Geoff Winkless wrote: > And on the subject of the accelerator board, I remember > Cookie showing one running Lemmings at the second Gloucester > show that someone (can't remember ... was it Nev? don't think > so) had built... what happened to that?
Well, here's a bit of history... I designed the basic way the accelerator works, and Martin designed the electronics to support it. What it was, was a 8MHz Z80 which I think we overclocked to 10 or 12MHz. The trick to getting an effective accelerator though is this: Most of the time you're doing reads. You don't do writes all that often (I think the ratio was about 1:7 or something like that). So I put in a mirror RAM - nice, fast RAM that the outboard Z80 could use. When you're reading from that RAM, it all happens with no wait states at the higher clock speed - and ignores the SAM's internal RAM entirely. When you're writing to that RAM, it writes to both the internal and external RAM, honoring wait states. IO, of course, needs to honor the waits. Works pretty well - you could actually run Lemmings at 50 frames a second that way. And because most apps sync to the frame sync, it has none of the "Turbo Mode" problems that PCs had when running on faster processors. Si
