The BBC Micro used a system where the microprocessor was clocked on only every second clock pulse (or something to that effect), so the RAM was accessed, on one pulse, by the processor, and on the alternate pulse, by the video circuitry.

I think it's kind of like the "Double-Port" thing everyone's on about.

All I know is-it worked beautifly, and didn't interfere with programming, as it was completely transparent to the processor.

(BTW-I've been designing some sort of bare-bones computer for a while now, and I'm planning to use this sytem for the video output.)