You can get the sourcecode
here in the demos/outline2023 directory of
my dos33fsprogs github repository: git clone https://github.com/deater/dos33fsprogs/
Vote for this at Pouet
Run-through of the Effects
Text Mode Scrolling
This really isn't an effect, but it sets up text page1 for later.
It uses the built-in COUT ROM routine to print the scrolling text
efficiently.
There is a graphic apple printed, this is from the "mousetext" character
set introduced with the later Apple IIe Enhanced system (yes, the
'e' in IIe also stands for Enhanced, so it's "enhanced enhanced", the
model so nice they named it twice)
Hi-res Background
This is some compact shape-table based code to draw a fancy background
in 16 bytes or so. We only wanted to loop around 64 bytes or so, for
a countdown timer we decrement a zero page location that the BASIC ROM
sets to $4C (jmp) at boot time.
Cycle-Counted Mid-screen Scrolling Window
The main event. I have been doing some cycle-counting recently with
some Atari 2600 demos, and was doing some fancy
double-graphics-mode switching
demos lately.
Cycle-counted mode-switching is complicated on the Apple II as there's
no standard way to find when VBLANK is to synchronize things. On older
models you actually have to do horrible hacks with the floating bus to detect
the beam location and I don't think you can do that in under 256 bytes
(though maybe I should try). The Apple IIe and IIgs models in theory
you can detect VBLANK by just reading a location in memory (though for
some reason the value is inverted in the IIgs). The IIc model does
something completely different. In any case that's why this demo is limited
to the IIe, there isn't room to support all possible models.
Even though VBLANK detection on IIe is just reading a single memory location,
you actually can only get the start within a 7-cycle range or so, which
is enough for per-line mode-switching (because you can hide this in the
HBLANK) but it's not good enough for exact mid-screen updates. This is
a trick though, described by Sather in his "Understanding the Apple IIe"
book from 1984. To do this you can repeat the detection 7 times, backing
off by a cycle each time, to find out where things exactly happen. This
is a pain though because you need 7 frames to find VBLANK, so you can't use
it each frame. I use a variant of this, but only once and then I have
to cycle count exactly the rest of the demo.
Once you have the VBLANK it's easy enough to change mid-screen to get
a rectangular window. For variety it switches the background between
hi-res (the design we drew earlier) and lo-res (which re-used the text
page, but shows the ``apple ii forever'' text as chunky colored blocks
instead.
Getting a nice scroll effect required a sine wave, which is actually
a big chunk of the code. I re-used a trick described at the
codebase64 site
that uses a parabola instead of a sine wave, as it's
much easier to calculate and close enough for our purposes.
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