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What is ROM? Types of ROM and how it works

If you’re unaware, the acronym ROM stands for Read-Only Memory. ROM chips are not only used in computers but also in other electronic devices, such as smartphones, as well. Also known as the storage for a hardware’s firmware, it is an integrated circuit that is programmed with specific data at the time it is manufactured.

ROM is a non-volatile memory, which means that unlike RAM it does not lose the data once the device is shut down, and that’s one of the main reasons it’s used as secondary storage on devices that contains fixed programs and data.

How does ROM work?

The Read-Only Memory operates like how an array does. The ROM contains a grid of rows and columns that are to turn on or off. If the value is 1, it uses a diode to connect the lines. When the value is 0, the lines are not connected at all. Each element of the array corresponds to the storage element present in the memory chip.

The address which is fed to the chip is employed to pick out a specific memory location. Then the value which is read from the memory chip corresponds to the contents of the selected element of the array. It is known that ROM consists of two basic components that are Decoder and OR gates.

In ROM, when there is input to the decoder, it will be in the binary form, and the output will be decimal equivalent to it. All the OR gates which are present in the ROM will take the output of the decoder as their input.

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Types of ROM

There are five basic types of ROM:

  • MROM (Mask Read Only Memory): These were the first ROM, which was hard-wired devices, and were pre-programmed with data.
  • PROM (Programmable ROM): Unlike regular ROMs, these chips come blank and can only be modified once by the user with the help PROM program. Once programmed, the chip can’t be overwritten or erased.
  • EPROM (Erasable Programmable ROM): As the name suggests, EPROM can be erased via a specific eraser that exposes the ROM to ultra-violet light for several minutes to delete the data. Once the data is erased, EPROM can be reprogrammed.
  • EEPROM (Electrically Erasable Programmable ROM): The immediate predecessor of the current ROM in use, similar to EPROM, this one is reprogrammable — up to 10,000 times — but a much quicker process erases it via an electrical charge. The chip can be erased selectively rather than having to wipe off the data, as is the case with the one mentioned above.
  • Flash (EPROM) memory: The youngest of the ROM family, flash memory can be erased more number of times — 1 million-plus — and rewritten faster than EEPROM. Far from the hard-wired device, this ROM features a silicon chip, and unlike its predecessors, the reprogramming is much more flexible and quicker.

Each type of ROM has unique characteristics; however, they have two things in common. First is that the data stored in these chips are non-volatile, which means that the data is not lost when the power is removed. Second is that the data is either unchangeable or requires a special operation to change.

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