Ken Shirriff does it again, namely dissecting a classic chip. This time, it’s about the Motorola MC14500B one-bit microprocessor, which was also one of the first CMOS processors. While processing a single bit may be considered a rather humble endeavor, it also poses a singular opportunity to inspect the implementation of a processor in close-up detail…
A one-bit processor explained: reverse-engineering the vintage MC14500B
The Motorola MC14500B is a 1-bit processor introduced in 1976. While a 1-bit processor might seem almost useless, it was marketed as an Industrial Control Unit for applications that made simple decisions based on Boolean logic, for example, air conditioning, motor control, or traffic lights. (…) The MC14500B has roughly 500 transistors, very few for a microprocessor. In comparison, the popular 8-bit Z-80 microprocessor, also released in 1976, had 8500 transistors. Even the first microprocessor, the 4-bit Intel 4004 (1971), contained 2250 transistors.
You might think that a 1-bit processor would only support two instructions, making it impractical. However, like many processors, the MC14500B uses different sizes for data and instructions. Although it used one bit for data, its instructions were 4 bits, giving it a small but usable instruction set of 16 instructions.
I played with the MC14500 in 82/83 in high school. My very forward thinking physics teacher decided to introduce it as part of the electronics option of the A-Level physics course. It was on this course that I was introduced to 4000 series CMOS logic.
It’s good to see Ken Shirriff cast some light again on this unusual device.
Motorola were right to market it as an industrial control unit, and their target market was to replace fixed function ladder logic with a programmable unit.
Using input multiplexers it could poll the state of a series of input switches or sensors, make a sequence of logic decisions and control the state of a series of output devices, often relays for switching motors, lamps or other electrical loads.
It was not a single chip processor, it required several support ICs including input multiplexers, output drivers (demultiplexers) and external 4-bit ROM to hold the program.
It could perform boolean operations one bit at a time. This was a subtle difference to bit-serial processors, which could perform arithmetic and logic functions on longer wordlengths - albeit at 1 bit at a time.
500 transistors is a very low count for any processor. The PDP-8/S which was the bit serial version of the PDP-8 was said to have 519 transistors, compared to the 1500 of the original PDP-8.