Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!bu-cs!encore!pinocchio!corbin
From: corbin@pinocchio.Encore.COM (Steve Corbin)
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: <4229@encore.UUCP>
Date: 18 Nov 88 17:05:56 GMT
References: <5065@whuts.ATT.COM>
Sender: news@encore.UUCP
Reply-To: corbin@pinocchio.UUCP (Steve Corbin)
Organization: Encore Computer Corp, Marlboro, MA
Lines: 32


 In article <5065@whuts.ATT.COM> jmbj@whuts.ATT.COM (BITTMAN) writes:
 >> |I read in a manual that the addresses above 0x0400 were unusable? 
 >
 >Base I/O addresses above this range cannot be used without claiming
 >the address equal to the lower 10 bits.
 >
 >> This is not true. What is true is that the motherboard and most
 >> plug-ins only decode (the low) 10 address bits. So if you use address
 >> 4f0, it will collide with the motherboard's 0f0. 
 >
 >> Every byte that you pick is equivalent to 64 bytes. If you pick byte
 >        ^^^^                                ^^^^^^^^
 >> 200, you can also use 600, A00, etc. No other hardware can possibly be
 >> broken by this since it is the same as if you only decoded the low 10
 >> address bits. You know all the address bits are there as 20 or more
 >> bits are needed for memory addressing. In theory this method should work. 
 >
 >Assuming a "byte" here is an I/O address, there are at least 1024
 >addresses available for each I/O address claimed, the 8-bit PC XT bus has
 >10 "extra" address lines (1024) and the AT has 14 "extra" for a
 >total of (16384).  Just use the lower 10 address bits like high-order
 >bits on any other system, to select your board. Then use the upper
 >bits as decodes or direct ram addressing, etc.

The original poster was correct.  64 bytes are available 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.


Stephen Corbin	    UUCP:	{bu-cs,decvax,necntc,talcott}!encore!corbin
		    Internet:	corbin@multimax.ARPA