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.