Memory Access over the 1Meg. boundry
dcr0801%ritcv at cs.rit.edu
dcr0801%ritcv at cs.rit.edu
Thu Dec 14 06:30:21 AEST 1989
I am currently writing my own little *nix type OS on a IBM type machine.
The hardware has 1 meg. on the system board, and 512k in an expansion
I/O slot. Currently running the processor in 286 mode, any descriptor
table entries where the base address is over the 1 meg boundry gets
truncated to the lower meg. This gives me only 640k of usable memory.
I am sure (99%) that my tables entries are correct, yet I still can't
access the 'expanded' memory. Is there some kind of bit somewhere that
enables the processor to generate address over 20 bits on the address bus??
I could only guess that DOS (yeck!) does something to map higher memory
into lower memory so that it could be access in real mode. I know that
there must be a way to be able to just generate the 24 bit address and get
the memory without mapping it, but, like I said, I haven't found the
'Golden Bit' yet. If anyone out there in net.land could give me a little
hint on this, it would be GREATLY apprieciated!!
Thanks in Advance,
Dave Rein
(dcr0801%ucss at cs.rit.edu) or (..!uunet.uu.net!ultb.cs.rit.edu!ritcv!dcr0801)
[Sorry, no cute byeline at the moment. Hard to be funny and frustrated!!]
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