Re: Apple video question?

[Holger Picker <pickerh@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
	...
which was very time consuming. IIRC, "Karateka" used something like this for
smooth scrolling (but tested for an AppleIIe before that). As someone
mentioned before in a different thread, the C64 in comparison had the
ability to program its video chip in such a way that it caused an interrupt
signal as soon as the position of the cathode ray reached a certain, given y
position.
Uh, what do you mean by "fake"?

> > Please note also the trick the programmers invented here: They used the
> > scrolling loop as a delay loop for the sound generation. By combining sound
> > and scrolling loop, they were able to reduce the amount of clock cycles a
> > lot. (This technique was also used e.g. in "Boulder Dash".)
> Sounds like Mel. ;) (see
> http://catb.org/jargon/html/The-Story-of-Mel.html for info)
You think that is weird? Well, you haven't seen nothing yet. :-)

> > (If you've got one of the following AppleII emulators for the PC, either
> > ApplePC or Dapple, you can take a look at this by switching the emulator
> > output to double view i.e. showing both hi-res pages at the same time.)
> BTW - Dapple ][ - the original Dapple couldn't do double view. ;)
Oops. Sorry. :-)

> I also think you can do double-view green monochrome in Dapple ][.
Yes, you can. Double-view works with all graphic modes. It is just not
possible to use it together with scanlines.

> > Another technique was dithering.
> Didn't a lot of graphical text adventure games do this?
Yes, but "Death in the Caribbean" is special in that way that it uses
patterns that stretch over 4 lines instead of only two. There are far more
different patterns found in this game than anywhere else (as far as my
knowledge is concerned).

Kind regards

  Holger