When it comes to computers and smartphones, 3 types of memory always appear in the discussion. The RAM, hard disk/internal memory, and ROM. ROM (Read-Only Memory) is a special type of memory that can be found in any computer device but used for special purposes. In this article, we’re going to discuss briefly the introduction and types of ROM.
What is ROM
ROM (Read-Only Memory) is what its name suggests. It’s a physical IC chip pre-loaded with information that rarely or aren’t changed in its entire lifetime. It may also be called “Firmware”. ROM chips are non-volatile forms of storage, unlike RAM, meaning that the data inside doesn’t change or erase when the power connection is turned off.
By default, the data stored in ROM chips don’t change or erase unless some specialized steps are taken to do so. Many electrical devices come with ROM chips pre-installed inside them. An external example of such ROM is the CD-ROM, a type of optical storage mainly used to load software and multimedia files for a long time. The CD-R (Write Once) can be written only once but read many times. The latest CD-RW can be rewritten multiple times. Since its invention, numerous types of ROM have been released.
Types of ROM
Unlike RAM, ROM comes with a less variation because of the same implementations of building them. In general, we can differentiate ROM in 4 main types. Most of these types are based on semiconductor technology.
- MROM (Masked Read-Only Memory)
- PROM (Programmable Read-Only Memory)
- EPROM (Erasable Programmable Read-Only Memory)
- EEPROM (Electrically Erasable Programmable Read-Only Memory)
Masked ROM is the original ROM type. This is not modifiable at all. The information is stored in the chip while manufacturing, and in the whole lifetime, the information can’t be erased or rewritten/modified. These are the oldest types, and so they’re cheap. They are seldom used in modern electrical devices, which makes them rare too. We can still see them in microprocessors or microcontrollers. CD-ROM is very similar to the MROM technology. Masked ROM is also much more compact per bit of info.
In this type, the data is non-modifiable, like the MROM. PROMs are used to store firmware of device which are constant. The main difference of it from MROM is, while data is stored in MROM during manufacturing, the PROM is manufactured as a blank one. Information is stored in it after manufacturing.
Designers input information into the PROM with a device called “PROM Programmer”. It’s a one-time process. The advantage of PROM is its good for making prototypes and run testing, while still being very cheap.
Erasable-Programmable ROM is an upgrade over PROMs in which, data can be erased after writing. After programming, EPROM can save its data for up to 35 years or more, which can also be written as many times as needed. To erase it, a specific wavelength of UV light is used for a period of 30-40 minutes.
EPROMs were often seen in previous generation computer BIOS systems. We can erase it for limited amounts of time (1000 times). There are some drawbacks to EPROM with the erasing strategy. Firstly, the erasure isn’t selective, meaning the whole EPROM is erased at once. We can’t erase a portion of EPROM and keep the other portion intact. The second problem is, erasing with UV light for a high volume of memory is much inefficient, and the presence of dust particles inside the erasure window can hamper the process, resulting in some valueless leftovers. For these reasons, EPROMs are losing their value and application fast.
Electrically Erasable Programmable ROM is one of the latest additions in a long line of different ROM chips. These are often seen in modern devices for their efficiency. To erase data, an electrical charge is given to the chip as opposed to the UV lighting. The endurance of data rewriting is much higher (1,000,000 times or more) than before.
Due to the usage of electrical charges, the erasure is selective. We can erase any portion of EEPROM and still keep the other information intact. Although the erasure speed is low, 1 bit each time, still the EEPROM is much more versatile and widely used today. We can take a look at 2 widely used forms of EEPROM:
- EAROM (Electrically Alterable Read-Only Memory)
A special form of EEPROM where at a time 1-bit of information is modified. EAROMs are used to partially rewrite a constant system or to store setup information of a critical system.
- Flash Memory
This is a more compact and yet flexible form of EEPROM, where data transfer (read, write, and erase) occurs much faster. This is done by writing data in packages of 512 bytes, as opposed to 1 byte of EEPROMs. Flash memories are widely used as temporary data storage, and although they’re not as reliable as the previous ones, flexibility is much helpful for quick transfers. Flash memories are the fastest type of ROMs with a transfer rate of up to 10GB/s.
Other Forms of ROM
In addition to the main types, for specialized applications, some additional types of ROM are still in use. Such as the previously discussed Optical storage types (CD-RW, CD-R, CD-ROM). In much older computers, Transformer Matrix ROMs were used. In small simple devices like calculators and PCBs, Diode Matrix ROMs can be found. These stores the basic mathematical or numerical functions that don’t change.
In this article, we’ve briefly discussed the general types of ROM used in different electrical devices, old or new. We may not encounter ROM technology in our daily lives as RAM or storage, this still is one of the most reliable ways of keeping information safe for prolonged times and used in almost all electronic devices.