A computer performs all calculation and comparison operations in the ALU. During processing of a job, the computer transfers data and instructions stored in its primary storage to ALU as and when needed. ALU does the processing and the computer temporarily transfers the intermediate result generated there back to primary storage unit needed later. hence, data may move back and forth several times between primary storage and ALU before processing of the job is over.
Almost all alus are designed to perform the basic arithmetic operations(add, subtract, and divide) and logic operations or comparisons such as, less than, equal to, and greater than.
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