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rom

is read only memory modifiable in some way (not PROM)

Just curious about ROM.

I understand some types of rom are editable a few times but for the ROM chips that claim they are unmodifiable I have a question.

Is there some kind of way to edit them? like a special hardware component or something?

And how are they able to be made as uneditable? can you give me a brief explanation of how this works.

Thanks in advance for your time.

Solution

  • There are a few different types available. ROM, PROM, EPROM, EEROM and EAROM.

    • ROM, Read-Only Memory is programmed at the factory and cannot be programmed or altered once delivered.
    • PROM, Programmable Read-Only Memory, can be written to once, after which there is no changing it.
    • EPROM, Erasable Programmable Read-Only Memory, can be written to. Erasure is typically done using ultraviolet light. The chip will have a window on it to allow the ultraviolet light into the chip. Typically the
      window is covered with a sticker or label to prevent accidental
      corruption of the data.
    • EEPROM, Electrically Erasable Programmable Read-Only Memory, can be
      written to and then erased by using particular voltages that are usually higher than the normal operating voltages and appropriately timing the signals.
    • EAPROM, Electrically Alterable Programmable Read-Only Memory, works a lot like EEPROM except that it can be changed without erasing the entire device.

    The various types of ROM do not require power to be applied to retain their contents. Programmer devices were and are still available for working with these parts. All of the programmable ROMS that can be altered after programming have a limited number of times that they can be reprogrammed before the reliability of the device starts to decay.

    In some devices, battery backed-up RAM was used to get around the need for abnormal voltages for programming and to allow for frequent changes to the contents.

    These days most of the original technology has been supplanted by various types of flash memory.

    There is also some confusion introduced through common language usage. For example, the BIOS for computers still being referred to as ROM sometimes, even though nowadays this is usually stored on a flash chip.