Summary of Inside the 74181 ALU chip: die photos and reverse engineering
The article details the reverse engineering of a 74181 ALU chip, revealing its internal TTL circuitry. It contains approximately 67 gates and 170 transistors built on silicon with aluminum interconnects. This chip was historically significant for performing arithmetic and logical operations in 1970s minicomputers before the microprocessor era.
Parts used in the 74181 ALU:
- TTL (transistor-transistor logic) circuitry
- Gates (approximately 67)
- Transistors (approximately 170)
- Silicon substrate
- Aluminum metal layer
- Bond wires
- External pins
- Doped silicon regions
- Resistors
What’s inside a TTL chip? To find out, I opened up a 74181 ALU chip, took high-resolution die photos, and reverse-engineered the chip.1 Inside I found several types of gates, implemented with interesting circuitry and unusual transistors. The 74181 was a popular chip in the 1970s used to perform calculations in the arithmetic-logic unit (ALU) of minicomputers. It is a moderately complex chip containing about 67 gates and 170 transistors3, implemented using fast and popular TTL (transistor-transistor logic) circuitry.
The 74181 die photo is below. (Click the image for a high-resolution version.) The golden stripes are the metal layer that interconnects the circuitry of the chip. (It’s not gold, just aluminum that looks golden from the lighting.) The white squares around the edge of the die are the pads that are connected by tiny bond wires to the external pins. Under the metal layer is the silicon that makes up the chip. Faint lines show the doped silicon regions that make up the transistors and resistors. While the chip may appear impossibly complex at first, with careful examination it is possible to understand how it works.
The 74181 chip is important because of its key role in minicomputer history. Before the microprocessor era, minicomputers built their processors from boards of individual chips. The arithmetic operations (addition, subtraction) and logical operations (AND, OR, XOR) were performed by the arithmetic/logic unit (ALU) in the processor.
for more detail: Inside the 74181 ALU chip: die photos and reverse engineering
- What is inside a 74181 chip?
It contains about 67 gates and 170 transistors implemented using TTL circuitry. - How many transistors are in the 74181?
The chip contains approximately 170 transistors. - What material makes up the interconnect layer on the die?
The golden stripes are actually an aluminum metal layer that looks golden from the lighting. - How were the external pins connected to the die?
They were connected by tiny bond wires to the pads around the edge of the die. - What role did the 74181 play in minicomputers?
It performed arithmetic and logical operations within the arithmetic-logic unit of the processor. - When was the 74181 popular?
It was a popular chip in the 1970s before the microprocessor era. - Does the chip contain resistors?
Yes, faint lines show doped silicon regions that make up transistors and resistors. - Can you understand how the chip works just by looking at it?
With careful examination of the die photo, it is possible to understand how it works.
