Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!jarthur!nntp-server.caltech.edu!tybalt.caltech.edu!toddpw
From: toddpw@tybalt.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: Deferred and Immediate mode
Message-ID: <1990Jul31.184844.10094@laguna.ccsf.caltech.edu>
Date: 31 Jul 90 18:48:44 GMT
References: <3529.apple.net2@pro-grouch>
Sender: news@laguna.ccsf.caltech.edu
Organization: California Institute of Technology
Lines: 36

rond@pro-grouch.cts.com (Ron Dippold) writes:

[ a somewhat involved routine to test the e flag and branch on it ]

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. A more robust way would be:

	php
	rep	$20	;should have no effect in emulation mode
	clc
	bit	#$3800	;sets carry flag to m bit which equals the e bit
	bcc	is_native
	bcs	is_emulation

note that both is_native and is_emulation should begin with PLP's.

You can also use CLC, BIT #$3800 at any time to branch on the accumulator size,
or CLC, LDX #$3800 to branch on the index register size.

All of this is irrelevant to the original question, of course, which has
already been answered.

Todd Whitesel
toddpw @ tybalt.caltech.edu