18 May
2000
18 May
'00
7:35 p.m.
On Thu, 18 May 2000, Charles P. Hobbs (SoCalTip) wrote:
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).
Ah. Thanks for clarifying that.
Well then this brings me back to wondering how those programs that allowed
mixed video modes on the Apple ][ worked. They required no hardware mods.
Hmmm.
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...
The programs I'm talking about I saw as demos on that Magazine Diskette
(Magazette) subscribtion service called SoftDisk (anyone remember that?
lots o' cool stuff on that). And they had hi-res animation mixed with
lo-res graphics and text-mode text. One day I will have to dig those
disks out and dissect the code to figure out how they did it. I don't
know why I didn't try to figure it out way back when.
There was one hi-res graphics game I was writing where I was going to try
to employ this trick and use a text line at the top of the screen to print
out the score. I knew it would involve having to keep excrutiating track
of each instruction cycle in the code to make the timing work. I never
completed this part.
Sellam International Man of Intrigue and Danger
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