Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!dlyons
From: dlyons@Apple.COM (David A. Lyons)
Newsgroups: comp.sys.apple2
Subject: checking native/emulation mode
Message-ID: <43491@apple.Apple.COM>
Date: 1 Aug 90 04:38:58 GMT
References: <3529.apple.net2@pro-grouch> <1990Jul31.184844.10094@laguna.ccsf.caltech.edu>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 32

In article <1990Jul31.184844.10094@laguna.ccsf.caltech.edu> toddpw@tybalt.caltech.edu (Todd P. Whitesel) writes:
>A simpler way to test for emulation mode is
>
>	clc
>	xce	;force native mode temporarily
>	php	;save the old e bit which is now in c
>	xce	;restore old e bit, carry is cleared
>	plp	;and get it back into the carry
>	bcc	is_native
>	bcs	is_emulation
>
>Unless I grossly misunderstand the stack effects of mode-switching, this
>routine should work under ANY conditions. However, there could be side effects
>if we are in emulation mode and the stack pointer is $100 -- namely, the
>interrupt info clobbers the top of zero page.
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

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.)

-- 
David A. Lyons, Apple Computer, Inc.      |   DAL Systems
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