What is ROM? And Their Types

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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 machines, VCRs, game consoles, car radios, and microwave ovens. ROM chips come built into an external unit like flash drives and other auxiliary memory devices or installed into the device’s hardware on a removable chip. Non-volatile memory like ROM remains viable even without a power supply.

What does ROM do?

ROM stores firmware, which is permanent software that’s hard-wired with integrated circuits. The manufacturer fits a specifically designed ROM chip into a device during the building process.

Unlike the non-volatile memory of a hard drive, it’s difficult and time-consuming to rewrite a ROM chip’s memory. In most cases, it’s impossible to alter integrated circuits, which may mean you can’t update more basic forms of ROM. You may be able to reprogram some modern types of ROM, but only at slow speeds and with special equipment. These hurdles make traditional ROM poorly suited for functions that require frequent feature updates or are prone to bugs or security issues.

Conventional ROM is pre-programmed to suit its intended hardware. It often controls functions that are both integral to the utility of the device and unlikely to need modification or personalization. One kind of data usually stored in a ROM chip is the Basic Input Output System (BIOS). A device’s BIOS controls the way the device responds to cues from the user at the most fundamental level, such as a computer’s “booting-up” process that occurs when the system powers on.

Let us now discuss the various types of ROMs and their characteristics.

Types of ROM

1. 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, which are inexpensive.

2. 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 that are burnt open during programming. It can be programmed only once and is not erasable.

3. 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.

4. 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 

• Nonvolatile 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

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