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ROM, PROM or EPROM are used to store information which must be retained when the power is off, typically they would be used to store the BIOS code.
ROM and PROM are memory types which cannot be erased by electrical signals (provided they are not sufficient to damage it!).
The difference between them is:
ROM is normally 'mask programmed' during manufacture. The data is part of the photographic image used to make the IC.
PROM is an empty memory which can be electrically programmed once only, it has no erase function. They allow latest software version to be programmed permanently.
EPROM can be erased and reprogrammed - for example when upgrading a BIOS.
EEPROM is quite different, it is a memory which is designed to be reprogrammed more frequenty, for example the BIOS configuration settings. It can be treated almost like RAM except it retains its contents when power is removed.
Most computers will use one of ROM, PROM or EPROM but probably not all of them.
Thanks for quick reply. How these memories look like? Are they simply IC chips? Also where are they physically present for example I can locate the RAM in one of the slots on my mother board. Also how can I know which of them are present on my PC.
the ROM equivalent in PC can be located by identifying the chip marked as 'bios'
usually a silvered paper pasted chip with marking like 'ami bios , award bios ' etc.
they are flash types in current pcs.
an eeprom or similar is there. but you cannot identify separately.
if you can locate rtc chip , then internally it has that.
Yes, they all look like little black plastic boxes! @srizbf's description is a good as it can be and quite right in saying that in modern systems the ROM/PROM/EPROM has largely been superceded by flash technology. Flash is similar to EEPROM in operation but is normally heavily protected to prevent accidental corruption and special software is needed to 'unlock' it so it's contents can be changed.
Besides the PC mainboard, also "intelligent" peripherals like graphic cards or supplemental interfaces have their own BIOS flash. Small capacity serial EEPROMS are used to hold the MAC address of ethernet cards.
We missed one: there is often a tiny eeprom on the RAM sticks. It holds information on the serial number and type of main RAM on the stick so the motherboard can adjust the signal timing automatically.