Introduction to Read Only Memory (ROM) and Its Types

What Is ROM

ROM stands for read only memory, a solid-state semiconductor memory that can only read data stored in advance. Its characteristic is that once the data is stored, it can no longer be changed or deleted. It is usually used in computer or other electronic devices, and even if the power is turned off, the data will not disappear.

The most widely used form of primary storage is the volatile form of random access memory (RAM), which means that any content contained in RAM will be lost when the computer is turned off.

While even ROM is a kind of non-volatile memory, it is not suitable for the use as primary storage due to some limitations. In general, non-volatile memories are more expensive, have lower performance, or have a limited lifetime compared to volatile random access memories.

So, what does ROM do? For its characteristics like the data stored in the ROM is generally written after manufacture so that it can only be read during the working process, instead of being rewritten quickly and conveniently like random memory.

Therefore, the data stored in the ROM is stable, and the stored data doesn’t change after power off; the structure is relatively simple, and the reading is convenient, thus it is often used for the task of secondary storage, or long-term persistent storage to store various fixed programs and data.

Types of ROM

Now let’s discuss types of ROM for the basic understanding.

MROM – Mask Read Only Memory

MROM is the short form of Mask Read Only Memory. It is inexpensive and is the very first ROM which is hard wired device that contains a pre-programmed set of data or instructions.

PROM – Programmable Read Only Memory

PROM is read-only memory chip that data can be written only once by a user. The difference between it and the read only memory is that PROM is manufactured as a blank memory, while the ROM is programmed during the manufacturing process.

The user buys a PROM, the user will need a special device called a PROM programmer or PROM burner to write the desired data onto the blank PROM chip. The process of programming a PROM is sometimes called burning the PROM. The memory can be programmed just once after manufacturing by “blowing” the fuses, which is an irreversible process.

EPROM – Erasable Programmable Read Only Memory

EPROM is a special kind of read only memory chip that has the opportunity to erase the programmed data, which the feature can be seen from its name. The programmable read-only memory can be programmed to write data with high voltage, and the data remains until it is exposed to ultraviolet light for lasting up to 10 minutes or longer.

Usually, an EPROM eraser can achieve this purpose, making it possible to reprogram the memory. For this purpose, a quartz transparent window is reserved on the package of the memory for easy exposure.

EEPROM – Electrically Erasable and Programmable Read Only Memory

EEPROM is also a kind of read only memory that the principle of operation is similar to EPROM which we have mentioned, but the ways to program and erase are done by exposing it to an electrical charge, so no transparent window is needed.

It can be erased and reprogrammed about 10,000 times. Both erasing and programming take about 4 to 10 milliseconds. In the EEPROM, users can selectively erase and program any location and it can be erased one byte at a time instead of being erased the entire chip. Therefore, the process of reprogramming can be flexible but slow.

Flash Memory

Flash memory (flash) is a modern type of EEPROM. Flash memory can be erased and rewritten faster than ordinary EEPROM, and newer designs has the feature that is very high endurance (exceeding 1,000,000 cycles).

Modern NAND flash memory can effectively utilize the silicon chip area, enabling individual ICs to have a capacity of up to 32 GB in 2007; this feature, along with its durability and physical durability, enables NAND flash to replace magnetic in some applications, such as USB flash drives.

Except these types, there are other types of non-volatile memory including optical storage media, such as CD-ROM (analogous to MROM). CD-R and CD-RW are both designed for backwards-compatibility with CD-ROM: CD-R which is write-once, read-many (analogous to PROM), while CD-RW supports erase-rewrite cycles (analogous to EEPROM).