System Memory

ROM (Read Only Memory)
(Pronounced 'rahm')

An acronym for Read Only Memory, ROM is computer memory on which data has been pre-recorded. The programming code and/or data on a ROM chip is written to the chip at the factory. It can be read, but it cannot be erased or removed. It's permanent. ROM retains its data or content even when the computer is turned off, unlike a computer's main memory (RAM), which needs a constant charge of electricity to keep its information. For this reason, ROM is considered to be 'non-volatile' and RAM is 'volatile'.

ROM chips are used in all kinds of electronic devices from calculators to video games. Most personal computers have several applications of ROM memory. These chips often store permanent and critical information and programs that don't need to be changed, or don't need to be written to. Most personal computers have a small amount of ROM that stores the code that starts up or boots the computer. Early computers also used ROM to store the BIOS (Basic Input Output System) which acts as a translator between the PC's hardware and the operating system.

The problem with using these ROM chips for BIOS information is that computer manufacturers had to build their systems around the available fabricated ROMs and their coding. Because the information was written to the chip during the fabrication process, changes to the chip would involve developing new assembly lines and purchasing new equipment. It would cost a small fortune if a single computer or motherboard manufacturer had visions of having the coding on these chips changed to accommodate new developments or enhancements they might want to incorporate into their product.

PROM (Programmable Read Only Memory)
(Pronounced 'prahm')

Remember, all a computer can understand is 1s and 0s. That's how a transistor works. It either allows electricity to pass or it doesn't. There's either an electrical charge, or there isn't. It's on or off (1 or 0). These memory chips are made up of millions of tiny transistors that hold the 1s or 0s. Just like millions of tiny switches or fuses.

With a ROM chip, these switches are permanently set at the factory in their respective on or off (1 or 0) positions. In the case of a PROM (Programmable Read Only Memory), the chip is manufactured with all its little switches or fuses intact, or set to '1' (blank memory). When these chips are programmed, with the use of a device called a PROM programmer (or burner), a high voltage electronic pulse destroys selected switches, burning in the settings that need to be change to a '0'.

So PROM is programmable, but only once. Like ROM, it's permanent, or non-volatile. It cannot be erased.