WDC 65C816

The WDC 65C816 (also known as W65C816S or 65816) is an 8/16-bit microprocessor developed and sold by the (WDC). Introduced in 1983, the W65C816S is an enhanced version of the MOS 6502 processor used in the Apple II series, but extended with 16-bit support. The 65C816 was used as the CPU in the Apple II GS and in modified form, the.

Description
The 65 in the part's designation comes from its 6502 compatibility mode, and the 816 signifies that the processor has selectable 8 and 16-bit sizes. In addition to the availability of 16-bit registers, the W65C816S features extended ing to 24 bits, supporting up to 16 megabytes of random-access memory, an enhanced instruction set, and a 16 bit, as well as several new electrical signals for improved system hardware management.

At, the 65C816 starts in "emulation mode", meaning it substantially behaves as a. Thereafter, the 65C816 may be switched to "native mode" with a two instruction sequence, causing it to enable all enhanced features, yet still maintain a substantial degree of with most 6502 software. However, unlike the version of the 65C02, which is a  replacement for its  ancestor, the PDIP40 W65C816S is not pin-compatible with any other 6502 family processor.

Related to the W65C816S is the W65C802, which had the same internal structure and 16-bit support, but used a 40-pin layout compatible with the original 6502. This allowed it to be used as a drop-in replacement in certain roles. However, the 65C802 could not emit a full 24-bit address, which limited it to 64 KB of memory. The 65C802 and its relatives are no longer produced.