Path: utzoo!attcan!uunet!pyrdc!pyrnj!rutgers!att!whuts!jmbj
From: jmbj@whuts.ATT.COM (BITTMAN)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Usable I/O Address range on the PC bus ...
Summary: Extending I/O space
Keywords: Limited to addresses up to 0x0400 only??
Message-ID: <5065@whuts.ATT.COM>
Date: 17 Nov 88 14:46:03 GMT
Distribution: na
Organization: AT&T Bell Laboratories
Lines: 28

> |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.

> I strongly recommend you grab the least number of bytes possible out
> of the range 0-3ff. It's a very limited resource that has to be shared
> with all the plug-ins in the world. 

Agreed.