Computers and their components

A computer system is built from four kinds of component, each with a clear job:

• Input devices let data and instructions enter the system for processing (keyboard, mouse, microphone, scanner, sensor).
• Output devices present the results of processing to the user (monitor, printer, speaker).
• Primary memory is reached directly by the processor and holds what is being used right now — this is RAM and ROM.
• Secondary storage keeps programs and data long-term, even when the power is off (magnetic hard disk, SSD, optical disc, USB flash drives). It is not accessed directly by the processor.

Why do we need both primary memory and secondary storage? RAM is fast but volatile (it forgets everything when switched off) and relatively small. Secondary storage is non-volatile and large, so it keeps your files permanently — but it is slower, so the processor cannot use it directly. Data is copied from secondary storage into RAM when needed.

Removable storage (USB pen drives, SD cards, external drives, optical discs) is secondary storage you can unplug and carry between machines — handy for backups and transferring files.

An embedded system is a complete little computer (microprocessor, memory and software) built into a larger device to perform a specific, dedicated function. You don't think of it as a computer at all.

Examples: the controller inside a washing machine, microwave, digital camera, car engine management unit, or central-heating thermostat.

Benefits

• Small and low cost → can be built cheaply into everyday products.
• Dedicated to one task → fast, efficient and reliable; it boots instantly and runs continuously.
• Low power consumption → suits battery or always-on devices.

Drawbacks

• Hard or impossible to upgrade or repair — the program is usually fixed in ROM.
• Designed for one fixed task — it cannot easily be repurposed for something else.
• Limited processing power and memory compared with a general-purpose computer.

Paper 1 expects you to describe the principal operation of specific devices — the key stages, in order, using the right words.

Laser printer (output). A rotating drum is charged; a laser writes the page image by altering the charge where it strikes; toner powder sticks to the charged image; the toner is transferred to the paper and fused (melted) on by heat.

3D printer (output). Builds a solid object layer by layer (additive) from a digital model, usually by melting plastic filament and depositing it, each thin layer hardening on top of the last until the object is complete.

Microphone (input). Sound waves make a diaphragm vibrate; this vibration is converted into a varying electrical (analogue) signal, which an ADC turns into digital data.

Speakers (output). A digital signal is converted by a DAC into an analogue current that moves a cone/diaphragm, creating vibrations in the air that we hear as sound.

Magnetic hard disk (storage). Data is stored as magnetised patterns on spinning platters; a moving read/write head floats just above the surface to read or change the magnetism. (Has moving parts.)

Solid state / flash memory (storage). Stores data electronically in EEPROM transistors — no moving parts, so it is fast, durable and silent; used in SSDs, USB drives and memory cards.

Optical disc reader/writer (storage). A laser reads the disc by detecting reflections from pits and lands on the surface; to write, a more powerful laser burns marks that change reflectivity (CD/DVD/Blu-ray).

Touchscreen (input/output). Resistive: two flexible conductive layers; pressure pushes them together and the contact point is detected. Capacitive: a charged layer; a conductive finger changes the electrostatic field, and the change is detected (supports multi-touch).

Virtual reality (VR) headset (output + input). Shows a slightly different image to each eye to create a 3D effect, and uses motion/position sensors to track head movement so the displayed scene updates to match where the user looks.

Different devices and processes work at very different speeds. A processor can send data far faster than a printer can print it, for example. A buffer solves this.

A buffer is a temporary storage area that holds data while it is being transferred between two devices or processes, compensating for the difference in their speeds.

Printing example: the computer pours the whole document quickly into the printer's buffer, then is free to do other work, while the printer takes data from the buffer at its own slower pace.

Buffers are also used when streaming video (data is buffered so playback is smooth even if the network slows down) and between the keyboard and the processor.

Both RAM and ROM are primary memory (reached directly by the processor), but they behave very differently.

RAM (Random Access Memory) is volatile — contents are lost when power is removed. It can be read from and written to, and it holds the programs, data and parts of the operating system in current use. More RAM lets a computer run more (or bigger) programs at once.

ROM (Read-Only Memory) is non-volatile — it keeps its contents without power. It is read-only in normal use, and it stores the start-up (boot) instructions / bootstrap and firmware the computer needs the instant it is switched on.

Two kinds of RAM — SRAM and DRAM:

	SRAM (Static)	DRAM (Dynamic)
Stores a bit using	flip-flops (transistors)	a capacitor (charge)
Needs refreshing?	No	Yes (thousands of times/sec)
Speed	Faster	Slower
Cost / density	More costly, less dense	Cheaper, denser
Typical use	Cache memory	Main memory

A capacitor's charge leaks away, so DRAM must be constantly refreshed; flip-flops hold their value while powered, so SRAM needs no refresh and is faster — which is why the small, fast cache uses SRAM and the large, cheap main memory uses DRAM.

Three kinds of programmable ROM:

• PROM (Programmable ROM): blank when made; can be written to once and then never changed.
• EPROM (Erasable PROM): can be erased and rewritten, but only by ultraviolet (UV) light, which wipes the whole chip, and the chip must be removed from the circuit.
• EEPROM (Electrically Erasable PROM): can be erased and rewritten electrically, in place, and byte by byte. Flash memory (in SSDs and memory cards) is a type of EEPROM.

Both kinds of system use sensors (input devices that measure a physical quantity such as temperature, pressure, infra-red or sound) feeding readings to a microprocessor, usually via an ADC. The difference is what happens next.

A monitoring system measures conditions and reports or warns, but does not change them — there is no actuator. Example: monitoring a patient's heart rate and sounding an alarm if it goes outside safe limits.

A control system measures conditions and then acts on the environment automatically through actuators (motors, valves, heaters, pumps) to keep a quantity at a required value. This needs feedback: the action changes what the sensor reads, and that new reading is fed back to be compared with the target again.

Without feedback, a control system could not detect the effect of its own actions and would over- or under-shoot the target. A monitoring system has no actuator, so there is nothing to feed back.