There are other chips like that which can be single-stepped. The one that I've driven with a push-button wired to the clock-input would be the Z80. The z80 is

There are other chips like that which can be single-stepped. The one that I've driven with a push-button wired to the clock-input would be the Z80. The z80 is very tolerant to slow clocks, letting you pulse every second or two to step forwards and watch the data & I/O lines in real-time.