Date   : Fri, 23 Jun 2006 21:25:03 +0100
From   : Sprow <info@...>
Subject: Re: Torch Graduate (Was: Re: PEDL Z80 Board)

In article <449A7EB2.3090701@...>,
   Jules Richardson <julesrichardsonuk@...> wrote:
> > > Doesn't the Graduate plug into the 1MHz bus?
> > 
> > I've got one ex-Torch contact who worked on some of their BBC-based 
> > stuff; I'll ask in case he remembers.
>
> 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).

Fred and Jim, Shiela's internal.

> 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.

I think the original intention was for the FIT or PET to take over. The test
equipment had a small ROM on it itself so as long as the CPU was running and
the first bit of the MOS ROM was OK you could hijack the machine to do the
remainder of the test sequence without needing a cassette port or disc.

The fact that it's a 256 byte window isn't of great concern since you can
just use the space for a loader to transfer paged ROM (in the true sense of
the word) into RAM for execution,
Sprow.