Hardware, the System Unit and Storage

What you need to know

• Hardware is the collection of physical, tangible components of a computer, the parts you can see and touch. Anything that holds instructions rather than circuitry is software, not hardware.
• Input devices send data into the computer. Learn these with their specific uses: keyboard for text and commands; mouse, trackball and touchpad for pointing; scanner for turning a printed page into a digital image; microphone for sound; digital camera and webcam for images; joystick for games and simulation; light pen and stylus for drawing directly on a screen; and touch screen, which is both input and output.
• Specialised readers matter in Nigeria and are frequently examined. OMR, the optical mark reader, reads shaded boxes on answer sheets, which is how multiple choice examinations are marked. OCR, optical character recognition, converts printed or typed characters in a scanned image into editable text. MICR, magnetic ink character recognition, reads the magnetic numbers at the foot of a bank cheque. A barcode reader reads the striped product code at a supermarket till, and a biometric scanner reads a fingerprint or iris, as used for BVN, NIN and INEC accreditation.
• Output devices present results. A monitor or visual display unit gives soft copy; its size is measured diagonally in inches and its sharpness in pixels. A printer gives hard copy. A plotter draws large engineering and architectural diagrams. Speakers and headphones give sound, and a projector throws the display onto a screen.
• Printers divide into impact and non-impact. An impact printer strikes an inked ribbon against the paper, so it is noisy, slower and lower in quality but can produce carbon copies; the dot matrix and the line printer are examples. A non-impact printer does not strike the paper, so it is quiet and of higher quality; the inkjet, laser and thermal printers are examples. Bank and payroll offices still keep dot matrix printers precisely because of the carbon copy.
• The system unit is the casing that houses the main electronics. Inside it are the motherboard or system board, the processor and its cooling fan, the memory modules, the power supply unit, the drive bays, the expansion slots and the ports that external devices plug into.
• The central processing unit is the brain. It has two working sections plus storage: the control unit, which fetches each instruction, decodes it, and directs all other parts in the right order; and the arithmetic and logic unit, which performs addition, subtraction, multiplication and division and makes comparisons such as greater than, less than and equal to. Registers inside the CPU hold the data being worked on at that instant.
• The machine cycle is fetch, decode, execute and store. Clock speed, measured in megahertz or gigahertz, tells how many cycles the processor completes per second, so a 3.2 GHz processor runs about 3.2 billion cycles each second. The buses that carry signals are the data bus, the address bus and the control bus.
• Primary memory sits inside or beside the processor and is directly accessible to it. RAM, random access memory, is read and write, holds the program and data currently in use, and is volatile, meaning everything in it is lost the moment power goes off. ROM, read only memory, is non-volatile, holds the permanent start-up instructions written by the manufacturer, and cannot normally be altered by the user. Variants of ROM are PROM, EPROM and EEPROM.
• Cache memory is a small, very fast memory between the processor and RAM that holds the data most recently or most frequently used, so the processor does not have to wait for slower RAM. The hierarchy from fastest and smallest to slowest and largest is registers, cache, RAM, then secondary storage.
• Secondary storage holds data permanently and is not directly accessed by the processor. The common media are the hard disk drive, the solid state drive, the flash or pen drive, the memory card, the CD, the DVD, the Blu-ray disc and magnetic tape, which is still used for bulk backup because of its low cost per gigabyte.
• Capacities worth knowing by heart: a 3.5 inch floppy disk held 1.44 MB, a CD holds about 700 MB, a single layer DVD holds 4.7 GB and a single layer Blu-ray disc holds 25 GB. Modern hard disks and solid state drives run from hundreds of gigabytes into terabytes.
• Storage access is either sequential, where the medium must be read from the beginning until the required record is reached, as with magnetic tape, or direct and random, where any location can be reached straight away, as with a disk or flash drive. That distinction explains why tape is cheap for backup but useless for day-to-day work.
• A peripheral is any device attached to but outside the system unit. The common ports that connect them are USB for most devices, HDMI and VGA for displays, RJ45 for a network cable and the 3.5 mm jack for audio. In Nigeria an uninterruptible power supply and a voltage stabiliser should be treated as part of the installation, because unstable mains supply damages power units and corrupts disks.