Path: utzoo!attcan!uunet!husc6!mailrus!ames!amdahl!amdcad!diablo!phil
From: phil@diablo.amd.com (Phil Ngai)
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: <23608@amdcad.AMD.COM>
Date: 22 Nov 88 01:04:53 GMT
References: <5065@whuts.ATT.COM> <4229@encore.UUCP> <1275@cfa.cfa.harvard.EDU> <3626@pt.cs.cmu.edu> <1276@cfa.cfa.harvard.EDU>
Sender: news@amdcad.AMD.COM
Reply-To: phil@diablo.AMD.COM (Phil Ngai)
Organization: Advanced Micro Devices, Inc. Sunnyvale CA
Lines: 34

In article <1276@cfa.cfa.harvard.EDU> ward@cfa.harvard.EDU (Steve Ward) writes:
|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

Steve, I think you have a very dangerous thing, i.e., a partial
understanding of the situation. My suggested method is not a
desperation technique, does not have any problems, and is also
approved by Lotus, Intel, and Microsoft as embodied in EMS. 

|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

This is a meaningless statement.

|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

No, there is no potential for clashing. Using 16-bit decoding is no
different from 10-bit decoding in terms of clashes. I think your
problem is that you think of 16-bit decoding as using something above
1K. The way that 16-bit decoding really works is that you pick a byte
WITHIN 1K. Then you distinguish all the addresses that would be
considered aliases of that address with 10-bit decoding. For example,
you distinguish between 240 and 640 and respond to both. But no one
else cares because when you use 10-bit decoding, you also claim both
240 and 640 and respond to both. 


--

Phil Ngai, phil@diablo.amd.com
{uunet,decwrl,ucbvax}!amdcad!phil