Date   : Fri, 12 Jan 2007 14:04:00 -0000 (GMT)
From   : splodge@... (Richard Gellman)
Subject: ADFS and DFS in a beeb

>> Jules Richardson wrote:
>>
>>> Can both ADFS and DFS ROMs exist in a BBC micro, with the user
>>> selecting
>>> between them?
>>
>> Yes, no problem - apart from PAGE being something like &1F00 as I
>> recall.

Correct. Note that M128 ADFS uses FS RAM (paged in under the MOS ROM) to
avoid shunting PAGE up so high. On the model B, there is no such luxury,
so page has to be lifted phenomenally high. In mode 0, this leaves you
with 4.25 K for programs :)

> Next question: apparently an 8271 *won't* work, which unfortunately is
> what's
> in the 'desktop' beeb...

Correct. ADFS only works with double density recording, which the 8271
can't do.

> What about a 1772? That's what the "nearest" other beeb has - it's a 28
> pin
> chip on a small adapter board along with three LS logic chips, plugged
> into
> the normal FDC socket on the beeb. Is it 1770-compatible?

It is. Sounds like you have a 1770-upgrade board fitted. These allow a
177x to be plugged into the 8271 socket. The 1772 has minor differences
from the 1770, and is a drop-in replacement. The only really noticeable
difference is one of the selectable step times is different, but is isnt
one that BBC disk drives typically use.

> I seem to be rather flush with 8271's - found around ten of them in a box
> of
> unknown provenance - but the only 1770's are apparently tucked away inside
> machines.

I had an M128 board once that had a 1772 (instead of a 1770) soldered to
it. Never realised until I opened the case.

> The other obvious question is whether Watford's DFS is compatible with the
> 1770 chip? (or the 1772)

I would imagine not. 16K isn't a great deal of space, and rarely afforded
the opportunity to operate two different FDCs (exception: the Solidisk
DFDC DFS will do this). You could give it a try (no harm will be done),
but I imagine the Watford DFS in question is an 8271-only variety (Watford
did produce a 1770 version of their DFS at one point).

-- Richard