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Main Memory Unit

Main Memory unit known as main memory or primary memory. Memory Unit is a internal memory. It is also known as semi conductor memory.

There are two part of memory unit

  • RAM- (Random Access Memory) It is a read and write memory. It is a volatile and temporary memory. Two Types of RAM. RAM is used by Users.
    • Static RAM is called static because it will continue to hold information without refreshment i.e it does not required refreshment
    • Dynamic RAM is called dynamic because it require refreshment. It loose information with time. 
  • ROM (Read Only Memory) It is a permanent memory, read only memory and Non-Volatile. ROM allows data and instruction to be read and does not allow user to write anything on it. The contents of ROM are written by the manufacturer at the time of manufacturing.
    • PROM (Programmable Read Only Memory):- PROM can be custom-programmed by the user using special circuitry. A PROM can be changed /updated only once.
    • EPROM (Erasable Programmable Read Only Memory):- This type of ROM can have its contents erased by ultraviolet light and then reprogrammed by a PROM programmer. This procedure can be carried out many times; however, the constant erasing and rewriting will eventually render the chip useless.
    • EEPROM (Electrical Erasable Programmable Read Only Memory):- This type of ROM works in a similar way to Flash memory in that it can its contents can be 'flashed' for erasure and then written to without having to remove the chip from its environment. EEPROMs are used to store a computer system's BIOS, and can be updated without returning the unit to the factory. In many cases, BIOS updates can be carried out by BIOS update.
    • Flash memory - Flash memory is non-volatile memory that is an integrated circuit that does not need continuous power to retain the data. It is much more expensive than magnetic storage. However, flash memory is widely used with car radios, cell phones, digital cameras, PDAs, MP3 players, and printers.

Difference between memories

  Access Time Storage Capacity Cost/bit of storage
Primary Memory Faster Smaller High
Secondary Memory Slower Higher Low

Based on access time, storage capacity and cost/bit storage, the memory devices can be categorized into three kinds of memory system.
  • Semiconductor memory such as RAM, ROM, PROM, EPROM, EEPROM and Flash Memory.
  • Magnetic Memory such as Hard disk, Floppy disk, and magnetic tapes.
  • Optical Memory such as Compact disk, Digital Video/Versatile disk (DVD), Blu-Ray disk.


Port is a connecting socket outside the system into which different type of cables are plugged. It is place where device are physically connected. There are many type of ports used in computer system. These ports are:-
  • Serial Port
  • Parallel Port
  • USB Port (Universal serial bus)
  • SCSI Port (Small Computer system Interface)
Serial Port:- Serial port transmits one bit of a byte, one at a time as a single stream of bits. It is used for transmitting slow data over long distances. E.g Dialup modem, serial mice use serial ports.
Parallel Port:- A Parallel port transmit 8 bit of byte of data in parallel. It is used for transmitting fast data over short distances. It is used to connect printer, monitor, projector etc.
USB Port:- A USB port can connect up to 127 peripheral devices such as a mouse, digital camera, scanners, speakers etc. It also permit to connect plug and play device.
Small computer System Interface (SCSI Port):- SCSI port allows data to be transmitted in a daisy chain to up to 7 devices at a higher speed (32 bit at a time). It is fast data transmitting device is used to connect HDD (Hard disk drive), CD-ROM drives etc with the computer.

A bus

A bus is a set (group) of parallel lines that information (data, addresses, instructions, and other information) travels on inside a computer. Information travels on buses as a series of electrical pulses, each pulse representing a one bit or a zero bit. The control unit and internal storage are linked together by sets of parallel electrical lines called bus.
  • An internal bus is a bus inside the processor, moving data, addresses, instructions, and other information between registers and other internal components or units
  • An external bus is a bus outside of the processor (but inside the computer), moving data, addresses, and other information between major components (including cards) inside the computer.

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