Computer Science Principles V11

Clive W. Humphris


The Arithmetic and Logic Unit (ALU) is the heart of a microprocessor. Under the control of the instruction register it performs the addition, subtraction, multiplication and division functions of the two binary numbers. Plus all the logic operations such as compare, copy, shift right, shift left and many more. Here we show an ALU that is 4 bits wide, eight, sixteen and thirty-two bit processors are available.

The arithmetic/logic function is selected from the 2-bit operation code, 00, 01, 10 or 11. The output of the 2-4 line decoder will then activate one of the analogue switches directing the result of that function to the output register.

After each instruction fetch cycle, the contents of the program counter must be incremented to point to the next address in the program.

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