ROM stands for read-only memory. Devices that use read-only memory, such as your computer, calculator, and printer, are using stored data that can't be altered by the user. That is, you're limited only to reading the information recorded – you can't write over what's already been recorded. This is mostly a protective feature to prevent you from accidentally writing over crucial data your computer needs to run properly. Most computers have a small amount of ROM in several fundamental programs that are used when the computer boots up. Some compact disks also use ROM as their primary storage format, but there are also computer CDs that can be rewritten. Random access memory, or RAM, on the other hand, is the main memory on the computer and can be changed after it has been recorded. ROM is considered to be non-volatile, or unable to be changed, whereas RAM is considered to be volatile. There are a few sub-categories of ROM. PROM and EPROM are similar to ROM in that they both contain read-only memory. However, PROM allows the user to write over previously recorded data if the user has a special device called a PROM programmer. However, with PROM you can't go back and re-program or write more data after you have already written over it once with the PROM programmer. In contrast, the EPROM, or erasable programmable read-only memory, allows you to erase or write over programmed data, erase what you have written, and write again as many times as you like.
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