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Read Only Memory

Read Only Memory (ROM)


ROM refers to computer memory chips containing permanent data. Unlike RAM, ROM is non-volatile, that is, the data is retained in it even when the computer is turned Off. Refer Table 1.2 to understand the key differences between RAM and ROM.

Differences between RAM and ROM

RAM

  • Data can be read as well as written.
  • Data is stored temporarily.
  • Data is stored while the computer is being used by users to hold their data.
  • It is required while the computer is being used by users to run their applications.

ROM

Data can only be read.

  • Data is stored permanently.
  • Data is stored during the time of fabrication.
  • It is required for starting the computer, and storing

Most computers contain a small amount of ROM that stores critical programs which are used to start the computer when it is turned On. Originally, ROM was actually read only. So, in order to update the programs stored in ROM, the ROM chip had to be removed and physically replaced by the ROM chip that has a new version of the program. However, today ROM chips are not literally read only, as updates to the ROM chip are possible. The process of updating a ROM chip is a bit slower as memory must be erased in large portions before it can be re-written.

Programmable read-only memory (PROM)

 also called one-time programmable ROM can be written to or programmed using a special device called a PROM programmer. The working of a PROM is similar to that of a CD-ROM recorder which enables the users to write programs just once but the recorded data can be read multiple times. Programming a PROM is also called burning.

Erasable programmable read-only memory (EPROM) 

is a type of ROM that can be erased and re-programmed. The EPROM can be erased by exposing the chip to strong ultraviolet light typically for 10 minutes or longer and then rewritten with a process that again needs higher than usual voltage applied.

Electrically erasable programmable read-only memory (EEPROM) 

allows its entire or selected contents to be electrically erased, then rewritten electrically. The process of writing an EEPROM is also known as flashing.


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