Re: Apple video question?

["Charlie" <charlied@NOSPAMbboard.com>]

"Holger Picker" <pickerh@uni-muenster.de> wrote in message
Pine.A41.4.44.0302202055160.86610-100000@zivunix.uni-muenster.de">news:Pine.A41.4.44.0302202055160.86610-100000@zivunix.uni-muenster.de...
> On 20 Feb 2003, Dosius wrote:
>
> > Date: 20 Feb 2003 08:32:43 -0800
> > From: Dosius <steve@dosius.zzn.com>
> > Newsgroups: comp.sys.apple2
> > Subject: Re: Apple video question?
>
> > (But the //e had something like the VBL - although AFAIK it worked
> > only on the //e and not even on the //c.  It was fake, however - there
> > was no real VBL. ;))
> Vertical Blank softswitch (AppleIIe, but not Apple//c): yes. Interrupt: no.
> You would have to write a waiting loop like
> blank1: LDA $c019
> BPL blank1
> blank2: LDA $c019
> BMI blank2
> ...

Isn't there a similar but undocumented way to get the VBL on an Apple II+ and
presumably an Apple II?  I seem to recall that it was a result of the same
clock signal being used to refresh the memory chips and the video.  I think
that you polled an unused location in the I/O area ($C000 - $CFFF) and somehow
derived the VBL from the garbage that you normally see there.  I don't have my
Apple II+ anymore so I can't check.
Or maybe I just dreamed all this?

Charlie