Answer 3—The first CMOS microprocessor was the COSMAC 1801, produced by RCA and introduced in 1975. It was a two-chip set—1801R and 1801U for registers and ALU, respectively. Early in the following year, RCA introduced the COSMAC 1802, a single-chip implementation that expanded the instruction set and ran at a higher clock speed.

In addition, the COSMAC family was the first microprocessor implementation to use a fully static design, which allowed the clock to run arbitrarily slowly, or even be stopped. This gave it a number of huge advantages for certain applications. CMOS power consumption is pretty much directly dependent on clock speed, so the 1802 was good for battery-powered systems, including satellites. It also made it easy for hobbyists to work with because with the clock stopped an extremely simple external logic circuit could be used to read and write the processor’s memory using its built-in DMA mechanism.

Contributor: David Tweed