ALU

Updated: 05/02/2021 by Computer Hope

Short for arithmetic logic unit, the ALU is a complex digital circuit with an AU (arithmetic unit) and a LU (logic unit). Multiple Arithmetic Logic Units are in a computer, including multiple ALU's in the CPU (central processing unit), GPU (graphics processing unit), and FPU (floating-point unit).

The computer central processing unit ALU performs both bitwise and mathematical operations on binary numbers and is the last component to perform calculations in the processor.

Machine cycle

The ALU uses operands and code that tells it which operations to perform for input data. After the information is processed by the ALU, it's sent to the computer's memory.

Note

The first and maybe the most famous ALU was the 4-bit Intel 74181 released in 1970. It was the first ALU placed in a single chip.

Arithmetic operations

The arithmetic unit handles all of the following computer numerical operations.

  • Add - add two bits.
  • Add with carry - add two bits with a carry-in.
  • Subtract - subtract two bits.
  • Subtract with borrow - subtract two bits with borrow from carry-in.
  • Negate - flip the bits values sign (- to + or + to -).
  • Increment - add 1 to a bit.
  • Decrement - subtract 1 from a bit.
  • Pass through - let bits through without modification.
Note

Simple ALU's have no divide or multiply operations. For these ALU's to perform these operations, they use the add and subtraction operations.

Logic unit operations

The logic unit performs logical operations (e.g., AND, OR, and NOT) and numeric tests like checking if a number is a negative number.

Architecture, Boolean, Computer acronyms, Control Unit, CPU terms, Logic gate, Machine cycle, Main memory, Von Neumann Architecture