ed read/write memory. Another type of internal memory consists of a series of read-only memory, or ROM, chips. The switches of ROM chips are set when they are manufactured and are unchangeable. The patterns on these chips correspond to commands and programs that the computer needs in order to boot up, or ready itself for operation, and to carry out basic operations. Because read-only memory is actually a combination of hardware (microchips) and software (programs), it is often referred to as firmware. Other devices that are sometimes used for main memory are magnetic-core memory and magnetic-bubble memory. Unlike semiconductor memories, these do not lose their contents if the powersupply is cut off. Long used in mainframe computers, magnetic-core memories are being supplanted by the faster and more compact semiconductor memories in mainframes designed for high-speed applications. Magnetic-bubble memory is used more often for auxiliary storage than for main memory. Auxiliary storage units supplement the main memory by holding parts of programs that are too large to fit into the random-access memory at one time. They also offer a more permanent and secure method for storing programs and data. Four auxiliary storage devices--floppy disks, hard disks, magnetic tape, and magnetic drums--store data by magnetically rearranging metal particles on disks, tape, or drums. Particles oriented in one direction represent 1s, and particles oriented in another direction represent 0s. Floppy-disk drives (which "write" data on removable magnetic disks) can store from 140,000 to 2.8 million bytes of data on one disk and are used primarily in laptop and personal computers. Hard disk drives contain nonremovable magnetic media and are used with all types of computers. They access data very quickly and can store from 10 million bytes (10 megabytes) of data to a few gigabytes (billion bytes). Magnetic-tape storage de...
