Read Only Memory: Unlike RAM, the Read Only Memory (ROM) retains its contents even when the computer is turned off. ROM is referred to as being nonvolatile, whereas RAM is volatile. Most personal computers contain a small amount of ROM that stores critical programs such as the program that boots the computer. In addition, ROMs are used extensively in calculators and peripheral devices such as laser printers, whose fonts are often stored in ROMs.

The Basic Input Output Software: The Basic Input Output Software (BIOS) is built-in software, which keeps track of all devices attached to the computer and provide inter-communication channel between listed devices. The devices include keyboard, monitor, disk drives, and a number of other miscellaneous devices. Older computers contained read-only BIOS that could not be altered. This means that new components could not be added to the computer because the BIOS would not know how to communicate with them.

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This seriously limits the user's ability to upgrade their computers. S0, Flash BIOS was introduced. Now when a new component is installed in the computer, the Flash BIOS can be electronically upgraded so that it can recognize and communic~te with the new devices. The BIOS is typically placed in a ROM chip that comes with the computer (it is often called a ROM BIOS). This ensures that the BIOS will always be available and will not be damaged by disk failures. It also makes it possible for a computer to boot itself. Because RAM is faster than' ROM, though, many computer manufacturers design systems so that the BIOS is copied from ROM to RAM each time the computer is booted. This is known as shadowing.