The Computer Inside-What is the ALU?

What is the ALU?

Simple processes are performed by the Arithmetic Logic Unit (ALU for short). The ALU is made up of devices called gates that receive one or more inputs and, based upon what function they are designed to perform, output a result. The ALU in our example performs one of seven functions: NOT, Left Shift, Right Shift, Add, Subtract, AND, and OR.

ALU Functions

NOT: Every bit in a word is changed into its opposite. Ones become zeros, and zeros become ones. For example: NOT 1010 yeilds 0101.

Left Shift: All of the bits in a word are shifted one position to their left. The left-most bit is lost, and the right-most bit is set to zero. For example: lshift(1011) yeilds 0110.

Right Shift: All of the bits in a word are shifted one position to their right. The right-most bit is lost this time, and the left-most bit is generally set to zero. For example: rshift(1011) yeilds 0101.

Add and Subtract: Adds or subtracts one value from another.

AND: Stands for the bitwise AND function. In a bitwise AND, each bit of the output is a one only if both of the input bits are a one. For example: 1010 AND 0110 yeilds 0010.

OR: Works much the same way as AND, only each output bit is a one if either of the input bits are a one. For example: 1010 OR 0110 outputs 1110.

The ALU takes two inputs, loaded from registers on the chip. For the commands Not through Right Shift, only the first input is used. The second is completely ignored. The ALU can also let its first input pass through unchanged. The function executed is determined by three additional input bits, specifying the number of the function. Depending on the result of the function, the ALU outputs two bits, called N and Z. N is a one when the result is negative (i.e. the left-moat bit is a one), and Z is a one when the result is zero. These bits are helpful for determining what the computer should do next. Then the answer is routed back to a register to be stored for future use.

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