hankd@pur-ee.UUCP (Hank Dietz) (11/30/88)
You don't even have to be memory-mapped to get operands to a functional unit which is out there somewhere. The old NorthStar Horizon FPB -- a BCD floating-point S100 board made of TTL (circa 1974) -- wasn't memory-mapped, but was controlled by a memory reference. First, you'd touch a location, then the FPB would actually watch bus references and would load things into the FPB as it saw them being loaded into the CPU... yes, I know that sounds like the bus loading is wrong, but it did work.... Supposedly, the idea was to save a few T-states over the usual memory-mapped technique: 8080-family CPUs were not good at multi-byte memory-to-memory moves, but could do a sequence of memory-to-register loads fairly fast. Results were read from the FPB in a similar way. Actually, I recall quite a few things being memory-controlled in the above sense, but not memory-mapped. I just thought it might be worth mentioning this alternative, however "kludgey" it might seem.... -hankd@ee.ecn.purdue.edu