37-ComptIA A+ 220-901 (ROM) by IT Training and Tutorials | English
ROM: Read Only Memory
ROM is only programmable once. For example, it could be programmed at the factory where they make the chip. And indeed, it’s usually used in firmly hardcoded chips made by the company.
Clarification from a reader: “ROM is not "programmed at the factory” in the same sense that you imply or even “programmed once”. It is programmed never. The data is hard-coded into the chip itself. Once the chip wafer is manufactured, that is it, it can never be changed, only tested before it goes out. The ones and zeros are hard-coded connections to +V and ground.“
PROM: Programmable Read Only Memory
This is similar to ROM except that you, the consumer, can program it. You can buy a blank chip and have a PROM programmer program it with your stuff. But, once you program it, you can never change it.
EPROM: Erasable Programmable Read Only Memory
So the cool this about this one is that you can rewrite this type of memory. To erase a previous program though, you need to set it under some form of UV light for a specified amount of time before being able to rewrite it.
EEPROM: Electrically Erasable Programmable Read Only Memory
So this one is the one that most people these days probably use (especially those working with microcontrollers on a hobby level). Essentially, it can be written, erased, rewritten electronically. No need for a fancy UV light thing.
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