RAM vs ROM vs EPROM
In: Technology
RAM is a kind of memory that a computer can “read” and “write”. That means it can store data on it, then retrieve that data later. Usually RAM is considered very fast, and it is often “volatile”, meaning if it loses power the data goes away. That means it’s just temporary storage.
ROM stands for Read Only Memory. The computer can’t write to it. The data inside of it is part of the circuitry and can’t be changed. It’s also pretty fast like RAM, but since it can’t be changed it’s only useful for things that won’t ever change. Cartridges for game consoles usually put the game code on a ROM chip, which is why emulator files for them are called “ROMs”.
EEPROM stands for something I’m too lazy to go look up. The ROM is still Read Only Memory, but it’s a bit of a lie. If you put an EEPROM in a machine called a “programmer”, it can write new data to the EEPROM. However, in the computer where the EEPROM is meant to be used, there is usually no capability to write it. So it’s still read-only inside the computer, even though with some work you could reprogram it to have new data. (I think the reason it usually can’t be reprogrammed by the computer is it takes a higher voltage to write to it than the voltages computers tend to use?)
EEPROMS are most often used in hardware labs where people are planning on using a ROM chip in the future but are still working on the program for that chip. Development cartridges for game consoles often used to have EEPROM chips on them so the developers could update the game code as they wrote it.
RAM is Random Access Memory. It is very fast to read and write to this memory. But the data does not stay for long and needs to be constantly refreshed. The controllers does this automatically but this means you need power to them in order for them to store data. ROM is Read Only Memory. These are programmed with the data at the factory and can never be changed in any way. Usually the data is stored in the wiring of the chip.
We then invented PROM which is programmable ROM. It allows you to program it with data in the field but you can never change the data. This is still used today for security reasons. Then we came out with EPROM or erasable PROM. These chips can be reset in order to upgrade the data. The reset procedure uses UV light to power the transistors in each cell so the chip package usually have a window to the chip, often covered with a sticker to prevent accidental exposure. We then invented EEPROM or Electronically Erasable PROM. These can be erased with an electronic signal so no sticker and UV light is needed.
It should be noted that an EEPROM have a limited number of times it can be erased. So you can not use it to store random bits of data like RAM. And the erasure procedure takes some time and power. In general EEPROM is good to store firmware or fixed data that is rarely updated.
The EEPROM technology have improved which is called Flash memory. This is the technology you find in SD cards and SSD hard drives and such. There are still limitations on the amount of times you can erase each block but you can erase several orders of magnitudes more often then an EEPROM and much faster. The controller also balances the erasing between the blocks and even have spare blocks for when you wear out the hardware.
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