According to their datasheets, the W65C02S and W65C816S, both still in production, can run up to 8MHz at 3.3V. They can also go down to 2MHz on 1.8V, or up to 1

According to their datasheets, the W65C02S and W65C816S, both still in production, can run up to 8MHz at 3.3V. They can also go down to 2MHz on 1.8V, or up to 14MHz on 5V. https://www.westerndesigncenter.com/wdc/documentation/w65c02s.pdf https://www.westerndesigncenter.com/wdc/documentation/w65c02... https://www.westerndesigncenter.com/wdc/documentation/w65c816s.pdf https://www.westerndesigncenter.com/wdc/documentation/w65c81...