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The system unit, or case, houses the motherboard, processor chip, memory chips, and power supply. The system unit also includes storage devices, such as disk drives, which are housed on shelves called bays. The motherboard contains sockets for expansion -- for adding new components, such as video cards -- or upgrading -- for changing to more powerful components, such as more memory chips.

The microprocessor: The most fundamental part of the motherboard, the microprocessor, is the miniaturized circuitry storing the program instructions that manipulate data into information. Two architectures for microprocessors are CISC chips, used mostly in PCs and mainframes, and RISC chips, used mainly in workstations. Most personal computers today use either Intel-type chips for PCs (made by Intel, AMD, Cyrix, and others for Compaq, Dell, Gateway, Hewlett-Packard, and IBM) and Motorola-type chips (made by Motorola for Apple Macintosh computers).

The speed of a microprocessor is determined by its system clock. For microcomputers, processing speeds are measured in megahertz (million cycles per second) or gigahertz (billion cycles). For workstations, such speeds are measured in MIPS (millions of instructions per second); for supercomputers, in flops (floating-point operations per second); and for all computers, in fractions of a second (milliseconds, microseconds, nanoseconds, and picoseconds). Another measure, word size, is the number of bits a computer can process at any one time, with 64 bits being faster than 32 bits.

A processor, also called a CPU (central processing unit), follows the instructions of the software (program) to manipulate data into information. The CPU consists of the control unit, which deciphers each instruction stored in it and then carries it out, and the arithmetic_logic unit (ALU), which performs arithmetic and logical operations. Both the control unit and the ALU contain registers, high-speed storage areas that temporarily store data during processing. Data transmission within the CPU and between the CPU and other components of the motherboard is by means of electrical roadways called buses.

Memory: Also on the motherboard are memory chips, of which there are four types: RAM, ROM, CMOS, and flash. (1) RAM chips (for random access memory) temporarily hold software instructions and also data before and after processing by the CPU. RAM is volatile; its contents are lost when the power goes off. Four types of RAM chips are DRAM, which must be constantly refreshed by the CPU or it will lose its contents; SDRAM, which is faster than DRAM; SRAM, also faster than DRAM and able to retain its contents without being refreshed by the CPU; and RDRAM which is faster and more expensive than SDRAM. RAM chips often appear on memory modules -- SIMM has chips on one side, DIMM has chips on both sides -- which can be plugged into expansion slots on the motherboard. (2) ROM chips (for read-only memory) contain fixed start-up instructions. (Read means to transfer data from an input source to the CPU or memory; write means to transfer data from the CPU or memory to an output device.) A variant is PROM, a ROM chip that allows users to load read-only programs and data, although only once. (3) Battery-powered CMOS chips don't lose their contents when the power is turned off; this quality makes them useful for holding times and dates. (4) Flash memory chips can be erased and reprogrammed more than once.

The processor searches for data or program instructions in the following order: first level 1 cache, then level 2 cache, then RAM, then hard disk (or CD-ROM). Cache temporarily stores instructions and data that the processor is likely to use frequently, thereby speeding up processing. Level 1 cache is built into the processor chip; level 2 cache resides outside the processor chip and consists of SRAM chips. Virtual memory is hard-disk space used to extend the capacity of RAM.

Ports and cables: A port is a connecting socket on the outside of the system unit into which are plugged different kinds of cables. Six types of ports are as follows: (1) Serial ports transmit bits one after the other, or slow data over long distances. (2) Parallel ports transmit 8 bits simultaneously, or fast data over close distances, as to printers. (3) SCSI ports transmit 32 bits simultaneously in a daisy chain of up to seven devices linked in a series. (4) USB ports are general-purpose ports that transmit data to up to 127 devices in a daisy chain. USB permits Plug and Play, which allows peripheral devices and expansion cards to be automatically configured while they are being installed. (5) Dedicated ports exist for special purposes, as for the keyboard and mouse. (6) Infrared ports enable cableless connection with infrared devices, as between the computer and some printers.

Expandability -- buses and cards: Closed architecture means a personal computer has no expansion slots; open architecture means it does. Expansion slots are sockets on the motherboard into which may be plugged expansion cards, circuit boards that provide more memory or that control peripheral devices. Expansion slots are connected to the CPU by expansion buses, such as ISA, the oldest and slowest at 8 or 16 bits; PCI, faster at 32 or 64 bits; and AGP, twice as fast as PCI and designed to support video and 3-D graphics. Types of expansion cards include the following: Graphics cards convert data into video images to display on a monitor. Sound cards transmit digital sounds; this includes music for video games created by wavetable synthesis, digitized sound samples taken from recordings of actual instruments. Modem cards are modems installed inside the computer. Network interface cards allow data transmission over a cable network. The PC card is used principally to expand the capabilities of laptops.

Floppy disks are removable flat pieces of mylar plastic in 3.5-inch plastic cases. Floppy disks have a write-protect notch, which can prevent accidentally recording over on a disk. Data is recorded in concentric circles called tracks; each track is divided into sectors, invisible wedge-shaped sections used for storage reference purposes. In the disk drive, the read/write head transfers data between the computer and the disk. Besides 3.5-inch floppy disks, which hold 1.44 megabytes, other forms of removable disks are floppy-disk cartridges, or higher-capacity removable disks -- Zip disks, SuperDisks, and HiFD disks. Zip disks are special disks with a capacity of 100 or 250 megabytes. SuperDisks have a capacity of 120 megabytes, and a SuperDisk drive can also read 1.44-megabyte floppies. HiFD disks have a capacity of 200 megabytes; the disk drive can also read 1.44-megabyte floppies.

Hard disks are thin but rigid metal platters covered with a substance that allows data to be held in the form of magnetized spots. Disks are sealed in a hard-disk-drive unit, which can be quite sensitive, susceptible to a head crash, when the surface of the disk touches particles or the read/write head, resulting in loss of data on the disk. Hard disks may be nonremovable or removable. (1) A nonremovable hard disk is housed in the microcomputer system unit and stores most programs and data. Ads for hard disks may specify the hard-disk controller, a circuit board that positions the disk and read/write heads. Popular hard-disk controllers are Ultra ATA (also known as EIDE), which allows fast data transfer and high storage capacity, and SCSI, which supports several disk drives as well as other peripheral devices. (2) A removable hard disk (Iomega's Jaz, SyQuest's SparQ) consists of one or two platters enclosed with read/write heads in a hard plastic case, which is inserted into a microcomputer's cartridge drive.

Optical disks are removable disks on which data is written and read through use of laser beams. Types of optical disks are CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD-RW, DVD-RAM and DVD+RW. CD-ROM is a read-only disk that holds prerecorded text, graphics, and sound. CD-R disks can be written to once but can be read many times. CD-RW disks can be written to and erased so the disk can be reused several times. DVD-ROM is a high-capacity CD, storing up to 17 gigabytes. DVD-R disks allow one-time recording by consumers. DVD-RW (for rewritable) , DVD-RAM (for random access memory), and DVD+RW (DVD+rewritable) can be recorded on and erased more than once.

Flash memory cards, or flash RAM cards, consist of circuitry on credit-card-size cards that can be inserted into slots connecting to the motherboard.

Magnetic tape is thin plastic tape coated with a substance that can be magnetized, with data represented by magnetized (1s) or unmagnetized (0s) spots. Large computers tend to use magnetic-tape reels, small computers tend to use tape cartridges, resembling audio cassettes. A smart card, such as a telephone debit card, contains a microprocessor and memory chip. An optical card is a laser-recordable, wallet-type card used with an optical card reader. A final type of storage is online secondary storage, in which secure online services provide backup storage.