Date   : Tue, 07 Jun 2005 23:56:08 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: BBC IDE Drive compatibility list

"neil f" <faz@...> wrote:
> > Summary information:
> >   http://mdfs.net/Info/Comp/BBC/IDE/Drives.txt
> >
> > Full information returned by IDE_Identify:
> >   http://mdfs.net/Info/Comp/BBC/IDE/DriveData
> 
> I get a 'not found' error on both those links.
 
Probably related to:
> > Sent: Tuesday, June 07, 2005 1:14 AM
                                 ~~~~    !
Add BBC/ in the appropriate place.
 
> Not sure what's involved in the 8-bit decoding, but wouldn't it be
> possible to use the other interleaved half of the disc simply by
> 'shifting' the address somewhere by one byte? Maybe you could have
> 'paged drives' like paged roms, just by switching to a different slot
> in a look-up table followed by a control/break. Probably complete
> nonsense, but it makes you think...
 
It's not interleaved, or at any other 'address' or anything. It's
in the other half of the data word.
 
The IDE bus uses 16 bits:  0123456789ABCDEF
 
The 8-bit card only accesses the bottom 8 bits, as the 1MHz bus
(and the 6502) uses an 8-bit data bus. Unless you put the top 8
bits somewhere temporarily /as you access the drive/, you lose
them.
 
The 16-bit interface latched the top 8 bits with an extra read or
write, but you still need to transfer data in whole sectors. All
8-bit filing systems use 256-byte sectors. If you want to use the
whole of a 256x16-bit word sector, you need to put those other
256x8 bits somewhere. Imagine loading 256 bytes. If that is the
first 128 words, packed two bytes per word, you need to load all
256 word - 512 bytes.
 
With writing, you also have to ensure you don't destroy the 'half'
of the sector that you aren't writing to. Again, imagine writing a
256-byte file. You can't just write 128 16-bit words and stop. The
drive would stall and write undefined data into the following 128
16-bit words, destroying whatever happens to be there. Such as the
beginning of the next file!
 
With an 8-bit filing system the simplest solution is to throw away
three pounds-worth of disk storage and just use half of each word.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
Opposition is about asking questions, and Government is about not answering
them.