Cool AppleSoft BASIC trick I never thought of before

From: Charles P. Hobbs <transit_at_lerctr.org>
Date: Thu May 18 17:59:20 2000

On Thu, 18 May 2000, Vintage Computer GAWD! wrote:
>
> The Apple ][ has an address that you can monitor that tells you when it's
> in the vertical blanking interval. People have done some neat stuff by
> timing their code around the interval, such as mixing all three graphics
> modes (text, lo-res and hi-res) on the same screen.

Most of these tricks were described in the following book (if you can
get your hands on it, it's long since gone out of print):

Lancaster, Don. Enhancing your Apple II /, by Don Lancaster ; [illustrated
by T.R. Emrick]. 1st ed. Indianapolis, IN : H.W. Sams, c1982-<c1985

Apple II's needed a simple hardware hack (a couple of wires or simple
components) in order to make one of the hardware memory addresses
correspond to the state of the vertical blanking interval (it would either
go high or low when vertical blanking was going on). IIe's and IIc's had a
memory location (hex C013?) that does that for you, no hardware hack
required. (Although, $C013 behaved in the opposite manner than Lancaster's
hack did--perhaps $C013 went high when Lancaster's memory location went
low and vice versa, so Lancaster's programs had to be modified a slight
bit in order to work).

In any event, once you had the vertical blanking information, you could
write machine language programs that would switch between Apple graphic
modes just about anyplace on the screen (within limits). You were more or
less limited to static displays because of the timimg requirements, but
you could still do some interesting stuff. I typed in a few of the
programs, and still have them sitting around on a disk if anyone is
curious...
Received on Thu May 18 2000 - 17:59:20 BST