Hardware

Paper 1 · Unit 3. Devices, memory, monitoring and control, and logic gates.

1. Inputs, outputs and storage

A computer system needs input devices to capture data, output devices to present results, primary storage (RAM and ROM) that the CPU accesses directly, and secondary storage for long-term, non-volatile storage. Secondary storage is needed because RAM is volatile and too small and expensive to hold everything.

An embedded system is a computer built into a larger device to perform one dedicated function, e.g. in a washing machine, car engine management or a thermostat.

Embedded systems: benefits	Drawbacks
Small, cheap to mass-produce, low power	Hard to upgrade or repair
Reliable and fast at one task	Limited functionality
Can be controlled remotely when connected	Connected devices can be a security risk

2. Devices and how they work

Device	Basic internal operation
Laser printer	A drum is given a charge; a laser removes charge where the page is blank (or draws the image); toner sticks to the charged areas; it is transferred to the paper and fused with heat
3D printer	Builds an object layer by layer from a digital model, e.g. by extruding melted plastic through a moving nozzle
Microphone	Sound waves vibrate a diaphragm; the movement (e.g. of a coil in a magnetic field) creates an electrical signal, which an ADC converts to digital
Speaker	A varying electrical current (from a DAC) passes through a coil in a magnetic field, moving a cone that produces sound waves
Magnetic hard disk	Platters spin; a read/write head on an actuator arm moves over tracks and sectors and magnetises tiny areas to store 1s and 0s
Solid state (flash) memory	No moving parts; data is stored by trapping charge in floating-gate transistors (NAND flash), arranged in pages and blocks
Optical disc	A laser reads pits and lands on a spinning disc; changes in the reflected light are read as 1s and 0s
Touchscreen	Capacitive: a finger changes the electrostatic field at a point; resistive: pressure makes two layers touch, completing a circuit
VR headset	Separate screens for each eye give a 3D image; sensors track head movement and update the view

3. Buffers

A buffer is a temporary area of memory that holds data while it is transferred between devices working at different speeds, e.g. data sent to a printer, or video being streamed. The CPU can fill the buffer and get on with other tasks instead of waiting for the slower device.

4. RAM and ROM

	RAM	ROM
Volatile?	Yes: contents lost when power is off	No: contents kept
Read/write	Read and write	Read only (in normal use)
Use	Data and programs currently in use	Firmware, e.g. the boot program (BIOS)

	SRAM	DRAM
Built from	Flip-flops	Capacitors and transistors
Refresh	Does not need refreshing	Must be refreshed constantly
Speed	Faster access	Slower
Cost and density	More expensive, less data per chip	Cheaper, more data per chip
Use	Cache memory	Main memory

• PROM: can be programmed once after manufacture.
• EPROM: erased with ultraviolet light, then reprogrammed.
• EEPROM: erased and reprogrammed electrically, without removing it (flash memory is a type of EEPROM).

5. Monitoring and control systems

A monitoring system uses sensors to collect data and warns or records, but does not change the process (e.g. a hospital patient monitor). A control system also uses actuators to change the conditions (e.g. a greenhouse that opens windows).

Sensor readings are converted by an ADC; the microprocessor compares them with stored values and, if needed, sends signals to actuators (via a DAC if they need analogue signals). Feedback means the output affects the next input: the loop repeats so the system keeps conditions at the set value.

6. Logic gates and circuits

Gate	Output is 1 when…
NOT	the input is 0
AND	both inputs are 1
OR	at least one input is 1
NAND	NOT AND: not both inputs are 1
NOR	NOT OR: both inputs are 0
XOR	exactly one input is 1

You must be able to build a circuit from a problem statement, logic expression or truth table, and produce a truth table or logic expression from a circuit. With 3 inputs, a truth table has 8 rows (000 to 111).

7. Examiner tips

1. Describe the operation of devices step by step; vague answers such as “it prints the image” get no marks.
2. SRAM vs DRAM: SRAM does not need refreshing, is faster and is used for cache.
3. Buffers exist because devices work at different speeds.
4. Control systems need actuators; monitoring systems do not change the process.
5. Truth tables: list inputs in binary order and check every row.
6. Embedded systems do one dedicated task inside a larger device.

8. Practice questions

9. Topic test: predict your grade