Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!bbn!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: a small point.....
	 64 images of same byte...
Keywords: Limited to addresses up to 0x0400 only??
Message-ID: <1275@cfa.cfa.harvard.EDU>
Date: 19 Nov 88 20:39:03 GMT
References: <5065@whuts.ATT.COM> <4229@encore.UUCP>
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 28

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 only 1024 I/O byte locations available due to the partial
  (10-bits out of the possible 16-bit I/O address space) I/O address
  decoding inflicted on us by IBM.  This 1024 byte I/O address space
  repeats itself 64 times within the 64K I/O address space.  Said
   another way, there are 64 distinct I/O addresses you can use to
  access exactly the same I/O byte.

  As has been pointed out, for purposes of I/O instructions on Intel 
  8088/86/286 micros (probably 386, too) only 16 address lines are
  used.  The memory address space is larger, being 20 address lines
  for the XT/AT's.

  I believe that Mr. Corbin is saying what I just said, but the spate
  of several postings needed at least one more followup :-)

Steve W.  ward@cfa.harvard.edu