Re: VBL on an Apple //c

[pim@planet.nl (Pim Blokland)]

In article <845m6s$o9a$1@server05.icaen.uiowa.edu>,
apple2pd@ground.ecn.uiowa.edu (ground.ecn AppleII Librarian) wrote:

> >In one of my BASIC programs, I use the VBL signal (PEEK(49177) 
[snip]
> >Apparently, on a //c you cannot see the VBL signal directly, but you have
> >to let the computer do it for you.

> tried to master any of its pecularities.  However, I have buried in my
> mindset the notion that the VBL signal is present on a //c, but with
> reversed logic.  That is, its logic state is opposite what we expect on a

No, that's the //e and IIgs you're thinking about: on a //e, the VBL
signal location ($C019) is low during a vertical retrace, in a IIgs it is
high.
This is alright, I can test this and all is fine.
But then there's the //c.
On a //c, you should not access $C019. That is, if a program accesses
$C019 when there's no interrupt handler installed, the computer hangs.
I do have the //c reference manual, but the information therein is very
scanty and I cannot build a reliable routine based on that, mainly because
I am not able to experiment.

Groetjes, Pim

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