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Re: FPGApple + ARM?

Jorge Chamorro Bieling wrote:

Michael J. Mahon <mjmahon@aol.com> wrote:



I guess the idea of *dynamically* changing the instruction set
still seems a little wierd.  I was involved in the design of the
Burroughs 1700/1800/1900 series, which had dynamic microcode and
multiple (application language dependent) instruction sets, but
it wasn't a great idea then, and it still isn't. 


That's because you are thinking of the 6502 as the main processor, but,
what this cpu is is a risc with fixed, not dinamically changing opcodes
in it.

Although, you can play around with the 6502 interpreter code, and do
those things if you wish for any reason, or don't do them if there no
reason to.

When you turn the power on, the arm boots and begins to run an arm
application that happens to be a 6502 emulator that is configurable.
There's always this safe outer shell with a fix, known instruction set,
just in case something goes wrong with the 6502 code that is being
interpreted.


OK, so what you're saying is that you'd like to have an ARM processor
in an Apple II bus environment...  Interesting, but still far from
mainstream.  Apple folks will find it foreign, and ARM folks will find
it terribly limiting.
You need good compilers for each variation that you design, and that is not an easy problem. 


It's not. But it's powerful. Better think of it as a configurable
coprocessor if you want to.


Been there, done that--and in practice it's not as useful as you
imagine.  Again, only very special problems merit such an approach,
and even they might be better served with an FPGA-based solution.


And the performance benefit is negligible, since its all being done in
software anyway.



Not necesarily true as these opcodes may run in native arm code and they
certainly would execute faster than interpreted 6502 code.


Yes, but, as I noted before, that's not an "extension" to the 6502;
that's a whole new processor.  The 6502 mode is only there for
compatibility!  It's not an Apple II!!

What I meant was that whatever you could do with this system, you
could do even better with a pure ARM software solution.  That is
not the case with an FPGA, where you could hope to do some things
even better than a general purpose processor.  (Not that I think
that increases its general attractiveness...)

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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