Date   : Thu, 25 Sep 2003 22:05:43 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: 6502 Co-Pro on a Master 128?

On Sep 24,  8:59, Matt.Forbes@... wrote:
>
> However, would one connect to an M128, and if
> so, what would be the advantages of doing so?  Do programs have to
> be written specifically to support the additional hardware, or will
> *anything* run with a 2nd 6502 connected?

Well, not *anything*, but most things will.  You don't need to do
anything special to make sure a program will run on a 6502 second
processor, and Acorn always gave guidelines about the sorts of things
you *shouldn't* do -- eg don't write directly into areas controlled by
the MOS, like addresses in JIM, FRED, and SHEILA (use the appropriate
OSBYTES/OSWORDS, etc).  A few programmers thought they knew better and
their software doesn't work as a result (and indeed some doesn't work
if on a Master rather than a Beeb), but most things are OK.

The advantages of using as 6502 Second Processor are speed (3MHz
instead of 2MHz) and memory.  PAGE always starts at &800 on a 2P,
because the space taken by sideways ROMs is in the I/O processor not
the 2P; top of usable memory will be &7FFF, or &BFFF (IIRC) if you use
HiBASIC, because the screen memory is also in the I/O processor.
 There's also a small gain by having the I/O processor execute some
tasks in parallel with the main program in the 2P, and (depending on
the language ROMs you use) the language will run at 3MHz instead of
rather less than 2MHz on a plain Beeb (ROM access was slower than RAM
access on a Beeb).


-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York