Computer Architecture

Discuss the arithmetic and logic unit (ALU)

Arithmetic and logic unit (ALU)

The arithmetic logic unit (ALU) of a computer system is the place where the actual execution of the instructions takes place during the processing operation. That is, it is also responsible for all calculations performed and all comparisons (decisions) made in the ALU. The data and instructions, stored in the primary storage prior to processing, are transferred as and when needed, to the ALU where processing takes place. Intermediate results generated in the ALU are temporarily transferred back to the primary storage until needed at a later time. Data may move from memory to ALU and back again to storage many times before the processing is over. There are a number of arithmetic and logic operations that a computer can perform. These are add, subtract, multiply, divide and logic operations or comparisons such as less than, equal to or greater than.


Fig. 2.1 shows, in general terms, how the ALU is interconnected with the rest of the processor. Data are presented to the ALU in registers and the results of an operation are stored in registers. These registers are temporary storage locations within the processor that are connected by signal paths to the ALU. The ALU can also set flags as the result of an operation. The flag values are also stored in registers within the processor. The control unit provides signals that control the operation of the ALU and the movement of the data into and out of the ALU.

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