Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!husc6!cfa!ward
From: ward@cfa.harvard.EDU (Steve Ward)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Usable I/O Address range on the PC bus ...
Summary: play it again, Sam...
Keywords: Limited to addresses up to 0x0400 only??
Message-ID: <1276@cfa.cfa.harvard.EDU>
Date: 21 Nov 88 07:10:06 GMT
References: <5065@whuts.ATT.COM> <4229@encore.UUCP> <1275@cfa.cfa.harvard.EDU> <3626@pt.cs.cmu.edu>
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 35

In article <3626@pt.cs.cmu.edu>, ralf@b.gp.cs.cmu.edu (Ralf Brown) writes:
> In article <1275@cfa.cfa.harvard.EDU> ward@cfa.harvard.EDU (Steve Ward) writes:
> }In article <4229@encore.UUCP>, corbin@pinocchio.Encore.COM (Steve Corbin) writes:
> }> ...there are 64 bytes at each I/O
> }> address if you decode the extra address lines.  There 20 address lines
> }> for memory on the 8088/8086 but only 16 lines are used for I/O.
> }> 
> }  Just to prevent misunderstanding, there are 64 images, or "copies"
> }  of each of the 1024 I/O byte-addressable registers, or I/O byte
> }  locations.  This means any I/O addressing done to any of these
> }  64 "imaged, copied, ghosted, etc" address locations will result
> }  in an access to the same I/O byte location.
> 
> There are 64 copies of each I/O address used by the motherboard.  All 16
> I/O address lines are available on the bus, and a board can choose to decode
> all 16 if it so desires (though many, if not most, don't)

I think I said that?!!!  Again, since somebody might get confused, sure,
you can decode all 16 address lines for I/O, but because of the partial
address decoding on the motherboard and on plug-in cards, you better
decode them carefully!  It buys you nothing to decode the extra lines
since the I/O addresses above 0x400 are not available except to the
extent that there is a "hole" in the first 1k addresses.  Sure, for
the very hard up, you can leave a hole in the first 1k (never ever use
10-bit decoding in the hole, 16-bit decoding is ok) and then you can
use 16-bit decoding in the 64 different (1k apart) slots within the
64kb I/O space.  The disadvantage is potential I/O address clashing
due to the non (IBM) standard decoding.  Of course, you always put
on dip switches so you can switch to somewhere in the first 1k :-)
Given the dozens of boards out there, all using 10-bit decoding, it
doesn't promise much return for decoding all 16 address lines for
I/O, though I have described a desperation method for garnering
I/O addresses.
ward@cfa.harvard.edu 
.