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TWO SECTIONS OF CPU
The Central Processing Unit (CPU) performs the actual processing of data. The data it processes is obtained, via the system bus, from the main memory. Results from the CPU are then sent back to main memory via the system bus. In addition to computation the CPU controls and co-ordinates the operation of the other major components. The CPU has two main components, namely:

  • The Control Unit -- controls the fetching of instructions from the main memory and the subsequent execution of these instructions. Among other tasks carried out are the control of input and output devices and the passing of data to the Arithmetic/Logical Unit for computation.
  • The Arithmetic/Logical Unit (ALU) -- carries out arithmetic operations on integer (whole number) and real (with a decimal point) operands. It can also perform simple logical tests for equality and greater than and less than between operands.
It is worth noting here that the only operations that the CPU can carry out are simple arithmetic operations, comparisons between the result of a calculation and other values, and the selection of the next instruction for processing. All the rest of the apparently limitless things a computer can do are built on this very primitive base by programming!

Modern CPUs are very fast. At the time of writing, the CPU of a typical PC is capable of executing many tens of millions of instructions per second.


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