Tuesday, April 17, 2012
ROM
§ ROM stands for Read only memory.
§ It is nonvolatile memory, i.e., the information stored
in it is not lost even if the power supply goes off.
§ It is used for permanent storage of information.
§ It also possesses random access property.
§ ROMs are much cheaper compared to RAMs when produced
in large volumes.
§ The stored information can only be read from ROMs at
the time of operation. Information cannot be written into a ROM by the
users/programmer. In other words ROMs are not accessible to users.
Mask-programmed ROMs- ROMs in which contents are written at the time of IC
manufacture are called mask-programmed ROM.
User-programmable ROMs- PROM, EPROM, E2PROM or any other kind of
PROM are user-programmable ROMs.
PROM:
§ PROM is a programmable ROM.
§ It contents are decided by user. The user can store
permanent programs, data or any other kind of information in a PROM.
§ PROMs are once programmable.
§ PROMs are more cost effective.
§ Example of PROM is 74S287.
EPROM:
§ An EPROM is an erasable PROM.
§ The stored data in EPROMs can be erased by exposing it
to high intensity short wave ultraviolet light for about 20 minutes.
§ EPROMs are cheap, reliable.
§ EPROMs employ MOS technology.
§ Examples of EPROMs are: Intel’s 87C257.
EEPROM (E2PROM):
§ EEPROM is an electrically erasable PROM.
§ Also known as EAPROM (Electrically Alterable PROM).
§ There is a limit on the number of times the EEPROMs
can be reprogrammed, usually 10,000 times. It not need be removed from the
computer’s board for erasing.
§ Examples of EEPROMs are: Intel 2816, Intel 2816A.
