Date   : Sat, 13 Nov 2010 00:23:12 +0000
From   : list-a_cloud9.bbc-micro@... (Theo Markettos)
Subject: ROMs

In article <4CDDC709.4020606@...> you wrote:
> [AMD Am28F256 / ST M28F256]
> 
> I think I ought to see if I can get myself some of those.

A 28F010 will cost you a fiver:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0540949
(a 28F512 is the same price)
or a 28F020 (256KByte - 16 ROM slots!) for 6.42:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0540977

Interestingly, socket IC51 (the OS ROM socket) on a Model B appears to be
wired for a 64KByte ROM (all A0-A15 are wired to the address bus).  Which is
a bit curious given the address decoding only allows 16KB to be selected
(though I admit I don't follow the function of all the various jumpers).

Presumably to attach 16 ROMs to a Beeb you tap off all 4 outputs from the
ROM select latch (IC76)?  Does this mean the OS ROM is always mapped in (at
&C000 to &FFFF) and the latch selects the language/FS ROM to page into
&8000-&BFFF?  In which case, put the flash ROM in the DNFS position (IC88)
and wire:

Flash    Beeb
A14      S22n
A15      S20n
A16      D4 anode
A17      D5 anode
/WE      IC77 pin 8 or IC78 pin 10
Vcc      +5V
Vpp      +12V

(on the 28F020 A17 occupies the position of the power pin on the socket, so
you have to bend that leg outwards so you can wire to it.  Likewise the Beeb
wires A14 and A15 on the ROM slot to +5V, so you'll have to do the same for
those)

Also set S32 to drive A13 to the ROM (the schematic doesn't say which way
this is), remove all the other ROMs except the OS, and wire S36w to S21se
(which means the DNFS ROM is always selected in the &8000-BFFF position and
disables the other ROM sockets).

There's a bit of a chicken-and-egg problem in that you need to load Basic
into the flash first time around - maybe you leave the Basic ROM in and plug
it into the ROM slot before doing the other wiring, and wire up just /WE
with all the extra address lines high (that would mean forcing selection of
paged ROM 15 - can Basic live there?)

Another advantage... when in read mode this ROM is write-proof, so software
that tries to write to sideways RAM for copy protection will think it's a
ROM.  No write protect switches required, just the special *SRLoad when you
really do want to write it.

Theo