Arithmetic Logic Unit

(Arithmetic Logic Unit) The high-speed CPU circuit that does calculating and comparing. Numbers are transferred from memory into the ALU for calculation, and the results are sent back into memory. Alphanumeric data are sent from memory into the ALU for comparing. The results are tested by GOTOs; for example, IF ITEMA EQUALS ITEMB GOTO UPDATE ROUTINE.

Some chips have multiple ALUs that allow for simultaneous calculations. For example, Chromatic Research's Mpact media processor chip had 450 of them. It allowed audio, video and other multimedia processes to be performed simultaneously. See Mpact chip.

The arithmetic-logic unit (ALU) performs all arithmetic operations (addition, subtraction, multiplication, and division) and logic operations. Logic operations test various conditions encountered during processing and allow for different actions to be taken based on the results. The data required to perform the arithmetic and logical functions are inputs from the
designated CPU registers and operands.

The ALU relies on basic items to perform its operations. These include number systems, data routing circuits (adders/subtracters), timing, instructions, operands, and registers. Figure 1-2 shows a representative block diagram of an ALU of a microcomputer. PRIMARY STORAGE (MAIN MEMORY) The primary storage section (also called internal storage, main storage, main memory, or just memory) serves four purposes:
* To hold data transferred from an I/O device to the input storage area, where it remains until the computer is ready to process it. This is indicated by the solid arrow on figure 1-1.
* To hold both the data being processed and the intermediate results of the arithmetic-logic operations. This is a working storage area within the storage section. It is sometimes referred to as a scratch pad memory.
* To hold the processing results in an output storage area for transfer to an I/O device.

ALU is a digital circuit that performs arithmetic and logical operations. The ALU is a fundamental building block of the central processing unit (CPU) of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers. The processors found inside modern CPUs and graphics processing units (GPUs) accommodate very powerful and very complex ALUs; a single component may contain a number of ALUs.

Mathematician John von Neumann proposed the ALU concept in 1945, when he wrote a report on the foundations for a new computer called the EDVAC. Research into ALUs remains an important part of computer science, falling under Arithmetic and logic structures in the ACM Computing Classification System.