Path: utzoo!attcan!uunet!husc6!uwvax!rutgers!rochester!pt.cs.cmu.edu!b.gp.cs.cmu.edu!ralf
From: ralf@b.gp.cs.cmu.edu (Ralf Brown)
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: <3626@pt.cs.cmu.edu>
Date: 20 Nov 88 01:12:52 GMT
References: <5065@whuts.ATT.COM> <4229@encore.UUCP> <1275@cfa.cfa.harvard.EDU>
Organization: Carnegie-Mellon University, CS/RI
Lines: 20

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).
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