The Central Processing Unit

The CPU is a main part of any digital computer. In addition to performing arithmetic and logic operations on data, it coordinates and controls the activities of other units of the computer system. It determines which operations should be carried out and in what order.

The CPU is a single chip device containing millions of tiny, interconnected electrical components. Mainframe and supercomputer CPUs sometimes consist of several linked microchips, called microprocessors, each of which performs a specific task, but most other computers require only a single microprocessor as a CPU.

Most CPUs have two functional sections: the arithmetic/logic unit (ALU) and the control unit (CU).

• The ALU is that part of the computer in which the four basic arithmetic operations (addition, subtraction, multiplication, division) and certain logic operations (such as the comparing of data and the selection of the required problem-solving procedure) are performed.

• The CU times and regulates the whole computer system. It obtains instructions from the program stored in main memory, interprets the instructions, and transmits signals that cause other units of the system to execute them. The CU has the following components: a counter that selects and retrieves the instructions from memory; a register that temporarily holds the instructions while they are being processed; and a decoder that takes the coded instruction and breaks it down into individual commands necessary to carry it out. A very fast clock times and regulates all the operations performed by a CPU. Every tick, or cycle, of the clock causes each part of the CPU to begin its next operation and to stay synchronized with the other parts. The faster the CPU's clock, the faster the computer can perform its tasks. The clock speed is measured in hertz (Hz). Today's desktop computers have CPUs with 1 to 4 GHz (gigahertz) clocks. The fastest desktop computers therefore have CPU clocks that tick 4 billion times per second. A CPU can perform a very simple operation, such as copying a value from one register to another, in only one or two clock cycles. The most complicated operations, such as dividing one value by another, can require dozens¹ of clock cycles.

 

The basic operation of the CPU is analogous to a computation carried out by a person using an arithmetic calculator, as illustrated in Figure 3. The control unit corresponds to the human brain and the memory to a notebook that stores the program, initial data, and intermediate and final computational results.

 

Figure 3