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Computer organization Notes

ROM

Read only memory: Its non volatile memory, i.e., the information stored in it, is not lost even if the power supply goes off. It’s used for the permanent storage of information. It also possesses a random access property. Information cannot be written into a ROM by the users/programmers. In other words the contents of ROMs are decided by the manufactures.
The following types of ROMs are listed below:
(i) PROM: It’s programmable ROM. Its contents are decided by the user. The user can store permanent programs, data etc in a PROM. The data is fed into it using a PROM programs.
(ii) EPROM: An EPROM is an erasable PROM. The stored data in EPROM’s can be erased by exposing it to UV light for about 20 min. It’s not easy to erase it because the EPROM IC has to be removed from the computer and exposed to UV light. The entire data is erased and not selected portions by the user. EPROM’s are cheap and reliable.
(iii) EEPROM (Electrically Erasable PROM): The chip can be erased & reprogrammed on the board easily byte by byte. It can be erased with-in a few milliseconds. There is a limit on the number of times the EEPROM’s can be reprogrammed, i.e.; usually around 10,000 times.

Difference Between RAM and ROM

RAM ROM
RAM is Random Access Memory. ROM stands for Read Only Memory.
RAM is volatile and is erased when the computer is switched off. ROM is non-volatile and generally cannot be written to it.
RAM is used for both read and write. ROM is used only for reading.
RAM needs electricity to flow to retain information. ROM is permanent.
RAM stores current executable program run by computer. ROM stores data permanently like booting etc.
RAM is used to store program and data that is subject to change. ROM stores program that are permanently in computer and do not change.
RAM is less reliable ROM is more reliable.
Its types are Static RAM and Dynamic RAM Its types are PROM, EPROM,EEPROM.





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