Re: Replacing the 6502 with a faster chip

[mjmahon@aol.com (Michael J. Mahon)]

Rob Jones replied:

>mjmahon@aol.com (Michael J. Mahon) wrote in message
>news:<20040820034253.26080.00002251@mb-m28.aol.com>...
>> Rob Jones wrote:
>> 
>> >While surfing the 'net, I found this adaptor
>> >(http://www.6502.org/mini-projects/816conv/conv.html) that replaces
>> >the 6502 with a 65C816 processor. I have not done any research on the
>> >65C816, or for that matter the 6502 and its variants, but is there a
>> >way to replace the Apple's 1MHz processor with something faster?
>> 
>> I expect you want to know if you can make an Apple II run faster.
>> 
>> The answer is yes, but it takes a lot more than a faster processor.
>> The Apple II video generation is timed by the same clock that
>> creates the processor timing, and the peripheral bus must
>> operate at 1.0204 MHz for compatibility.  Some I/O cards
>> also require precise code timing for correct operation, most
>> notably the Disk ][ Controller for 5.25" disks.
>> 
>> So the trick is speeding up processing without changing the
>> bus or video timing and only when speeding up will not cause
>> other difficulties--not a simple problem.

<snip>

>Is the 65C02 a pin-for-pin compatible with the 6502? I could very
>easily make a 3X PLL (3MHz) to clock the "C" while keeping the bus at
>1X (1MHz).

Simply clocking it faster will not allow it to interface with the
1MHz bus and memory interface.  You need to make the rest
of the Apple II _completely unaware_ that the processor is
running faster, which means that the fast processor must have
its own faster memory, either as an associatively mapped cache
or as a complete copy of the content of standard Apple RAM
(including any expansion memory if you want to run faster on
those accesses).

As I said, it is not a simple problem.  Modern FPGA's have
enough speed and complexity for a few-chip solution, but
the logic design is non-trivial.  I'm not aware of any publicly
available designs.

As Roy pointed out, anyone who manages to make, say, an
8MHz or faster Apple II accelerator for less than $80 will be
able to sell quite a few (I'd guess 50-100).  Of course, that
price point would not be easy to hit...

-michael

Check out parallel computing for 8-bit Apples on my
Home page:  http://members.aol.com/MJMahon/