11 Apr
2024
11 Apr
'24
6:21 p.m.
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