A20 gate on the 386 
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 A20 gate on the 386

        Okay to all you 386 gurus, I have a question.  I am interested in
writing a Protected Mode OS type thing, but I don't understand exactly
what the A20 Gate is.  I believe it is something that was put in to make
the adressing of the 386's carry-over compatible with the 286, but I
don't know what too do with it.  Does anybody out there know what the
solution the the A20 problem is?  Thanx in advance.


Wed, 09 Oct 1996 14:46:42 GMT  
 A20 gate on the 386
The A20 gate is an AND gate inserted in the A20 address line.  Its purpose
is to make memory wrap around to 0 at the 1MB mark like a real 8086/88,
since some real-mode programs (such as DOS itself) depend on this behavior.
In protected mode having A20 gated off would mean that you couldn't address
odd megabytes anywhere in the address space (they'd just be duplicates of
the preceding even megabyte), so you generally want to gate A20 back on
in protected mode.

John Wilson

Thu, 10 Oct 1996 00:26:02 GMT  
 [ 2 post ] 

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