On 4/11/24 11:01, Fred Cisin via cctalk wrote: > Then, there was the "64-256KB" motherboard. It had one row of 4164s > soldered in, and three rows of sockets. Populating those with 4164s > gave you 256K of RAM. BUT, there was an empty socket on the board, that > you could populate; I don't know whether it was a PAL or some 74xx > logic, that then let you use two rows of 4164s (one row of which was > soldered in) and two rows of 41256, giving 640K! 640K was all of the > RAM that could be easily used, other than some upper memory space of the > other video or bits in between other stuff.
Bipolar PROM. A few years ago, I published a way for one to use a 22V10 GAL as a substitute to fill in the D000 segment as well as the lower 640K by using 256Kb DRAM. Although the 22V10 is a 24 pin DIP and the PROM being replaced, 16, things could be arranged to let the "tail" end of the 22V10 hang out of the socket and have no changes made to the planar traces themselves. So completely reversible. --Chuck
