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From: Søren A.Møller
Subject: Re: HDD address decoding
Date: Tue, 26 Nov 2002 21:32:36 +0100
NNTP-Posting-Date: Tue, 26 Nov 2002 20:30:14 +0000 (UTC)
X-Newsreader: MicroPlanet Gravity v2.60
In article , paul.9753
> I am designing an embedded board that takes an ISA bus and provides a box
> header to take a standard PC IDE hard drive. As far as I can see, I can
> connect up most of the IDE signals to the ISA bus. I need to generate
> the two IDE chip selects, CS0 and CS1. What addresses do these need to
> decode to?
> I have got a PC memory map which says:
> 1F0-1F8 Fixed disk controller 1
> 170-177 Fixed disk controller 2
> Only one disk will ever be plugged into my system. Is CS0 for fixed disk
> 1 and CS1 for fixed disk 2?
Short answer: No.
Long answer can be found in the ATA specifications at
The ATA3 spec ( http://www.t13.org/project/d2008r7b.pdf ) is probably
the easiest to read - the newer contains a lot more words (and
information) and the older are obsolete.
> Is there a reason why fixed disk 1 reserves 9 addresses when there is
> only 3 address lines (and therefore can only address 8 locations) going
> to the IDE connector?
CS0 is used for the 8 addresses for the interface itself (eg. 1F0-1F7)
Note that address 0 is 16 bit, the rest 8 bit.
CS1 is not needed to make an ATA interface for an embedded computer if
you are writing the drivers yourself.
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