Computers and their components

Computer Science AS · Hardware. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A computer needs input devices, output devices, primary memory that the processor reaches directly, and secondary storage that keeps data when the power is off. An embedded system is a small computer built into a larger device to do one fixed task, such as the controller in a washing machine.

Primary memory is RAM and ROM. RAM is volatile: it loses its contents without power. ROM is non-volatile. RAM comes as static (SRAM) or dynamic (DRAM), and ROM chips differ in whether and how they can be reprogrammed.

Computers also watch and manage the outside world. Sensors supply readings; in a control system the computer acts on them through an actuator.

Rules to remember

RAM: volatile, can be read and written, holds the programs and data in use. ROM: non-volatile, read only, holds the start-up instructions
SRAM: uses flip-flops, needs no refresh, faster and more expensive, used for cache. DRAM: uses capacitors that leak and must be refreshed, cheaper, more bits per chip, used for main memory
PROM: programmed once, cannot be changed. EPROM: erased with ultraviolet light, then reprogrammed. EEPROM: erased with an electrical signal while still in the circuit
Buffer: a temporary storage area that holds data on its way between two devices that work at different speeds
Monitoring system: sensors take readings and the computer only records them or warns. Control system: the computer uses the readings to change the conditions through actuators
Feedback: the output changes the conditions, so it affects the next sensor reading (the next input)
Magnetic hard disk: magnetised spots on spinning platters. Solid state (flash): electrons trapped in floating-gate transistors, no moving parts. Optical disc: a laser reads pits and lands

Common mistake

Students mix up SRAM and DRAM. Dynamic RAM is the one that must be refreshed, and the refreshing makes it slower, so the faster SRAM is used for cache.

Worked example

The program in a car's engine controller must stay when the power is off. A garage must be able to update it without removing the chip. Which type of memory is suitable?

The program must survive without power, so RAM is not suitable; a type of ROM is needed.
PROM can be programmed only once, so it cannot be updated.
EPROM needs ultraviolet light, so the chip would have to be taken out.
EEPROM is erased and rewritten electrically while it stays in the circuit.

Answer: EEPROM.

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