Date   : Sat, 18 Dec 2010 02:07:04 +0100
From   : rick@... (Rick Murray)
Subject: Harddisc fakery using a microcontroller - back on

On 17/12/2010 04:59, Alan Williams wrote:

> I have been vaguely thinking about a 1MhzE bus interfaced SD card
> interface or about an emulator for the whole host adaptor ACB4000/4070
> chain.

Wow, back on the topic! ;-)


> the dev tools is some what draconian.

It seems this is all too often the case.


> This makes writing one sector quite a time consuming job and it also
> requires a big (in 8 bit terms) ram buffer in reach of the micro
> controller to backup all the other sectors that have not changed.

Excuse me - but if you use an SD card (instead of, say, a flash chip), 
can't you just say "write this sector here"? It *ought* to be optimised 
to write a sequence of sectors, but we don't have that luxury.

It is time consuming and might be a factor when you want to stream video 
to it, but a FileStore? Is it really slower than an *old* SCSI drive, 
given you'll be buffering it sector-by-sector?


> for use with a Filestore rather than a BBC that's not all that possible.

Indeed, that's why I'm looking at a dirt-simple SCSI->SD hack. It seems 
the easiest way, as opposed to major firmware patches. It'd be different 
if we had the sources, but so much of the disassembly is "unknown 
function" and suchlike. What is safe to yank out? What isn't? Can we 
even rearrange the code? There's blanks in EPROM - does this serve a 
purpose in live mode? Well, it must as firmware and operational memory 
are all together in the RAM. But what/where/how? There's just so many 
questions that a different (hardware) approach seems simpler.


If I ever get my A5000 going again, I might try wiring up the parallel 
and I/O podule user ports to be a SCSI interface, and see if I can put 
together some code to make it work. How this goes should tell me if an 
AVR is viable, basically, as glue logic, and what needs to be done to 
achieve this.

Just a damn shame the user port podule doesn't fit into the RiscPC.


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...