ROM Types: PROM, EPROM, and EEPROM for GCSE Computer Science

What is mask ROM and why is it called "read-only"?

Mask ROM is the original form: the data is literally burned into the chip during manufacturing by selectively connecting or disconnecting circuit paths on a physical "mask." Once manufactured, the content cannot be changed. The chip is genuinely read-only.

Mask ROM is still used in high-volume, cost-sensitive products (game cartridges, microwave controllers, cheap toys) where the code never changes and the volume justifies a custom chip run. For GCSE purposes, this is the ROM students first encounter.

What is PROM?

PROM (Programmable ROM) ships as a blank chip with all bits in one state (usually all 1s). A programmer uses a PROM burner (a device that applies a high voltage) to permanently blow tiny internal fuses to set bits to 0. Once a bit is blown, it cannot be restored.

Key characteristics:

• Can be written once by the end user (not just the manufacturer).
• After programming, it behaves like mask ROM — permanently fixed.
• Useful when a small number of customised chips are needed without paying for a custom mask.

Think of PROM as a one-time-use message: you choose the content once, then it is sealed forever.

What is EPROM?

EPROM (Erasable Programmable ROM) can be erased and reprogrammed. Erasing requires exposing the chip to ultraviolet (UV) light through a small transparent quartz window on top of the chip package.

EPROM was widely used in the 1970s–1990s for firmware development: engineers could reprogram chips as software was revised, without discarding expensive hardware. The UV-erase step was a practical inconvenience — chips had to be removed from the board, exposed under a UV lamp, and reprogrammed — but it was far cheaper than manufacturing new mask ROM chips for every firmware revision.

What is EEPROM?

EEPROM (Electrically Erasable Programmable ROM) removes the UV light requirement. It can be erased and reprogrammed using ordinary electrical signals — and crucially, individual bytes or pages can be erased rather than the whole chip.

EEPROM is used wherever a small amount of data must persist without power and may need updating occasionally: storing user preferences in a washing machine, calibration data in a sensor, the VIN code in a car's engine control unit.

How does flash memory relate to these types?

Flash memory is a high-density descendant of EEPROM, developed in the 1980s. It erases in larger blocks (not individual bytes) for efficiency but achieves far higher storage density and lower cost per bit than EEPROM.

Flash is what your phone's storage, USB drives, and SSDs use. It is not quite the same as EEPROM (erasing by block rather than byte), but the underlying transistor technology is the same family.

Where are EPROM and EEPROM used today?

Frequently asked questions

Why does the original ROM have a quartz window but EEPROM does not?

The quartz window is only needed for EPROM, which requires UV light to erase. UV light must pass through the window to reach the floating-gate transistors inside. EEPROM uses a different electrical mechanism (Fowler–Nordheim tunnelling) that does not need light; the chip can be erased in-circuit with standard voltages. Mask ROM and PROM need no erase mechanism at all.

Do modern computers still use ROM?

Yes — the term "ROM" is still used, though the underlying technology is usually flash memory. Your computer's UEFI firmware (the modern replacement for BIOS) is stored in a flash chip on the motherboard. It is non-volatile (survives power loss), updateable (unlike original mask ROM), but slow to write compared to RAM — exactly the characteristics of flash.

What is the difference between EEPROM and flash for GCSE purposes?

At GCSE level, the key distinctions are: (1) EEPROM can erase and write individual bytes; flash works in larger blocks. (2) Flash offers much higher density and lower cost. (3) Both are non-volatile and electrically erasable in circuit. Unless a question specifically asks about block vs byte erasure, describing both as "electrically erasable non-volatile memory used for firmware" is sufficient.

Why would a fridge use EEPROM?

A modern fridge's control board stores user temperature settings and calibration data. When you change the temperature, those preferences must be saved even when the fridge is unplugged — that requires non-volatile storage. The amount of data is tiny (a few hundred bytes), and it may need to be updated thousands of times over the appliance's lifetime. EEPROM is ideal: cheap, non-volatile, supports many write cycles, and does not need a complex file system.

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Want to go deeper on how memory works — from transistors to storage hierarchy? Professor Turing at aitutors.me can build your understanding from the ground up.