Ken Shirriff Reverse-Engineers the Motorola MC14500B, an Unusual One-Bit CPU for Industrial Use

Designed for industrial control, though "the opposite of a microcontroller in many ways," Motorola's unusual chip gets dissected.

Gareth Halfacree
3 years agoRetro Tech

Noted die-level reverse engineer Ken Shirriff has turned his attentions to a most unusual part, first introduced in 1976: a one-bit processor, the Motorola MC14500B.

The pinnacle of computing technology in the 1970s was the eight-bit microprocessor, exemplified by devices like the popular Zilog Z80, after Intel had opened the market with the launch of the four-bit Intel 4004 in 1971. While Motorola had its own eight-bit parts, though, it also launched a one-bit part — targeting industrial usage.

"The Motorola MC14500B1 is a 1-bit processor introduced in 1976," Shirriff explains. "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."

"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."

Looking at a high-resolution die shot of an MC14500B, Shirriff was able to identify a range of functional blocks: An instruction register, instruction decoder, a combined instruction decoder and logic unit, registers, latches, and output drivers. "The MC14500B has an unusual architecture," he notes, "making it more of a building block than a complete microprocessor."

"In particular, the chip doesn't include any support for memory or addresses; it didn't even have a program counter. The program counter, instruction fetches, jumps, subroutine calls, and I/O needed to be implemented with external circuitry. This is a key reason that the chip was so simple. (The other reason, of course, was that it only supported one bit.)"

Shirriff's full analysis, which includes a look at how the CMOS logic is implemented and how the "rather unusual" logic unit works, is available on the Righto.com website.

📷 Main article image courtesy of siliconpr0n and Ken Shirriff.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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