Page 0 was Re: [Gllug] BBC and acorn bits

[Nix]

On 28 Nov 2006, Pete Ryland spake thusly:

> On 26/11/06, Andrew Black <andrew-li at black1.org.uk> wrote:
>> Page 0 having registers looks like a recipe for registers accidently
>> getting corrupted (eg a pointer set to 0 or a low value).
>> I am most familiar with the VAX and Alpha architecture where the lowest
>> page is always (at least under VMS) no access.  I have many times been
>> saved by processes access violating rather than overwritting data and
>> plowing on regardless.
>
> IIRC, on the C64, page 0 was more like IO than registers.  Byte 0, of
> all the things they could have put there, decided whether the IO
> port's pins were read or write!  The IO port was of course only
> present with the 6510, not the 6502.

Bytes 0 and 1 on the 6510 were magic control registers: about half their
space was devoted to weird I/O crap and the other half to the 6510's
peculiar variant of paging.

The other 253 bytes remained `ordinary' page 0 `registers' and had no
magic I/O capabilities.

-- 
`The main high-level difference between Emacs and (say) UNIX, Windows,
 or BeOS... is that Emacs boots quicker.' --- PdS
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