Read Only Memory ( ROM )

Read Only Memory ( ROM )

ROM or Read Only Memory is a special type of memory which can only be read and contents of which are not lost even when the computer is switched off. It typically contains manufacturer’s instructions. Among other things, ROM also stores an initial program called the ‘bootstrap loader’ whose function is to start the computer software operating, once the power is turned on.
Read-only memories can be manufacturer-programmed or user-programmed. While manufacturer-programmed ROMs have data burnt into the circuitry, user programmed ROMs can have the user load and then store read-only programs. PROM or Programmable ROM is the name given to such ROMs.
» ROM, as the name implies, has only read capability and no write capability. After the information is stored in ROM, it is permanent and cannot be corrected.
» ROM comes programmed by the manufacturer. It stores standard processing programs that permanently reside in the computer. ROM stores the data needed for the start up of the computer. The instructions that are required for initializing the devices attached to a computer are stored in ROM.
» The ROM memory chip stores the Basic Input Output System (BIOS). BIOS provides the processor with the information required to boot the system. It provides the system with the settings and resources that are available on the system. BIOS is a permanent part of the computer. It does not load from disk but instead is stored in a ROM memory chip. The program code in the BIOS differs from ordinary software since it acts as an integral part of the computer.
The number of re-writes and the method used for the re-writing ROM’s are following types :
1. Programmable ROM (PROM)
2. Erasable Programmable ROM (EPROM)
3. Electrically Erasable Programmable ROM (EEPROM)

1. Programmable ROM (PROM)

PROM can be programmed with a special tool, but after it has been programmed the contents cannot be changed. PROM memories have thousands of fuses (or diodes). High voltage (12 V) is applied to the fuses to be burnt. The burnt fuses correspond to 0 and the others to 1.

2. Erasable Programmable ROM (EPROM)

EPROM can be programmed in a similar way as PROM, but it can be erased by exposing it to ultra violet light and re-programmed. EPROM chips have to be removed from the computer for re-writing.

3. Electrically Erasable Programmable ROM (EEPROM)

EEPROM memories can be erased by electric charge and re-programmed. EEPROM chips do not have to be removed from the computer for re-writing.

Flash Memory

Flash Memory is a kind of semiconductor-based non-volatile, rewritable computer memory that can be electrically erased and reprogrammed. It is a specific type of EEPROM.
» It combines the features of RAM and ROM. It is a random access memory and its content can be stored in it at any time. However, like ROM, the data is not lost when the machine is turned off or the electric power is cut. Flash memory stores bits of data in memory cells.
» Flash memories are high-speed memories, durable, and have low-energy consumption. Since flash memory has no moving part, it is very shock-resistant. Due to these features, flash memory is used in devices such as digital camera, mobile phone, printer, laptop computer, and record and play back sound devices, such as MP3 players.