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
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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