9 Nov
2006
9 Nov
'06
2:56 p.m.
I read something in an early BYTE stating that as
time went on, the dip switches on the pc's mobo had
little consequence on the systems ability to determine
the amount of ram installed. Am I in error to state
that the dip switches are read by code? Perhaps
someone can confirm, deny, elaborate, denounce...
Yes, the DIP switch settings can be read in software, and for most of the
switches, all the affect is the value read by the software. But IIRC the
memory size swtiches are also wired to the memory address decoder PROM,
and will thereforee control the address range where the mainboard memory
data buffers are enabled, etc.
In othor words, if you only have (say) 64K on the mainaboard, you can add
memory up to 640K on expansion cards _but_ you'd better set the DIP
switches on the maiboard correctly or you'll end up with a bus contention
(beteen the buffers on your memory expansion card and the mainboard data
buffer) for, say, the addtress range 64K -- (256K-1).
Incidentally, that address decoder PROM contains 4 addressing tables.
IIRC, in som order, they are :
256K (with 64K*1 chips in all 4 banks)
640K (with 256K*1 chips in banks 0 and 1, 64K*1 in banks 2 and 3)
64K (with 16K*1 single-rail (4816s) in all banks)
512K (with 12kK*1 chips in all banks).
The 2 DIP switches basically select how many of the banks will be used.
-tony