Path: utzoo!attcan!uunet!mcsun!unido!sbsvax!roeder
From: roeder@sbsvax.UUCP (Edgar Roeder)
Newsgroups: comp.sys.atari.st
Subject: Re: How to call the bios ...
Summary: one more solution
Message-ID: <2116@sbsvax.UUCP>
Date: 12 Jan 90 17:49:42 GMT
References: <9001060449.AA23484@TIS.COM> <1937@atari.UUCP>
Organization: Universitaet des Saarlandes, Saarbruecken, W-Germany
Lines: 34

In article <1937@atari.UUCP>, apratt@atari.UUCP (Allan Pratt) writes:
> I think it's a reentrancy problem.  The OS hooks seem to use external
> storage, meaning they're non-reentrant.  Worse, they all use the SAME
> external location, so in a given program, you can't call BIOS from
> BIOS, but you also can't call XBIOS from BIOS! 
> [...]
> Hooks like this can be made reentrant by changing how they work:
> [example how to do it by copying the arguments]

There is another solution: use your own stack for saving the return address.
(This is also the method used in the ROM when bios is called.)

	move.l	stackpointer,a0
	move.l	(sp)+,-(a0)
	move.l	a0,stackpointer
	trap	#$x
	move.l	stackpointer,a0
	move.l	(a0)+,-(sp)
	move.l	a0,stackpointer
	rts

You have to initialize the variable stackpointer to point to the upper end of
some memory space reserved for this new stack. Although this does only allow
a limited level of reentrancy (the stack-space divided by the amount needed for
every call), you don't have to copy your arguments around.

	- Edgar
-- 

Mail:  Edgar R\"oder			E-Mail: roeder@cs.uni-sb.de
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