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From: toddpw@nntp-server.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: Bug contest
Message-ID: <1991Feb5.044339.25263@nntp-server.caltech.edu>
Date: 5 Feb 91 04:43:39 GMT
References: <9102050238.AA18080@anaconda.cis.ohio-state.edu>
Organization: California Institute of Technology, Pasadena
Lines: 49

meekins@CIS.OHIO-STATE.EDU (timothy lee meekins) writes:

>Anyways, about a half hour ago, I was working on Z3D. To make things short,
>the entire screen began scroll, almost like a screwed-up vertical hold on a
>TV. very strange. I think the electron beam location is getting screwed
>big time. If anyone has the foggiest notion of how this could happen, let
>me know, it would be neat to take advantage of. The entire screen was
>scrolling, border and all.

This is what happens if you set PAL video mode (50 hz vertical sync).
When you do a control-option-reset, one of the options is to set system
standards and 50 Hz -- this is for european/australian PAL video equipment.
In the hardware it is activated by bit 4 (mask $10) of $c02b, which is
described in the firmware reference (page 281 of the hardcover).

>Oh yeah, how did ACS italicize the text in
>their "HAM mode" demo? They switch from graphics to text mode, so the
>text is hardware text, not graphics, so, how was it italicized? Makes
>me wonder if their are ways to fool the hardware to perform strange effects.

What they are doing is switching from graphics to text mode at the same place
on the screen consistently. SHR is controlled by the VGC chip, and all other
graphics modes are controlled by the Mega II chip -- the two are not perfectly
in sync, the VGC tracks the Mega II and lags it by about 5/4 of a microsecond.
(I had hoped that Apple would combine the Mega II and VGC into a single chip
driving VRAMs when they made the ROM 3, but they didn't :( .) As a result, when
you switch in and out of SHR mode, the sync signal 'slips' by that 5/4 of a
micro second. The monitor hasn't totally lost the signal, but it has been
knocked away from stable synchronization, and as it re-stabilizes itself the
displayed image appears to warp for ten lines or so, and this creates the
italic effect. (Actually, I think the ACS demo does something a bit trickier
to obtain a better italic effect, but I buried the disk somwhere so I can't
go check.) The following code will demonstrate what I'm talking about:
	lda	#$80
	ldx	#$90
	ldy	#$c0
lp1	cpx	$c02e
	bne	lp1
	tsb	$c029
lp2	cpy	$c02e
	bne	lp2
	trb	$c029
	bit	$c000
	bpl	lp1
	bit	$c010
	rts

Todd Whitesel
toddpw @ tybalt.caltech.edu