ROM stands for Read Only Memory. It is a class of computer memory which could only be read. Such that the contents of the memory could be read multiple times but could not be written (modified), once written. ROM is a non-volatile type of memory, therefore data in it is retained even after the unavailability of power. Read-only memory is useful for storing software/Data that is rarely changed throughout the life of the system, also known as firmware. ROM is cheap to produce as compared to other non-transient storage mediums and hence experienced widespread adoption of the technology.
Types of ROM
Although, as the name suggests the memory isn’t strictly read only as other variants of the same technology exists which do offer features such as writing data to it, after the data has been written initially. The reason for them using a different underlying mechanism as then a ROM. A ROM contains integrated circuits that physically encode the stored data. This makes the data impossible to be rewritten and hence maintains the integrity of the data. But other types of ROM use semiconductor-based solid-state memory, which allows for modification of the written data. Types of ROM technology are:
- MROM – Mask ROM is a kind of read-only memory, that is masked off at the time of production. Like other types of ROM, mask ROM cannot enable the user to change the data stored in it. If it can, the process would be difficult or slow.
- PROM: PROM stands for Programmable read-only memory. A PROM is shipped as a ROM without any data/software inside it. Therefore the user is free to program any data inside it. But once the data has been written to the PROM, it could no longer be written or modified. It could only be used to read the data which was initially written to it. The way PROM differs from a regular ROM is that a generic ROM has data prewritten to it (BIOS, Embedded systems, etc), and the user (or device containing the ROM) could only read it. But a PROM is empty at the beginning, and therefore custom data could be written to it as our requirement.
- EPROM: EPROM stands for Erasable Programmable read-only memory. The data inside could be erased if the EROM chip is exposed to strong ultraviolet light for some duration. After which the contents of the EPROM are lost, and newer data could now be rewritten to it.
- EEPROM: EEPROM stands for Electrically Erasable Programmable Read Only Memory. EEPROM builds upon the principle of EPROM (ability to re-write data). EEPROM allows the stored data to be erased electrically. After which the data could be rewritten Electrically. Since, the data is written electrically the longevity of the EPROM chip increases.
Advantages of ROM
- Non-Volatile: Does not require a continuous uninterrupted power supply to maintain the data in the device, once the data has been written. Therefore preserves information even after the loss of power.
- Cost: Cost-efficient. Hence, could be integrated into devices with an economical budget.
- Circuitry: Incorporates a simple underlying mechanism, and could easily be reproduced without much trouble.
- Data Security: Since the data could only be written once, the mechanism inherently provides write protection to the data. Therefore, accidental deletion of data could not happen (assuming ROM is used rather than one of its writable semiconductor variants).
Disadvantages of ROM
- Reading is faster (nanoseconds per bit) as opposed to writing data to ROM (milliseconds per bit) (assuming it is a writable variant of ROM)
- Has low storage capacity, and hence could only be used to store small (but critical) data