Need Docs for XT RAM Board...

[Tony Duell]

>  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