# The Arithmetic and Logic Unit

## The Arithmetic and Logic Unit

The Arithmetic and logic unit performs all computations (addition, sub­traction. Multiplication, and division) and all logic operations (compar­isons).

Examples of computations include the payroll deduction for Social Se­curity, the day-end inventory, and the balance on a bank statement. A logic operation compares two pieces of data. Then, based on the result of the comparison, the program "branches" to one of several alternative sets of program instructions. Let's use an inventory system to illustrate the logic operation. At the end of each day the inventory level of each item in stock is compared to a reorder point. For each comparison indicating an inven­tory level that falls below (<) the reorder point, a sequence of program in­structions is executed that produces a purchase order. For each compari­son indicating an inventory level at or above (= or >)the reorder point, another sequence of instructions is executed .

The arithmetic and logic unit also does alphabetic comparisons. For example, when comparing Smyth and Smith, Smyth is evaluated as being greater alphabetically, so it is positioned after Smith.

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