74181

The 74181 is a 4-bit slice arithmetic logic unit (ALU), implemented as a 7400 series TTL integrated circuit.

The 74181 represents an evolutionary step between the CPUs of the 1960s, which were constructed using discrete logic gates, and single-chip microprocessors of the 1970s.

Although no longer used in commercial products, the 74181 later was used in hands-on computer architecture courses and is still referenced in textbooks and technical papers.

The 4-bit wide ALU can perform all the traditional add / subtract / decrement operations with or without carry, as well as AND / NAND, OR / NOR, XOR, and shift.

Many variations of these basic functions are available, for a total of 16 arithmetic and 16 logical operations on two four-bit words.

For example, sixteen 74S181s and five 74S182 look ahead carry generators can be combined to perform the same operations on 64-bit operands in 28 nanoseconds (36 MHz).

The 74181 was used in various minicomputers and other devices beginning in the 1970s, but as microprocessors became more powerful the practice of building a CPU from discrete components fell out of favour and the 74181 was not used in any new designs.