How the CPU works


The Central Processing Unit (CPU)


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 How the CPU works


The CPU is centrally located on the motherboard. Since the CPU carries out a large share of the work in the computer, data pass continually through it. The data come from the RAM and the units (keyboard, drives, etc.). After processing, the data is sent back to the RAM and the units.

The CPU continually receives instructions to be executed. Each instruction is a data processing order. The work itself consists mostly of calculations and data transport.

The Instruction-Execution Cycle

Many types of personal computers can execute instructions in less than one-millionth of a second; supercomputers can execute instructions in less than one-billionth of a second.

The CPU performs four steps in executing an instruction:

    1. The control unit gets the instruction from memory.
    2. The control unit decides what the instruction means and directs the necessary data to be moved from the memory to the arithmetic logic unit.
    3. The arithmetic logic unit performs the actual operation on the data.
    4. The result of the operation is stored in memory or a register.

The first two instructions make up what is called the instruction time. The last two instructions make up what is called the execution time.

The combination of these two is called a machine cycle.

Each central processing unit has an internal clock (or system clock), which produces pulses at a fixed rate to synchronise all computer operations. A single machine cycle instruction is made up of a number of subinstructions, each of which must take at least one clock cycle.

Each type of CPU is designed to understand a specific group of instruction called the instruction set.

How the CPU finds Instructions and Data

The location in memory for each instruction and each piece of data is identified by an address, or a number that stands for a location in the computer memory.

An address may be compared to a mailbox in everyday life, except that the address can hold only one item - a fixed amount of data, a number or a word - at any one time.

The following is an example of a simple case of adding two numbers together and placing the result in a location X.
The command executed is - Let X = N1 + N2. See the diagram below.


Adding 2 Numbers


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