The 65c816 XL/XE OS revision

65c816 extension vs. 6502 ?

WDC 65C816S, or shortly 65c816, is a microprocessor designed and produced by Western Design Center. The CPU is a successor of the 6502. Various versions of the 6502 were used in computers like Apple II, Atari 400/800/XL/XE, Commodore C-64, C-128 etc.

The difference between 6502 and 65c816 is that while the former one is a fully 8-bit CPU, the latter one, eventhough it is equipped with 8-bit data path to the memory, has an internal structure that allows 16-bit data processing. 16-bit wide registers, up to 64 kB of the stack, many new addressing modes and instructions (including memory-to-memory moves, unknown to the 6502). And, which is one of the most important things, thanks to the 24-bit address bus the CPU allow you to use 16 MB of directly addressable memory. It can also work with higher frequencies, up to 14 MHz.

All this stuff makes the 65c816 an interesting extension, that we can put into 8-bit Atari. 

Details: http://drac030.krap.pl/ (Polnisch & Englisch)