65816 Overview

The W65C816 (also 65C816 or 65816) is a 16-bit microprocessor (MPU) developed and sold by the Western Design Center (WDC). Introduced in 1983, the W65C816 is an enhanced version of the WDC W65C02 8-bit MPU, itself a CMOS enhancement of the venerable MOS Technology 6502 NMOS MPU.

The 65C816 is a CPU option supported by the Foenix F256, that can be dropped-in, in place pf the default W65C02 processor supplied with the F256.

The "65" in the part's designation comes from its 65C02 compatibility mode, and the "816" signifies that the MPU has selectable 8-bit and 16-bit register sizes. In addition to the availability of 16-bit registers, the W65C816 processor itself extends potential memory addressing to 24-bits, supporting up to 16 megabytes of random-access memory. It has an enhanced instruction set and a 16-bit stack pointer, as well as several new control signals for improved system hardware management.

Despite allowing 24-bit addressing, a 65816 installed in a Foenix F256 still utilises the same 16-bit physical address space of the original W65C02 processor.

At reset, the W65C816 starts in "emulation mode", meaning it substantially behaves as a 65C02. In fact, a 65816 in default power-up "emulation mode" is cycle-accurate with the original MOS 6502 processor, whereas the W65C02 differs on some instructions.

User programs can then switch the W65C816 to "native mode" with a two instruction sequence, causing it to enable all enhanced features, yet still maintain a substantial degree of backward compatibility with most 65C02 software.

However, unlike the DIP40 version of the W65C02, which is more-or-less a pin-compatible replacement for its NMOS ancestor (excepting mostly pin 1 - Vss / VPB), the DIP40 W65C816 is not directly pin-compatible with any other 6502 family MPU.

The W65C816 pin differences are mostly around the clock pins and some control pins. Despite this, the W65C816 can be a drop-in replacement for the W65C02 on the Foenix F256 series. No FPGA upgrade is necessary when in emulation mode (#).

(#) This point (noted from the main site F256K ordering page), needs more explanation by someone more knowledgable on F256 operation with a W65C816 processor installed.