WDC 65C816

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The W65C816S (also 65C816 or 65816) is a 16-bit microprocessor or CPU developed and sold by the Western Design Center (WDC). Introduced in 1985, the W65C816S is an enhanced version of the WDC 65C02 8-bit CPU, itself a CMOS enhancement of the venerable MOS Technology 6502 NMOS CPU. The 65C816 is the CPU for the Apple IIGS and, in modified form, the Super Nintendo Entertainment System. The 65 in the part's designation comes from its 65C02 compatibility mode, and the 816 signifies that the MPU has selectable 8- and 16-bit register sizes. In addition to the availability of 16-bit registers, the W65C816S extends 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 electrical signals for improved system hardware management. At reset, the W65C816S starts in "emulation mode", meaning it substantially behaves as a 65C02. Thereafter, the W65C816S 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 backward compatibility with most 65C02 software. Unlike the PDIP40 version of the 65C02, which is a pin-compatible replacement for its NMOS ancestor, the PDIP40 W65C816S is not pin-compatible with any other 6502 family MPU. The W65C802 or 65802 is completely software-compatible with the 65C816 and it is also electrically compatible with the 6502 and 65C02. Hence the W65C802 could be used as a drop-in replacement in most systems equipped with a 6502 or 65C02. Since the W65C802 has a limited number of pins and does not use multiplexing, it cannot emit a 24-bit address, which limits it to a 64 KB address space. The W65C802 is no longer produced.