logden@fluke.UUCP (09/17/87)
I have another question related to geos. The 1764 RAM Expander, is it just memory chips on a board? Or is it more complicated than that? What I am really driving at is: can I build one of these puppies at home? I remember in the old VIC 20 days, I built up a board that had additional memory that when populated gave me around 35k (3K +16K +16K). I also built an expansion board that allowed me to have 4 or 5 of these memory boards, plus other cartridges all plugged in at once, and then I could chose memory sizein blocks of 8k or cartridges. Anyway, a board I can make (no wirewrap here!), But is it just a matter of putting more chips on the expansion bus? Any pointers would be appreciated. Thanks, Logden
fred@cbmvax.UUCP (Fred Bowen) (09/18/87)
In article <1791@sputnik.COM>, logden@tc.fluke.COM (Leonard Ogden) writes: > > I have another question related to geos. The 1764 RAM Expander, is it just > memory chips on a board? Or is it more complicated than that? > > What I am really driving at is: can I build one of these puppies at home? Yes, it's more complicated than that. The 17xx RAM expanders are controlled by a DMA device, which provides both DRAM refresh and the interface to the host system. The host's processor does not access the expansion RAM- you have to program the DMA controller and it will read or write the chunk of memory you specify. This sounds complicated, but what it provides in return is very fast data transfer- much faster than the processor can provide by addressing each byte. The DMA controller is a custom IC which, I beleive, is not generally for sale to the public. The RAM expanders are reasonably priced, however, especially for the C64 version which comes with a beefier power supply necessary for supporting the card. The more adventuresome techie can probably stuff more RAM (up to 512K- the DMA controller's limit) onto the board however. -- -- Fred Bowen uucp: {ihnp4|rutgers|caip}!cbmvax!fred arpa: beats me tele: 215 431-9100 Commodore Electronics, Ltd., 1200 Wilson Drive, West Chester, PA, 19380
simon@ukma.UUCP (09/25/87)
Speaking of the 1764 Ram expander, can the 128 use the RAM it it while in 64 mode? Can it access it in 128 mode? What about in CP/M mode? Can it be a substituted for the 128's RAM expansion or is the DMA chip different??? For that matter, if they are different, can a good techie wire in both expanders on a 128 for more then 640k? simon.
fred@cbmvax.UUCP (Fred Bowen) (10/01/87)
> Speaking of the 1764 Ram expander, can the 128 use the RAM it it while > in 64 mode? Can it access it in 128 mode? What about in CP/M mode? > Can it be a substituted for the 128's RAM expansion or is the DMA chip > different??? > For that matter, if they are different, can a good techie wire in both > expanders on a 128 for more then 640k? The boards for the 17xx RAM expanders are basically the same, except for one resistor and, of course, the amount of RAM. There were several different layouts of the PCB, the most noticeable difference being the packaging of the DMA controller (8726), DIP vs. quad pack. A good techie could, indeed, modify a 1764 for use on a C128. All you gotta do is add one 390 ohm resistor at location R4. This is necessary to tweek the relationship of the dot clock to phi-2, which differs slightly between C64 and C128's. Once you have either a 1700 modified as above, or a 1700/1750, you can use it in any of the 3 modes of the C128: 64, 128, or CP/M. CP/M makes good use of it as a RAM disk- simply address as device M:. In native C128 mode, BASIC provides 3 commands (FETCH,STASH, and SWAP) specifically for talking to the DMA controller. The upgrade 128 ROMs enhance this further by allowing full access to all internal banks of memory. -- -- Fred Bowen uucp: {ihnp4|rutgers|caip}!cbmvax!fred arpa: beats me tele: 215 431-9100 Commodore Electronics, Ltd., 1200 Wilson Drive, West Chester, PA, 19380