How to enable 24-bit addresses (Answer to Access over 1MB)
dcr0801%ritcv at cs.rit.edu
dcr0801%ritcv at cs.rit.edu
Sun Dec 17 05:04:35 AEST 1989
I'd like to thank everyone who responsed to my question about accessing
memory over 1 Meg. in protected mode on an IBM. Here is the answer:
System Control Port A (0x92) can be used to enable or disable the
Alternate Gate Address 20 signal. When Bit 1 of this port is set to
0, address lines 20-23 are forced to 0. This way, any address
generated over 20 bits (access over 1MB) will be wrapped around to
the lower Meg. Enabling this signal (setting Bit 1 on) will allow
the full address (24 bits in 80286) to be placed into the address
bus. (IBM Tech. Ref. , Compatability Section, page 3-26,3-23,3-38,3-50
for the PS/2 Models 50,60,70, and 80 respectivily.)
Thanks again for everyone that replied. I hope this information will help
someone else out of a bind in the future.
Dave Rein (dcr0801 at ultb.isc.rit.edu) "I just goes to show what you can do
if you're a total psychotic"
- Woody Allen
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