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The Basic Components of a Digital Computer

Input Devices:
These are the devices using which the user provides input instances. In a programmable computer, input devices are also used to input programs. Examples: Keyboard, mouse.

Output Devices:
These devices notify the user about the output of a computation. Example: screen, printer, speaker.

Processing Unit:
The central processing unit (CPU) is the brain of the computing device and performs the basic processing steps. A CPU typically consists of:

An arithmetic and logical unit (ALU): 
This provides the basic operational units of the CPU. It is made up of units (like adders, multipliers) that perform arithmetic operations on integers and real numbers, and of units that perform logical operations (logical and bitwise AND, OR etc.).
A control unit: This unit is responsible for controlling flow of data and instructions.

General Purpose Registers:
A CPU usually consists of a finite number of memory cells that work as scratch locations for storing intermediate results and values.

External Memory:
The amount of memory (registers) resident in the CPU is typically very small and is inadequate to accomodate programs and data even of small sizes. Out-of-the-processor memory provides the desired storage space. External memory is classified into two categories:

Main(or primary) memory: 
This is high-speed memory that stays close to the CPU. Programs are first loaded in the main memory and then executed. Usually main memory is volatile, ie, its contents are lost after power-down.

Secondary Memory:
This is relatively inexpensive, bigger and low-speed memory. It is normally meant for off-line storage, ie, storage of programs and data for future processing. One requires secondary storage to be permanent, ie, its contents should last even after shut-down. Examples of secondary storage include floppy disks, hard disks and CD-ROM disks.

A bus is set of wires that connect the above components. Buses are responsible for movement of data from input devices, to output devices and from/to CPU and memory.