William David Mensch, Jr. (born February 9, 1945) is an American electrical engineer born in Quakertown, Pennsylvania. He was a major contributor to the design of the Motorola 6800 8-bit microprocessor and was part of the team led by Chuck Peddle that created the MOS Technology 6502. He also designed the 16-bit successor to the 6502, the 65816.^[1]

Mensch is the founder, chairman, and chief executive officer (CEO) of the Western Design Center (WDC) located in Mesa, Arizona. Before founding WDC in 1978, Mensch held design engineering and management positions at Philco-Ford, Motorola, MOS Technology, and Integrated Circuit Engineering.^[2] At WDC, Mensch worked mainly on extending and expanding the 6502 architecture. His designs are used widely in embedded systems and implantable, electronic, life-support devices.^{[citation needed]}

Mensch graduated with an associate degree (A.S.) from Temple University in 1966 where he was a member of Sigma Pi fraternity.^[3] He received his Bachelor of Science (B.S.) degree in electrical engineering from the University of Arizona in Tucson in 1971. He has taught classes at Arizona State University, including courses on system on chip (SoC) integrated circuit (IC) design. Mensch is a Senior Member of the Institute of Electrical and Electronics Engineers (IEEE). In 2004, he was inducted in the Computer Hall of Fame (hosted by the San Diego Computer Museum, part of the San Diego State University Library), and in 2005 was presented with a Lifetime Achievement Award from the University of Arizona's College of Engineering.^[2]

Based on his participation in the basic circuit design, definition, and system design of the Motorola 6800 microprocessor and supporting computer chips, Mensch is a co-holder of several 6800 family patents, including the 6800 central processing unit (CPU), 6820/21 Peripheral Interface Adapter (PIA), 6850 Asynchronous Communications Interface Adapter (ACIA), and 6860 modem chip. He was the sole IC design engineer of the 6820/21 PIA, which was the first peripheral IC to have bit-programmable input/output (I/O).^[2]

Before the 6800 family, Mensch had worked on the Mostek 5065 processor.^{[citation needed]}

Shortly after Mensch founded the Western Design Center (WDC) in 1978, Commodore International contracted the company to develop what they called a "macro-micro", a macro-programmed complementary metal–oxide–semiconductor (CMOS) processor that could be used in a small and powerful calculator. However, WDC was unable to complete the project, and Commodore ended their relationship with the new company.^[5] Mensch claimed that Commodore never intended to use his design, and were instead seeking leverage to get a better deal for competing chips from Japanese producer Toshiba.^[6]

The first major effort of Mensch and his team was developing the WDC 65C02, an enhanced version of the NMOS 6502 microprocessor. The 65C02, in addition to being implemented in CMOS circuit technology that reduced power use and improved noise immunity, added some new instructions, and corrected several defects in the NMOS 6502. The 65C02 was subsequently adopted for use in the Apple IIc computer and, later, in an enhanced version of the Apple IIe.

Mensch's next design, which was to become an important product at WDC, was a 65C02-compatible 16-bit microprocessor, the 65C816, later designated W65C816S. The 65C816's design came about following consultation with Apple and was adopted by them for use in the Apple IIGS computer. The 65C816 was later chosen as the core of the Ricoh 5A22 processor that powered the Super Nintendo Entertainment System.

Mensch developed the Mensch Computer as a means to promote the W65C816S microprocessor. It was a system designed around the WDC W65C265S microcontroller, which contains a W65C816S core. The computer, which includes the Mensch Works software suite, was produced for a time by WDC and was made for hobby and education uses.

Chuck Peddle and Bill Mensch are regarded as personal computer pioneers, because both the 6502 technology and business model were instrumental in helping launch the personal computer revolution.^[7][8]

Mensch has five children and resides with his wife, Dianne, in Superstition Mountain, Arizona.^[2]