Date   : Mon, 21 Sep 1998 23:18:46 +0100
From   : James Fidell <james@...>
Subject: Re: 65C02 vs. 65C12 and Acorn ROMS

Quoting Stuart William McConnachie (stuart@...):

> In message <Pine.SUN.3.95.980911091454.11562A-100000@...>, Carlo
> CONCARI <concaric@...> writes
> >The only big difference I'm aware of is that the 65C02 has four
> >instructions that are not implemented on the 65C12: BBR, BBS, RMB, SMB.
>
> Could you tell me what their op-codes are and what they do!

BBS/BBR are "Branch Bit Set/Reset".  They take a zero page address and
test a specific bit of the value at that address, branching if it's set
or clear.

SMB/RMB are "Set/Reset Memory(?) Bit".  Similar to the above, but they
set or clear a bit in ZP memory.

I was under the impression however that BBS and BBR *were* available on
the 65C12 and only SMB and RMB were 65C02 only.  Can anyone categorically
confirm or deny this ?  I've probably got all of the documentation that I
used for my CPU emulation somewhere, but half of my stuff is still in
cardboard boxes from when I moved.

> >The 65C12 treats these four instructions as NOPs.
> As it does all other illegal op-codes.

> >By doing some experimentation on my machine, I have found that they
> >increment the Program Counter by one and elapse only one clock tick.
> >BTW, I have found out the number of bytes and clock ticks consumed by
> >all of the unallocated opcodes of the 65C12, which are all treated as
> >NOPs; if someone is interested, just let me know and I'll post them.
>
> Just a little curious.  I could measure them myself, but if you've
> already done the work it would be useful to see.  Cheers.
> Do you also know the cycle counts for the legal instructions.  I belive
> these are different than for the original 6502.  BTW is that correctly
> called a 6502A, and if so why have I never heard of a 6502B (Or is that
> a stupid question?)

I've done what I believe to be correct in my latest code.  All 65C12 NOPs
are single-byte and take one clock cycle.  For the other two CPUs it's not
so straightforward.  You should be able to extrapolate the number of CPU
cycles from the instruction type though -- they do follow the rules.

I *think* the 6502A was a successor to the 6502, but I really couldn't
be sure without digging out more documentation I don't have to hand at the
moment :(

> >Just another thing: is the ROM matter still unsolved? I've read the
> >archives of this mailing list and I've found some discussion two years
> >ago about the perverse behaviour of Acorn still wanting to hold their
> >rights on the ROMs. Even a guy from Acorn was subscribed to this list,
> >whose name was William Turner, who was supposed to be an intermediate
> >between emulator writers and Acorn bosses, but I haven't been able to
> >find anything on the archives about what he did.
> >Are you still here William? What are the current willings of Acorn's
> >high-located people? I think that by distributing the ROMs Acorn would
> >make more people able to use emulators, thus selling again some copies
> >of their games! 
>
> But aren't these all owned by Superior?
> 
> Maybe we'll see some progress in this area now that I hear Acorn have
> haulted development of any further new machines (Shame :-(((().  Since
> they currently only licence use of the ROMs on Acorn machines this
> policy now seems pointless (I guess the whole idea was to encorage
> people to stay with the Acorn platforms?).

It's definitely got to be worth trying this one again.  To that ends,
I've set up an e-mail address bbc-micro-roms@...  If you want
the ROMs to be freely distributable, just drop an e-mail to that address
saying that you'd like the ROMS to be freely distributable.

Some time soon I'll get some WWW pages up with more information.  Given
a few months, perhaps we can go to Acorn with a large list of people and
lean on them some more.

Feel free to distribute the address anywhere you think it might help,
especially your own WWW sites :)  It would obviously be preferable that
e-mails be tracable back to the sender though, rather than coming through
a CGI gateway or something similar...

James.
-- 
 "Yield to temptation --             | Consultancy: james@...          
  it may not pass your way again"    | http://www.cloud9.co.uk/james
                                     |
        - Lazarus Long               |              James Fidell