Another thought struck me: why not use unused portions of video s-ram for CPU cache to speed up program execution? In future hardware upgrade perhaps?

Sprinter has 64K FastRAM (one S-RAM chip like videRAM chips) already. Today Sprinter can use 16K of FastRAM only. Read about it above.

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PETERS PLUS LTD

Actually, now i notice the Fast Ram in the Sprinter architecture document, also described as cache. Does this mean the Sprinter already has a CPU cache? Sorry, i'm a bit confused. Sprinter architecture is very new to me.

No, it is not cashe like PC CPU cashe.
CPU cache is unvisible for program. So, program use slow RAM but work more quickly.
Sprinter's FastRAM is part of RAM, during usage FastRAM. For example, today, 16K of FastRAM must set to PAGE 0. Program must use this PAGE 0 for quick work.

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PETERS PLUS LTD

So in Figure 2 in the Architecture document, RAM_00 is always Fast Ram?

Not always. Program must set FastRAM to the PAGE 0.
see more info at http://forum.petersplus.com/forum/showthreaded.php?Cat=&Board=program_e&Number=347&page=&view=&sb=

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PETERS PLUS LTD

Thanks. I look forward to experimenting with it once i get my sp2000 mainboard set up. Letting the programmer choose what to run fast and what to run slow is a great idea.