ROM-Read only memory. Volatile or Non volatile

ROM (read-only memory) is a non-volatile memory type. This means it receives data and permanently writes it on a chip, and it lasts even after you turn off your computer. The data is coded to not be overwritten, so it's used for things like your printer software or your startup programs. You can't change your ROM data just by doing ordinary computer activities.

In fact, you can’t actually directly access data stored in the ROM. If you need data from the ROM, you must first transfer it to the RAM, where your processor can then access it.

                  Block diagram of ROM

The diagram above displays the block diagram of a ROM containing m output lines and n input lines. Every possible combination of the input variables bits corresponds to an address. Every combination of bits received from the output lines is known as a word.

The number of output lines (m), is the same as the number of bits per word (t). The address represented by a binary number specifies one of the addresses of n variables. The total number of possible distinct addresses that can be generated using n input variables is 2^n

 , as each variable has the choice of being included or not, which results in 2*2*... n times.

An output word is selected through a unique address. Since there are a total of 

2×2 ^n

  distinct addresses in a read only memory, there are a total of  2×2^ n

  distinct words in the read only memory. The address value applied to the input lines determines the word that will be available, at any given instance, on the output lines.

How does ROM work :-

    ROM is sustained by a small, long-life battery in the computer. It contains two basic components: the decoder and the OR logic gates. In ROM, the decoder receives input in binary form; the output will be the decimal equivalent. The OR gates in ROM use the decoder's decimal output as their input.

ROM performs like a disk array. It contains a grid of rows and columns that are used to turn the system on and off. Every element of the array correlates with a specific memory element on the ROM chip. A diode is used to connect the corresponding elements.

When a request is received, the address input is used to find the specific memory location. The value that is read from the ROM chip should match the contents of the chosen array element.

                    Fig: Types of ROM

Types of ROM:- 

1. MROM

MROM is an abbreviation for Masked Read Only Memory. It is a read only memory chip that is programmed (data is stored in it) during it's manufacturing. MROMs cost relatively low.

MROMs were the first ROMs to be developed and were hard wired, contain a pre-programmed set of instructions or data. During the design phase in the manufacturing process of a MROM, a software mask is burned directly onto the chip.

The customer receives the specifications of the MROM in a certain format and in tabular form. It's the job of the manufacturer to make the corresponding mask for the paths to generate the required output.

2. PROM

PROM is an abbreviation for Programmable Read Only Memory. PROMs are also called as PLD, as they are used in logic designs. It's a device that includes both the OR plane as well as the AND plane within a single IC package. AND plane is fixed and OR plane is programmable. A plane is a sequence of gates such as AND, OR, etc as shown in the diagram above. it is also known as an array.

The AND array (AND plane) of a PROM acts as a decoder. It will decode the data from the address lines. The circuits in a PROM can be designed and modified easily and fast.

3. EPROM

EPROM stands for Erasable Programmable Read Only Memory. It is a chip that is non-volatile in nature and was invented in 1971 by Dov Frohman at Intel. An EPROM can be reprogrammed if required by exposing it to ultraviolet light. But otherwise, an EPROM does not save or accept any new data. EPROM chips are not used in modern computers as they have been replaced by EEPROM chips.

EPROM is used by hardware manufacturers when it is required that the data stored in the ROM be changed. A distinguishing feature of EPROM chips is a small quartz crystal circle window exposing the chip so that it could be reprogrammed.

4. EEPROM

EEPROM is an abbreviation for Electrically Erasable Programmable Read Only Memory. It is non-volatile in nature and is used for storing small amounts of data in computer systems or some other electronic devices. In an EEPROM, write and erase operations are performed one byte at a time In an EEPROM an individual byte of data could be erased and reprogrammed entirety, by application of electrical potential.

EEPROM technology was developed in 1978 at Intel by George Perlegos. EEPROMs have been replaced PROMs and EPROMS. An EEPROM needs not be ejected from the computer system for making modifications to it. A drawback of EEPROM chips is that they perform read and write cycle slowly in comparison to primary memory (RAM).

5. Flash Memory

Flash memory is non-volatile in nature and is therefore has several use cases in which short to medium-term data storage is required. Flash memory technology implementations range from flash USB memory sticks (also known as pen drives) to camera memory cards like compact Flashcards or CF cards and SD memory cards and solid state drives (SSDs) in a computer system.

Flash memory is one of the most widely used forms of memory. Flash memory storage was born out of a combination of the traditional EPROM and E2PROM (another type of memory). Flash memory utilizes the same method of programming as does EPROM and the erasure method as that of E2PROM.

Conclusion

• ROM, also known as firmware, is an integrated circuit programmed with certain data during the time it is manufactured.
• ROM consists of both the decoder as well as the OR gates within a single IC package.
• ROMs are of five types: MROM (masked read only memory), PROM (programmable read only memory), EPROM (erasable programmable read only memory), EEPROM (electrically erasable programmable read only memory), and Flash Memory.