Date   : Mon, 22 Nov 2010 15:50:46
From   : heyrick.beebsoc@... (Rick Murray)
Subject: [BeebSoc] Harddisc fakery using a microcontroller

Hi,

An idea I'm (lightly) toying with, as an example, and if you'd like to 
take this as a thread on its own, great:

The FileStore. Has a harddisc. A 256 byte sectored smallish SCSI drive 
of a fairly specific type. Which is practically impossible to find 
nowadays. I wanted to write an emulator (which is sort of in limbo at 
the moment) to look at hacking the firmware to permit an SD card to be 
used off the parallel port, as a form of storage.

Somebody on the (proper) list mentioned the idea of using a small MCU to 
read off of some sort of storage media, and "pretend" to be the correct 
sort of harddisc. Now I'm thinking this idea might actually have some 
mileage to it, and it ought to be "reasonably" simple to implement. A 
card, say SD, hooked to a MCU with some on-board memory and on-board 
firmware, which 'fakes' a SCSI bus. Given it is '80s tech, it is neither 
overly complicated, nor especially fast (in the case of the FileStore, 
treacle speed).

Now, I have sod-all experience with microcontrollers so I couldn't tell 
you if this is something the AVR is good at, or a PIC. The way I see it, 
it would be best if there is something adept at handling SD cards 
(perhaps moreso than bit-bashing I/O lines, if possible), and has a LOT 
of digital I/O lines capable of running at TTL levels. This will be for 
the SCSI.

Internally, I do not imagine it would need to do a lot. There'll be no 
requirement for a "view" of the disc as this will be provided by the SD 
card. It should simply map reads and writes as seen by the 8 bit kit 
into reads and writes to the SD. If we're writing to the SD in raw mode, 
then read the low 256 bytes and discard the high; write the same 256 
bytes twice. Binary shift all addresses to account for sectors being 
twice as large. Simplest method.

As a possible alternative, is it viable for a small embedded MCU to work 
with the DOS filesystem? If so, without overly complicating things, 
perhaps work using "images"?

Something that would be *very* useful, at any rate, is a button or 
special command sequence or somesuch which can trigger the device to 
self-format - lay down the basic AFS0 structure and write the default 
$.Passwords file.


All musings, but I think it might be an avenue to consider. Certainly it 
is rather less destructive to older hardware than trying to botch in an 
SD interface!


Best wishes,

Rick.