimed that you can call Setscreen from within
such a handler.  He was indeed correct.  (I was a bit skeptical after
the 36th crash, but now I Believe.)

HOWEVER, my frustrating attempts to get this to work indicate that the
OS hooks are different in his compiler than in Sozobon C.  Calling
Setscreen with Sozobon's standard xbios() call will cause an exception.
I.e., adding *just*

	Setscreen(-1L, -1L, 1);

to my process termination handler causes the handler to RTS to an
illegal instruction.  Note that this call is #defined in
<osbind.h> and turns into 

	xbios(5, -1L, -1L, 1)

(i.e., it calls the xbios hook in dstart.o, provided with Sozobon).

For the hell of it, I tried writing my own xbios hook, assuming
Sozobon's stack checking was causing the problem.  It wasn't the
stack checking.  (At least, that wasn't the only thing.)

Saving the stack pointer before the trap and restoring it afterwards
fixes things.  I don't know why the bios trashes the stack when called
from a termination handler.  Perhaps a wizardly Atari person could
comment.

Also, I'm wondering if dstart.o is in error for not saving the 
stack pointer, or if Atari is in error for writing bios code
that trashes the stack pointer, or if somebody else is in
error (maybe me).  (Well, _somebody_ has to be in error!  Who
can we flame if no one's in error?)

In any case, anyone who's having trouble getting bios calls to work
from a termination handler (and I believe this is a problem in Laser C
as well) should try using the OS hooks below.  They work with Sozobon,
and as far as I know should work with other C's too.

Dave Baggett
dmb@TIS.COM

----------------------------------- >8 ---------------------------------------

;
; Operating system hooks for C with stack pointer saves.
; These are useful for calling bios routines from within a process termination
; function; i.e., a function installed at GEM process termination vector 0x102
;
; (Modified from Dlibs code in dstart.o)
;

.globl	__gemdos __bios __xbios

;
; long _gemdos(function, [parameter, ...])
;   int function;
;
__gemdos:

	move.l	(A7)+, traprtn		; save return address
	move.l	A7, saveA7		; save stack pointer
	trap	#1			; do gemdos trap
	move.l	saveA7, A7		; restore stack pointer
	move.l	traprtn, -(A7)		; put return address back
	rts				; return

;
; long _bios(function, [parameter, ...])
;   int function;
;
__bios:
	
	move.l	(A7)+, traprtn		; save return address
	move.l	A7, saveA7		; save stack pointer
	trap	#13			; do bios trap
	move.l	saveA7, A7		; restore stack pointer
	move.l	traprtn, -(A7)		; put return address back
	rts				; return

;
; long _xbios(function, [parameter, ...])
;   int function;
;
__xbios:
	move.l	(A7)+, traprtn		; save return address
	move.l	A7, saveA7		; save stack pointer
	trap	#14			; do xbios trap
	move.l	saveA7, A7		; restore stack pointer
	move.l	traprtn, -(A7)		; put return address back
	rts				; return

		bss
		even

traprtn:
	ds.l	1			; storage for return PC in trap hooks

saveA7:
	ds.l	1			; storage for stack pointer in traps