Date   : Sat, 17 Oct 2009 13:54:07 +0200
From   : rick@... (Rick Murray)
Subject: ROM upgrades on the cheap

Dave Curran wrote:

> OK, this is from memory, but the sideways rom decode is based on 4
> bits that are latched in ic76, giving a possibility of 16 addresses.

Thanks for the explanation. I'll take this with the schematic this 
afternoon...


> I'm not sure why they didn't do that on the original beeb. No extra
> components are required, only a bit of extra tracking, it maintains
> compatibility with 2764 and  27128 roms, but gives considerable scope
> for expansion.

I was going to post in the first place "why the hell didn't Acorn...", 
but there are two thoughts:

   1. Perhaps they had the B+ in mind already. After all, looking at
      Micro Men, and knowing the history, the ARM was already a concept
      when the Masters were rolling off the production lines.
      Why is this important? If you can stuff your B full of EPROM
      images, up to the capacity of the OS/hardware, then what value a
      B+?

   2. More importantly, this destroys the possibility to buy and insert
      ROMs. This is where things like the Watford expansion board came
      in userful. Generic user can purchase, install, use.
      The alternative? Purchase a ROM, "rip" (!) it, burn it to a bigger
      EPROM in groups of four, then install that. It might be possible
      to leave gaps in the EPROM for future use as the MOS will skip
      stuff that doesn't hold a valid header...
      ...however all of this requires an amount of technical know-how,
      which might be out of the scope of many - especially the generic
      educational user... my school's teacher/net admin used to turn off
      the entire room of Beebs with a big switch on the wall, rarely
      waiting for everybody to *BYE. And no wonder the poor FileStore
      was "unreliable" and oft complained "Too many users"...
      Would you want HIM messing with a bunch of EPROMs????


> Ah, you found my hidden pictures, curse the inquisitive viewer!

Sort of - your server provides directory listings and the main index 
page is a generic holding page telling you what to call the index page! :-)


 > I might get around to writing this up, but it is on a very long
> list of things to do.

Mmmm,  I know that feeling. I'm back on earlies next week which means a 
week of nothing much being done. <sigh>

One of these days I plan to get around to rewriting my 65C02 code. 
Something is going wrong with the FileStore start-up, and I rather 
suspect its a subtle bug in the emulator. I'm not good enough with 6502 
to trace the EPROM - the partial disassembly I have doesn't cover the 
operation of the system initialisation. So I figure it might be useful 
to just try reimplementing the processor core. Might give me a chance to 
look again at how I designed it.

[if anybody is interested - the FileStore emulator is open source, have 
a look... http://www.heyrick.co.uk/econet/fs/emulator.html ]


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...