Subject: Re: I/O maps and user-level device drivers
To: John Kohl <>
From: Brett Lymn <>
List: tech-kern
Date: 12/12/1995 16:35:28
According to John Kohl:
>This is incorrect; my reference [1] says that ports which have no bitmap
>position (i.e. their bit offset into the bitmap is beyond the end of
>the bitmap) are protected from access.

John is correct - I used the same reference as he but got the meaning
of the bit around the wrong way.  Intel say that a 0 means you can use
the port and a 1 will cause an exception - I skipped that bit and
assumed it was the other way round, so when I got to the bit about a
short table behaving as if all the bits were set I took this to mean
all access was allowed whereas it _really_ means all access denied.
What can I say?  DOH!!!!

>[1] Nelson, Ross. Microsoft's 80386/80486 Programming Guide, 2nd
>Edition.  Microsoft Press, Redmond, WA, 1991.

Brett Lymn, Computer Systems Administrator, AWA Defence Industries
  "Upgrading your memory gives you MORE RAM!" - ad in MacWAREHOUSE catalogue.