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Re: What do you get when you access RAM that isn't there?---Video RAM

IIRC you actually read the last video memory byte accessed.
The processor and video display circuitry alternate access to RAM.
When there is no RAM driving the bus, you could fairly reliably read
the last byte display latched by the video circuitry-which mide be a text
byte or hi-res byte.
The ][+ didn't have a softswitch for the vertical sync (or blanking
interval).
I vaguely remember using this technique to synch to the video signal for a
light pen I built.

Boy, that's a long time ago.

Joel


"Dosius" <steve@dosius.zzn.com> wrote in message
9307085f.0210211337.68cea360@posting.google.com">news:9307085f.0210211337.68cea360@posting.google.com...
> Let's say you have a 32K system and you access byte $8000 of RAM.
> Obviously there's nothing there, so what happens?  I'm just curious,
> but perhaps I can see what happens when one uses a 32K, 16K, or even
> 4K Apple ][, just for the hack value. :)
>
> - The Usotsuki http://sourceforge.net/projects/dapple