WDC 65816
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The W65C816S (also 65C816 or 65816) is a 8-bit/16-bit microprocessor (MPU) developed and sold by the Western Design Center (WDC). Introduced in 1983, the W65C816S is an enhanced version of the WDC 65C02 8-bit MPU, itself a CMOS enhancement of the venerable MOS Technology 6502 NMOS MPU. 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 features extended memory addressing to 24-bits, supporting up to 16 megabytes of random access memory, 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 essentially behaves as a 65C02. Following reset, 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. However, 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.
Related to the W65C816S is the no-longer-produced W65C802, which is discussed below.
Development of the W65C816S commenced in 1982 after Bill Mensch, founder and CEO of WDC, as well as the designer of the 65C02 microprocessor, consulted with Apple Computer on a new version of the Apple II series of personal computers that would, among other things, have improved graphics and sound. Apple wanted an MPU that would be software compatible with the 6502 then in use in the Apple II but with the ability to address more memory, and to load and store 16 bit words.
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