Path: utzoo!attcan!uunet!husc6!mailrus!uflorida!beach.cis.ufl.edu!seeger
From: seeger@beach.cis.ufl.edu (F. L. Charles Seeger III)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Usable I/O Address range on the PC bus ...
Keywords: Limited to addresses up to 0x0400 only??
Message-ID: <19186@uflorida.cis.ufl.EDU>
Date: 16 Nov 88 00:33:31 GMT
References: <3552@hubcap.UUCP>
Sender: news@uflorida.cis.ufl.EDU
Reply-To: seeger@beach.cis.ufl.edu (F. L. Charles Seeger III)
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Organization: UF EE Dept
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In article <3552@hubcap.UUCP> oolidjr@hubcap.UUCP (Joe Moll) writes:
|What is the deal with the usuable I/O address range on the IBM PC bus??
|
|I read in a manual that the addresses above 0x0400 were unusable?  Is
|this true??  I don't see how this could be, since all the address lines
|and control signals off the 8088 are available on the bus.
|
|Could someone shed some light on this please???  I was about to wire
|a project into I/O address 0x0400 and it needs to be done soon.
|
|Thanx Much ...
|Joe

Only the first ten bits of the address are decoded on the motherboard.
Hence, any addresses that the motherboard responds to will be shadowed
into the upper regions of the address range.  You can make use of 
higher addresses on IO cards, but you must be certain that the lower
ten bits of the addresses do not conflict with any devices on the
motherboard.  Choose whatever address range is appropriate in the
ten bit range, then make sure that all your addresses, both above
and below 0400, are within that range for the lower ten bits.  The
conventions of IO addresses are published, and you should consult
them.  They're not within a few feet of my keyboard, but I can
provide references or the info, if needed.  I suggest using PALs
for address decoding, if possible, because they're so easy to change.
Be aware that some IO cards may not decode all address bits, either.

Good luck,
Chuck