Date   : Thu, 18 Aug 2011 17:10:27 +0100
From   : robert@... (Rob)
Subject: Tube ROM

On 18 August 2011 15:46, Rick Murray <rick@...> wrote:
>
> Ah, but the logic here is that the HiBASIC simply *will* *not* work if
> the co-processor isn't connected. It isn't like the Master's
> auto-relocate one, it just... plain won't work.

I think the theory was that the co-processor became "the computer" and
the BBC Micro was just a means by which you interacted with it.  In
this case, having a frequently used piece of software held in ROM
rather than on removable media made considerable sense.  Bear in mind
that for any given ROM, the header tells the MOS if it's a "high"
version, and indeed what processor family it is for, and so it can be
ignored if it's inappropriate in the circumstances. (e.g., if the
relevant co-processor is turned off!)   And acorn did have the
foresight to allow up to 16 ROMs to be used, albeit with add-on
boards, so it's not as if it would be always be impossible to have
both versions fitted.

> Therefore, I'd consider it to be more logical held within the co-processor.

It would have complicated the co-pro startup sequence, though. If you
have a language ROM in there too, then in what circumstances do you
want to invoke it?  Power up, ctrl-break, standard break?  What if you
aren't actually running BASIC, but another language (or a ROM that
pretends to be a language) and want to continue to use it?  (Normally
it just restarts the last thing copied across.)  What if the default
language on the host isn't even BASIC?

>
> However - Sprow has a point with the size of the EPROM. There's no
> reason why a bigger EPROM couldn't have been used, however 16K of BASIC
> and 2K of Tube code will leave a lot of space in the chip, and I'd guess
> in those days size=cost.

That's certainly an argument!  I know in the early days 16K EPROMs
were a significant expense - which is maybe why acorn arranged the
hardware to accept 4x4K EPROMs for MOS 0.10 instead of supplying it on
a 16K EPROM; it must actually have been cheaper!

>> (Interesting though just occurred to me - was there anything other
>> than BBC Micros that acted as a tube /host/ ? )
>
> In software alone, there's 65Tube.
>
> In hardware... good question!

It would be interesting to hang a 6502 second processor off a ZX
spectrum, say. Somebody was writing a Beeb emulator on one; that could
act as host... :-)

Rob