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 that are required to start a computer. This operation is referred
to as bootstrap. ROM chips are not only used in the computer but also in
other electronic items like washing machine and microwave oven.
Let us
now discuss the various types of ROMs and their characteristics.
MROM (Masked ROM)
The
very first ROMs were hard-wired devices that contained a pre-programmed set of
data or instructions. These kind of ROMs are known as masked ROMs, which are
inexpensive.
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 program. 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)
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 10 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)
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
(millisecond). 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 reprogramming is flexible but slow.
Advantages of ROM
The
advantages of ROM are as follows −
- Non-volatile in
nature
- Cannot be
accidentally changed
- Cheaper than
RAMs
- Easy to test
- More reliable
than RAMs
- Static and do
not require refreshing
- Contents are
always known and can be verified
