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Path: utzoo!watmath!clyde!cbatt!ihnp4!meccts!ahby
From: ahby@meccts.UUCP (Shane P. McCarron)
Newsgroups: net.micro.apple
Subject: Re: Apple IIe and IIc raster scan synchronization.
Message-ID: <532@meccts.UUCP>
Date: Sun, 28-Sep-86 12:09:32 EDT
Article-I.D.: meccts.532
Posted: Sun Sep 28 12:09:32 1986
Date-Received: Tue, 30-Sep-86 07:50:25 EDT
References: <2869@sdcc6.ucsd.EDU> <4160@reed.UUCP>
Reply-To: ahby@meccts.UUCP (Shane P. McCarron)
Organization: MECC Technical Services, St. Paul, Mn
Lines: 40
Keywords: IIe IIc video

In article <4160@reed.UUCP> kamath@reed.UUCP (Sean Kamath) writes:
>On the //e and //c there
>is no need to solder a pin. It has already been done for you.  Read
>$C019.  When it goes low (<128), it is in VBL.  For some really neat
>programs that take advantage of that, look into "Enhancing Your Apple ][
>Vol. 2" by Don Lancaster.
>
>On the //c it is possible to generate interrupts when VBL ($C019) goes
>low.  I KNOW this.  I THINK the same can be done on a //e with a mouse
>card, but I'm not altogether sure.  If anyone is dying to know, I'll do
>that before I work on my color printing routine...

Actually, you really can detect VBL on the //e using $C019.  And yes,
if you have a mouse card, you can cause it to generate interrupts at
each VBL.  I think you can have it generate interrupts at all sorts of
interesting events, like mouse movement, button down, button up,
etc...

However, you cannot detect VBL using $C019 on the //c or //gs.  The
only way to do it is using interrupts.  The line in the Apple //c
Technical Reference Manual reads:

	R7	$C019	RstVBL	9.1.3	See if VBlInt off(1); reset it

And the Apple //e Reference Manual reads:

	VBL	Read vertical blanking	$C019	Read, 4

This is just another example of Apple 'fixing' something so that half of 
their vendors software doesn't work.  Now when I want to detect VBL, I 
need to case for the machine I'm on; On a //e I wait for $C019 to go low.  
On a //c (//gs) I enable VBL interrupts and then loop infinitely (until 
interrupt, of course).  This works, but because of the overhead involved with
processing an interrupt, there is less time during the interval to
perform operations.  This is very annoying!
-- 
Shane P. McCarron			UUCP	ihnp4!meccts!ahby
MECC Technical Services			ATT	(612) 481-3589

"They're only monkey boys;  We can still crush them here on earth!"