ROM
ROM
stands for Read Only Memory. The memory from which we can only read but cannot
write on it. This type of memory is non-volatile. The information is stored
permanently in such memories during manufacture.
A
ROM stores such instructions as are required to start computer when electricity
is first turned on, this operation is referred to as bootstrap. ROM chip are
not only used in the computer but also in other electronic items like washing
machine and microwave oven.
Following are the various types of ROM:
MROM (Masked ROM)
The
very first ROMs were hard-wired devices that contained a pre-programmed set of
data or instructions. These kinds of ROMs are known as masked ROMs. It is
inexpensive ROM.
PROM (Programmable Read only Memory)
PROM is read-only memory that can be
modified only once by a user. The user buys a blank PROM and enters the desired
contents using a PROM programmer. Inside the PROM chip, there are small fuses,
which are burnt open during programming. It can be programmed only once and is
not erasable.
EPROM (Erasable and Programmable Read Only
Memory)
The
EPROM can be erased by exposing it to ultra-violet light for a duration of up
to 40 minutes. Usually, an EPROM eraser achieves this function. During
programming, an electrical charge is trapped in an insulated gate region. The
charge is retained for more than ten years because the charge has no leakage
path. For erasing this charge, ultra-violet light is passed through a quartz
crystal window (lid). This exposure to ultra-violet light dissipates the
charge. During normal use the quartz lid is sealed with a sticker.
EEPROM (Electrically Erasable and Programmable Read Only Memory)
The EEPROM is programmed and erased
electrically. It can be erased and reprogrammed about ten thousand times. Both
erasing and programming take about 4 to 10 ms (milli second). In EEPROM, any
location can be selectively erased and programmed. EEPROMs can be erased one
byte at a time, rather than erasing the entire chip. Hence, the process of
re-programming is flexible but slow.
Advantages of ROM
- Non-volatile in nature
- These can not be accidentally changed
- Cheaper than RAMs
- Easy to test
- More Reliable than RAMs
- These are static and do not require refreshing
- Its contents are always known and can be verified
