How many computers?on```````````

From: John Ott <jott_at_rottweiler.ee.nd.edu>
Date: Tue Jul 21 13:01:10 1998

>
>
>
> Another follow up about the Graduate boot process...
>
> OK, in short, it works like this... (apologies in
> advance for being a bit vague, or even inaccurate on
> a few points - it's been well over a decade since I
> worked on the Graduate...)
>
> When powered on and booted, the Graduate asserts the
> NMI on the BBC Micro 1MHz bus. Thanks to an
> undocumented feature placed in the MOS by Paul Bond,
> if the 1MHz bus NMI is active when the BBC Micro is
> switched on, the MOS does a JSR to &FE00 (I think -
> the 1MHz data page).
>
> 256 bytes of the Graduate's 6502 ROM is paged into
> here, and the code wedges into the BBC Micro "call
> this OSBYTE on boot" call (I think). This is called
> several times on boot, and the Graduate allows the
> BBC Micro to completely boot, before using the
> bootstrap it copied into memory to copy over yet
> more 6502 assembler into BBC memory to act as the
> "client" for keyboard and screen operations.
>
> (The 6502 Graduate ROM can be paged into the 1MHz
> bus "data" space 256 bytes at a time by poking a
> register in the 1MHz bus "address" space. The
> other communication goes over the TAP, which was
> used to exchange the screen and keyboard data etc.
> Used a protocol sort of like the Torch Z80 "tube"
> calls, I think, but don't ask me more than that -
> I'll have to find the notes...)
>
> And if you thought the Graduate DFS was cr*p, it's
> because I developed it after each normal working day
> (8am - 4pm Torch, 4.30pm - midnight Data Technologies,
> go home, get up at 6am again...) over about six weeks
> and since it was not a sideways ROM, it had to
> execute entirely in 256 byte pages on the 1MHz bus.
> It's a miracle it worked at all (even if I do say so
> myself :-) )
>
> --
> Kevan
>
> Old Computer Collector: http://www.heydon.org/kevan/collection/
>
>
>
>


-- 
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* John Ott                         *  Email: jott_at_saturn.ee.nd.edu     *
* Dept. Electrical Engineering     *                                  *
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Received on Tue Jul 21 1998 - 13:01:10 BST

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