Date   : Thu, 22 Jun 2006 11:27:46 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Torch Graduate (Was: Re: PEDL Z80 Board)

>> Doesn't the Graduate plug into the 1MHz bus?
> 
> Spot on - dug the docs out and indeed it does plug into the 1MHz bus. 
> Why, I'm not sure - I only leafed through the docs but they didn't seem 
> to say. Maybe Torch intended you to own one of their other copros (Z80 / 
> 68000) and so they kept the Tube free.
> 
> I've got one ex-Torch contact who worked on some of their BBC-based 
> stuff; I'll ask in case he remembers.

Here we go:

<<<
Anyway, the way it was told to me was that the Graduate people
(founded by the man that founded TORCH, Martin Vlieland-Boddy) knew
Paul (?) Bond, one of the developers of the BBC Micro MOS, and so
were aware of the special feature in the code. If you assert the
interrupt line on the 1MHz bus early on in the boot cycle the MOS
will execute the code in the 256 byte memory mapped window in the
1Mhz bus space in the memory map (was it "Shiela" - I can't
remember).

This could enable the machine to bootstrap itself into the BBC Micro
memory and take over the machine entirely without the need for a
sideways ROM to be installed.

I think the principle was that you could sell the device more
easily, especially to businesses, if you didn't have to open the BBC
up to do it. Plus, the ability to take over the BBC was important
given the low level at which it all worked - the BBC became the
memory mapped screen RAM rather than an actual computer.

Of course, the down side is that the 1MHz bus wasn't all that quick
(neither was the Tube in the cosmic scheme of things) so performance
wasn't all that special. Also, it made writing a DFS to use the
Graduate drives (where the BBC was left active) much more
challenging as all the code had to be fitted into a 256 byte page
and subroutines were difficult as that would involve paging in
another 256 byte page to run them.

The reason that the machine shipped with a limited DFS was that was
all we had time to write. An Advanced DFS which did fully work was
subsequently produced by Tony Sherman, one of the other developers.
 >>>