Date   : Tue, 06 Jan 2004 22:04:29 +0000
From   : Mike Tomlinson <mike@...>
Subject: Re: IDE Interface for BBC

In article <040106025502@...>, Jonathan Graham Harston
<jgh@...> writes

>I'm building a prototype IDE interface for the BBC. I think if I'm careful
>I can get it down to a two-chip solution.

Two chips is great.  What about software - would you write your own FS
ROM, or could you write a "shim" ROM to make your interface work with
the Acorn ADFS?

>  However, before I commit the
>design, I'd appeciate a few comments on some options.
> 
>1: The IDE interface uses a 16bit data bus. The usual solution to get at
>this through an 8bit data bus is to have two i/o locations, one for the
>bottom byte, one for the top byte. Does anybody have any opinion on
>whether I should put the two data registers next to each other in the i/o
>map, or 8 locations apart?

I prefer the A3 option as the locations are all consecutive, apart from
the disk data.

>2: How useful would it be to be able to remove the hard drive from the BBC
>and plug it into an Arc and access it natively with RISC OS ADFS?

Not for me, but others may find it useful.

>It is easier to program accessing those 512-byte sectors by either:
> 
>* Ignore the high byte.  Each 256-word sector holds 256 bytes of data in
>the low byte, and the high byte is ignored.

Hard drives are large enough nowadays that the loss of half the capacity
isn't a problem.  This simple 8-bit IDE interface for the Sinclair
Spectrum does it like this (with one chip) - this may give you some
ideas:  http://members.tripod.com/~piters/simpif.htm

Hope your soldering is a bit better than his, though:

http://members.tripod.com/~piters/pics/HD_interface.jpg

And if you liked that, this picture should give you nightmares:

http://members.tripod.com/~piters/pics/Speccy.jpg

(note how the memory has been replaced with two chips.  Wonder what the
breadboard in the top LH corner of the Spec, where the modulator would
have been, is?  An RGB interface?)

Rgds
Mike