Re: [N8VEM-S100:1028] A new S-100 6502 CPU Board

[Ian May <fps1...@gmail.com>]

Hi John, you are probably too far down the design path for the new
board to change CPU now. What I was suggesting was using the W65C816S
instead of a 6502. At reset the W65C816S behaves exactly like a 6502
but when you switch to native mode it turns into a 24 bit addressing
machine with 16 bit index registers and accumulator. The program
counter is still 16 bits, the extra 8 address bits come from the
program bank register. To get beyond the bottom 64K there are JMP and
JSR instructions that have 24 bit address operands (much like far
calls on an 8086). There is a data bank register that generates the
top 8 address bits for data instructions.

Jameco has them for $7.95 versus $6.95 for the W65C02 so the price
difference isn't much. Unfortunately the W65C816S is not pin
compatible with the 6502 so you can't drop it in later. If you were to
use the W65C816S you would have a board that behaves exactly like a
6502 but has the ability to transform into a machine capable of
natively generating 24 bit addressing without messing around with
external page registers.

In hindsight I should have mentioned the W65C816S when you announced
the first 6502 S100 board.
Cheers,
Ian.