Mia Magnusson wrote: > Oh, I thought that all those buffers/latches on the 8088 board did half > of this, and the stuff on the mainboard did the other half, i.e. the > 8088 board connected directly to the RAM IC:s. Too bad that it isn't. Well they do. They correspond to a similar set of buffers/latches on the 6509 side. Then all these latches feed into multiplexers which determine which processor has access to the RAM. Unfortunately these multiplexers are on the mainboard too. > Does all known CBM-II have the same layout of the mainboard? If so you > might be able to make a card that sits in the sockets for dram and some > other circuits, and has larger ram ICs (or more ram ICs) and the > required logic. There are two types of mainboards - high profile and low profile. Low profile needs hardware modifications to run the 8088 board anyway. My high profile has sockets where the high 128 kB of RAM should go, and I think others have them too. So it should be indeed quite straightforward to remove the 4164 chips from sockets, and insert a board with a few 44256 chips instead plus logic. > It seems easier to do something with the F****-decoder so it disables > access to CBM-II mainboard for all adresses above 3FFFF and then just > add a 512k static ram IC decoded from 40000 to BFFFF. Such an > add-on-board could sit in the BIOS socket with some wires to the 8088 > board. Sounds very reasonable. > Maybe it would be good to do something with the adress bus so local ram > and shared ram end up at other places, for example shared ram at > B0000-BFFFF? That's easy to do. The board outputs 4 highest address bits directly, so it's just a matter of changing $C to $4 to move the upper 64kB of shared RAM to segment B000. > Oh. If memory is added locally to the 8088 then atleast some I/O needs > to be double buffered. I saw that IO.SYS always double buffers disk sectors for some reasons, and when I tried disk access myself, it hung when I trid to use buffers higher up in memory. It may be quite possible that the software has soe bugs which require using double-buffering anyway. > To me it seems like something that scans B0000 or B8000 memory for > changes would be the best place to start - since it will allow almost > all programs to run. If some special program feels too slow and is used > often then it might be a good idea to patch that specific program. Originally I thought that something on the 6509 side should scan this area periodically, but then I remembered that 6509 cannot access DRAM when 8088 is running. So my idea is that whenever INT 16h is called to check for keyboard input, this routine should scan video memory and instruct the 6509 to redraw it if it was changed. Alternatively, the 50 Hz TOD clock interrupt could do it, but I am not sure it it could check it fast enough. Regards, Michau. Message was sent through the cbm-hackers mailing listReceived on 2017-10-17 14:00:02
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