The fix you refer to has been in place since before I started maintaining the OS. It will stop the OS from using the page but is only a software solution -- it is best to ensure the card cannot/does not respond to the memory space from a hardware perspective as well. This may include one or more pages in the peripheral bus space 0x88-0x8f
What are all the memory holes that a 512k card should leave unmapped?
1. with AMA,AMB,AMC high, >4000 (page >BA)
2. with AMA,AMB,AMC high, >8000 - 9fff for memory mapped peripherals like sound, speech? (page BC)
3. dip switch option to open holes at any AMA,AMB,AMC in case of any nonconforming card in PBOX
pages 80 - C0 are external 512K in pBOX
AMC is most significant address line
>BA must be a hole. This is >4000 in the top 64K.
AMA,AMB,AMC are pulled high in the PBOX.
DIP switch option to open the hole at >4000 for all values of AMA,AMB,AMC
dsr = !a0 & a1 & !a2
mmap = a0 & !a1 & !a2
high = ama & amb & amc
hole = (high | dip) & (dsr | mmap)
CS* = memen* | hole
OE* = memen* | dbin = dbin
WE* = memen* | we* = we*
lcv245a to data bus:
A side = SRAM
B side = PBOX
dir = memen* | dbin (high: A to B. low: B to A)
oe* = memen* | hole
lvc245a will output high level voltage 3.3V on data pins. This should be greater than VIH of the LS245 of the Geneve. Should be OK.
all other input signals buffered one way through lvc245a
Edited by FarmerPotato, Tue Nov 27, 2018 10:33 AM.