Path: utzoo!attcan!uunet!samsung!usc!jarthur!nntp-server.caltech.edu!tybalt.caltech.edu!toddpw
From: toddpw@tybalt.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: checking native/emulation mode
Message-ID: <1990Aug1.225059.7606@laguna.ccsf.caltech.edu>
Date: 1 Aug 90 22:50:59 GMT
References: <3529.apple.net2@pro-grouch> <1990Jul31.184844.10094@laguna.ccsf.caltech.edu> <43491@apple.Apple.COM>
Sender: news@laguna.ccsf.caltech.edu
Organization: California Institute of Technology
Lines: 23

dlyons@Apple.COM (David A. Lyons) writes:

[ about some code I posted ]

>What?  Yeah, if your stack pointer is $100 that's Not Good.

Actually, what I meant to point out was that in emulation mode the stack is
supposed to wrap around (if that ever happens; I know the O/S tries to prevent
it but it still can happen) and since the CPU just switched into native mode
an interrupt would clobber the very top of zero page.

>If you just meant the stack = $1xx, that's no problem, because switching to
>NATIVE mode doesn't fiddle with the stack pointer.  (If you were switching
>to emulation from native & the stack was not already in $01xx, *then* there
>would be problems.)

This is all true; however my real point was that one should ALWAYS program for
an interruptible environment. This means being careful about the stack & the
P register flags -- and the second piece of code I posted neatly avoids all
that because it doesn't do anything potentially dangerous.

Todd Whitesel
toddpw @ tybalt.caltech.edu