--Video RAM

["Joel" <joelbuckley54.nospam.@hotmail.com>]

My previous post wasn't exactly accurate,
I never accessed the phantom video RAM in empty RAM space.
My Apple had full 48K RAM.
I accessed it with a read of empty I/O space, somewhere in the $Cxxx region.
I suspect empty RAM space might give the same result, but I never did that.
And I didn't discover the technique.
I'm sure I read it in some Apple mag, maybe by Bob Bishop?


"Joel" <wrote
> 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.