Read Only Memory

There are various kinds of memory available. Broadly, we can categorize memory in two ways:

(1) Memory which does not have the ability to store the data once the power is OFF (called "volatile memory", also know as "Random Access Memory").

(2) Memory which does have the ability to store data once power is OFF (called "non-volatile memory", also known as "Read Only Memory (ROM)"). The additional characteristic of ROM is that it can't be modified/changed once manufactured.

-- Vishvesh Raiter

Except that non-volatile memory isn't always called read-only memory - NVRAM (non-volatile random access memory) for example. Oh, and a common type of read-only memory is PROM (programmable read-only memory), where programmable means "modifiable by a program", presumably.

PROM typically requires that you do something special, like asserting a `programming pin` at an `interesting` voltage in order to write. May or may not be erasable. There are a number of variants on this idea:

EPROM (Erasable PROM) typically works via a `window` in the top of the package. Certain wavelengths of light falling on the chip cause it to be erased. These are typically in the UV range.

EEPROM (Electrically Erasable PROM) is "modifiable by a program", but functions along the different-voltage lines. You *could* use this like very-slow-to-write RAM, but its endurance isn't typically very good. With *all* these technologies, the act of writing a cell causes a small amount of permanent damage.

Then there's modern things, Flash - similar to EEPROM, but now bigger, faster, cheaper. Endurance seems better too. If someone wants to fill in how it *actually works*, that would be awesome.

FRAM - similar again to EEPROM, except the write speed is almost as fast as reading, and I've never seen any endurance issues. Expensive compared to Flash though!