[Coco] Using a CoCo disk drive with a BBC computer

[farna at att.net]

It was common for DECB ("RS-DOS") users to modify the ROM to address floppy drives in the same manner. It was a very popular feature of ADOS. I believe some people actually hard wired the drive to work that way, but the easiest way is to modify the ROM code. When the ROM was modified , OS-9 could be booted and still use the two double sided 40 track drives as full 360K drives instead of two 35 or 40 track drives. Another common option used with ADOS was making the drive 40 instead of 35 track as well. A 40 track formatted disk can still be read on a 35 track drive,  just the data on the extra five tracks couldn't be read. Of course a 40 track drive would read a 35 track disk. 

The double sided drive as two drives and 40 track option can be poked into memory on the CC3 or on the CC2 (when in all RAM mode). That's good for temporary use only, of course. Some people just saved the pokes in a short BASIC or ML to make the mods when needed. 

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 -------------- Original message ----------------------
Message: 8
Date: Fri, 16 Feb 2007 21:50:30 -0800 (PST)
From: Dan Olson <dano at agora.rdrop.com>
Subject: Re: [Coco] Using a CoCo disk drive with a BBC computer

> The BBC uses standard drives either jumpered as DS0 or DS1 (it can only
> normally have 2 drives, a restriction of the firmware), however the way
> these are handled is quite clever.
>
> side 0 of first drive is drive 0
> side 0 of second drive is drive 1
> side 1 of first drive is drive 2
> side 1 of second drive is drive 3

Very strange, it seems like this would have the disadvantage of limiting 
the max file size and adding more complexity to file management, with no 
real advantage.  Is this a result of the DOS or is it hard coded in a ROM 
somewhere?