On 5/7/2018 12:43 AM, Mia Magnusson wrote: > Hi! > > > On the net there are some descriptions of more or less the same > modification to expand the C128's memory in a way that is somewhat > useable by BASIC and 100% useable by the KERNAL funcctions. However all > theese use an extra MOS 8722 MMU (the same as the original one in a > C128). It seems like a waste to use an extra MMU (which probably comes > from scrapped C128's). I assume you are referring to this doc? ftp://www.zimmers.net/pub/cbm/documents/projects/memory/c128/1028/1028.html http://www.ktverkko.fi/~msmakela/8bit/memory/memory-c128.pdf > How hard can it be to make a modern MMU add-on which makes it possible > to expand RAM without using another 8722? I doubt it's very hard at all. Is there a datasheet for the MMU anywhere? I looked at: ftp://www.zimmers.net/pub/cbm/documents/chipdata/index.html but did not see it. > I'm not > really sure why it uses gates to combine A6+A7 and A4+A5 to two inputs > though. I suspect they ran out of pins on the package. > > So, using some kind of modern programmable it seems like we would need > all adress lines, D0, D1, D6, D7, R/W, AEC, phi 0 and reset from the > socket for the MMU. We would also need the CAS signals from the > existing MMU. Then it seems like it would probably be easiest to > implement all ram bank selection related stuff in programmable logic, > but use the existing MMU for everything else, and just combine the two > CAS signals from the MMU to one "RAM access is requested" signal and > let the programmable logic select which of four RAM banks to use. I think the MMU is socketed, so can you put a daughtercard in the socket and get all the signals you need? It may be easy to just replicate the existing MMU logic and dispense with the original. It *looks* like one could make a daughtercard that plugs in where the 8722 goes, with RAM on board that could replace the existing DRAM. You'd need to pick up the missing address lines, and get some information from the PLA, but if it can be done, that would open up a ton more options. The 6809-based TANDY color computer supports a nice MMU functionality that offers the ability to expand to 2MB RAM easily, in 8kB pages, with a hardware based task switcher as well. Such a design is trivial to add to the C128, even with adding a relocatable zp and stack. If the RAM cna be put on board the daughtercard, such a design would be trivial. Such a design would also be portable to the C64 as well, I think. JimReceived on 2018-05-07 09:00:02
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