I'm presently trying to decide on how to implement memory expansion from 64KB ot 256KB on an 805x's external bus. Since I have a CPLD at my disposal, I'm considering a 4-bit "segment register" internal to my CPLD that's added to the topmost two bits of the data memory address. It's pretty easy to implement in a CPLD, as it only requires a four-bit adder and a latch.

I need this because there's a relatively large, fast SRAM buffer required, and the external memory is written at 40 MHz, and the 805x has to process it once it's loaded, then send it off to someone else in parallel form. I'm planning, for now, to map the 256K buffer into a 16KB window in the top of data space and use the segment register to do that.

What schemes have you guys used to do this sort of thing? What did you find "good" and what did you find "bad" about the scheme you used?

RE