Read Only Momory, Types Of ROM and Advantages Of ROM – The memory from which we can only read but cannot write on it. These are 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 can also be used to store that software which is frequently needed by the computer.
ROM is also referred to as firmware, since it combines characteristic of both hardware & software, that is to say, software embedded in hardware. Note that the programs stored on ROM is called firmware. Other term used for firmware are microcode & microprogram. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.
Types of ROM
(a) 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. The contents of such ROMs have to be specified before chip production, so the actual data could be used to arrange the transistors inside the chip. It is inexpensive ROM.
(b) PROM (Programmable Read only Memory) – PROM is read-only memory (ROM) that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM programmer (PROM burner). Inside the PROM chip there are small fuses which are burnt open (cut) during programming. It can be programmed only once and is not erasable.
As a result, PROM is also known as one-time programmable (OTP) device. Blank PROMs are economical. PROM chips are valueable for companies that make their own ROMs from software they write, because when they change their code they can create new PROMs without requiring expensive equipment.
(c) EPROM ( Erasable and Programmable Read Only Memory ) – The EPROM can be erased by exposing it to ultra violet light for a duration of upto 30 minutes. Usually, a 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.
An EPROM eraser is not selective; it will erase the entire EPROM. Although EPROMs are more expensive than PROMs, their ability to be reprogrammed makes them an essential part of the software development and testing process.
(d) 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 3 to 10 ms (Milli seconds). 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 fiexible but slow. The biggest advan t slow. The biggest advantage of EEPROM is that it is non-volatile memoy and connot be updated easily while disadvantages are the high cost and at present they are not completely non-volatile and the write operation takes considerable time. Both of these disadvantages are disappearing with the growth in technology. You can also learn about Random Access Memory.
Advantages of ROM
There are 8 advantages of ROM –
- Non-volatile in nature
- Easier to interface than RAMs
- 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