Two observations, I's agree with Peter, why do many people seem to think that they need 64K of RAM, when often the 128/256 bytes in the MCU would actually do.

As for a suitable method of mapping memory and I/O, I have to agree with Bruce, I've been using the PSD 8xx and 9xx devices for about 6 months or so and they make building even a reasonably complicated system easy, AND if you must have ram then they make a version with 32K which must surely be enough for just about everyone.

Having said that I actually want several Mb as a buffer for reads from a ide hdd using DMA while the MCU does other tasks! I don't think that I'll be doing this with a 8051, more probably use a Coldfire as I have access to the necessary resources.