Hardware

1. Input & Output Devices

① 3D Printer — basic internal operation

Idea: builds a solid 3D object layer by layer from melted plastic. A very time-consuming process (one layer at a time).

1. The object is designed using computer-aided design (CAD) software. Any CAD package; Windows even has a 3D-printer/3D app. You draw the 3D shape here.
2. The software splits the object into slices. = thin horizontal layers (layer 1, 2, 3 …) — e.g. a human face is cut into many thin layers. This is the software's job. (Not "symmetry lines" — just ordinary layers.)
3. The data about the slices is sent to the printer. The coordinates of every slice go to the 3D printer.
4. The solid plastic is melted and transferred to the nozzle.
5. A stepper motor moves the nozzle into position. Stepper motors on the x- and y-axes (and z for height); the nozzle is joined to them. Basic maths/coordinate algorithms decide where to move.
6. The nozzle extrudes the molten plastic. The plastic loaded in step 4 now comes out at that position.
7. The previous steps are repeated until the layer is complete.
8. A fan cools the layer.
9. Each layer is printed by following the same steps. Repeat for every slice ⇒ finished 3D shape.

② Laser Printer — basic internal operation

Parts to know: drum, laser, moving mirror, toner, paper tray, fuser. Paper travels: tray → drum → fuser → exit.

1. The revolving drum is initially given an electrical charge. + or − (either).
2. A laser beam bounces off a moving mirror and scans back and forth across the drum. Mirrors move ⇒ laser sweeps the drum.
3. Discharging certain points (the letters / images to be printed) as electrical charges. e.g. to print "A" the laser discharges exactly the shape of A on the drum.
4. The drum is coated with oppositely charged toner, which only sticks to charged areas. Toner = the ink. e.g. drum +, toner − ⇒ sticks only to the charged pattern.
5. The pattern on the drum is transferred to the paper. Paper comes from the paper tray, passes the drum.
6. The paper is passed through the fuser to seal the image.
7. The electrical charge is removed from the drum and excess toner is collected. Drum is cleaned, ready for the next page.

③ Microphone — basic internal operation

• The microphone has a diaphragm.
• The incoming sound wave causes (it to) vibrate — diaphragm moves back, forward, back, forward following the wave.
• causing a coil to move past a magnet. Diaphragm is attached to the coil ⇒ coil moves ⇒ current is induced (it behaves as an electromagnet near the permanent magnet, attracted/repelled).
• An electric signal is produced. …and this electrical signal is then stored / sent on.

④ Speaker — basic internal operation

Parts: cone (= diaphragm), coil of wire (voice coil), permanent magnet, dust cap (dust cap keeps dust out).

1. Takes an electrical signal and translates it into physical vibrations to create sound waves.
2. An electric current in the coil creates an electromagnetic field.
3. Changes in the audio signal cause the direction of the electric current to change.
4. The direction of the current determines the polarity of the electromagnet. polarity = N or S pole (north on top then south, flips every time the current flips).
5. The electromagnet is repelled by or attracted to the permanent magnet.
6. Causing the coil to vibrate.
7. The movement of the coil causes the cone to vibrate (coil is attached to the cone).
8. The vibration is transmitted to the air in front of the cone. Cone pushes & pulls the air ⇒ sound wave. (Put small balls on a speaker and they jump — that is the cone vibrating.)
9. The amount of movement will determine the frequency and amplitude of the sound wave produced.

⑤ Resistive Touch Screen

• Consists of two charged plates.
• Pressure causes the plates to touch, completing the circuit.
• Point of contact is registered.
• Coordinates are used to calculate the position.

⑥ Capacitive Touch Screen

Still a touch screen, just a different name/technology.

• Made from materials that store electric charge. (a surface material that holds a charge)
• When touched, charge is transferred to the finger.
• Sensors at the screen corners detect the change.
• Point of contact is registered.
• Coordinates are used to calculate the position.

⑦ Virtual Reality (VR) Headset

• Video is sent from a computer to the headset using a cable. (computer, or even a mobile)
• Two feeds are sent to an LCD. Sometimes two screens are used (one per eye); sometimes one LCD with two different feeds.
• Lenses are used for focusing and reshaping the image. The two lenses in front of the eyes.
• Most headsets use a 60 to 120 frame rate and 110° field of view to make it realistic. frame rate = number of images shown per second (60–120 Hz/fps).
• Gyroscopes or accelerometers are used to detect head movement. Two kinds of sensors measure where your head moves so the video can move with it.
• Headsets use binaural sound. Different sound to each ear: sound from the right ⇒ the character is on your right (e.g. footsteps in a game, 3D-sound videos on headphones).

2. Primary Memory — RAM & ROM

Computer memory is of two types: primary memory and secondary memory. (Primary vs secondary was O-Level; the new A-Level bit is the types of RAM and ROM.) Today: primary memory first.

RAM = Random Access Memory · ROM = Read Only Memory

Difference between RAM and ROM

e.g. 3 Chrome tabs + a Word doc open ⇒ switch the PC off ⇒ all lost, because it all sat in RAM. ROM keeps its content with power off — e.g. the BIOS (Basic Input Output System) = the boot-up instructions needed to start Windows. ROM chips come empty from the maker; the manufacturer puts the data in once (can't remove it in plain ROM).

Sir's analogy: secondary memory = the big serving dish (all the food/data stored); primary memory = your plate. To use food you must first move it from the dish onto the plate — likewise data must be brought from the hard drive into RAM (not ROM) before the CPU can work on it.

Uses of RAM and ROM — scenario questions

3. Types of RAM — DRAM vs SRAM

DRAM = Dynamic Random Access Memory (main memory) · SRAM = Static Random Access Memory (cache). Diagrams only help you relate — they have never been asked.

Flip-flop = a logic circuit that traps (latches) one bit, either 0 or 1. (all types — SR, JK … with truth tables — are studied in A2; here just know the idea). Logic-gate symbols are only drawn to understand the concept: really it's transistors, capacitors and wires on a board that behave like AND / OR / XOR.

Why refresh? a DRAM bit is a charge/voltage and with time it leaks away; if it isn't recharged the data would be lost. Why cheaper? fewer transistors per bit (transistors cost money). Why more storage? a flip-flop holds only one bit and takes several transistors, so SRAM holds less in the same space.

4. Types of ROM — Read Only Memory

Careful: PROM here is not "PRAM". All types start initially empty.

5. Secondary Memory — Types of Storage Media

Primary memory is done. Secondary memory stores data in three ways: magnetic, optical or solid state media. If asked to describe types of storage media, write these points.

6. Basic Internal Operation of a Hard Disk

Parts: platter, spindle, actuator arm (+ actuator axis), read/write head; also jumper block, IDE & power connectors on the board.

• HDD has one or more platters made of aluminium. The disks where the data is stored.
• Each surface of the platter is ferrous-oxide, which is capable of being magnetised. Magnetic properties are altered to store data.
• The platters are mounted on a central spindle (a rod; platters stacked on it).
• The disks are rotated at high speed. That PC noise you hear = hard disk. This is why it is called a hard drive.
• Each surface of the disk has a read/write head mounted on an arm, positioned just above the surface. Disk keeps revolving; when the data passes under the arm's head it is read.
• The surface of the platter is divided into concentric tracks and sectors. Concentric = a circle within a circle (like running tracks).
• One track in one sector is the basic unit of storage, called a block. (the yellow part in the figure)
• The data is encoded as a magnetic pattern for each block.

7. Flash Memory & Basic Internal Operation of SSD

The SSD (solid state drive) uses flash memory, so learn flash first; then ~90 % of the SSD points are identical — you only add the controller.

What is flash memory? (has been asked in past papers)

• Most are NAND-based flash memory. Built from logic gates (NAND); flip-flops/latches trap each bit.
• There are no moving parts. (compare HDD: disks rotate at high speed ⇒ moving parts)
• Uses a grid of columns and rows that has two transistors at each intersection. very important point!
• One transistor is called the floating gate and the second transistor is called the control gate.
• Memory cells store voltages which can represent either a 0 or a 1.
• Essentially, the movement of electrons is controlled to read / write.
• No overwriting data — first erase, then add. You can't overwrite a cell; remove the old data first, then store the new data in that location.

SSD — basic internal operation

• No moving parts. · Non-volatile.
• Uses NAND gates, NOR gates (transistors) and integrated circuits. IC = the chip that combines transistors into logic gates.
• SSD controller manages the components. the extra point for SSD — like a processor managing all the programs in a computer, the controller manages all the SSD's components.
• Uses a grid of columns and rows that has two transistors at each intersection.
• One transistor is called the floating gate; the second is called the control gate.
• Memory cells store voltages which can represent either a 0 or a 1.
• Essentially, the movement of electrons is controlled to read / write.
• No overwriting data — first erase, then add.

8. Optical Discs — CD, DVD-RW, DVD-RAM

Optical disc — operation

• Drive motor is used to spin the disc.
• Tracking mechanism moves the laser assembly. The laser is used for both reading and writing.
• A lens focuses the laser onto the disc.
• Laser beam is shone onto the disc to read / write. The disc has a coating; the laser burns a pattern of pits and land (areas that reflect less or more light) = 0s and 1s. To read, shine the laser again: sensors measure the intensity of the reflected light ⇒ 0s and 1s.

Disc quality degrades with time — scratches and temperature affect it; performance (access time, speed, space) slowly drops.

Spiral track vs concentric tracks

9. Embedded Systems

Special-purpose computers are still computers because they contain a microprocessor — it holds programs/instructions, decides what to do and controls the components, just like a normal computer, but it is built to do only one job. Special purpose computers = embedded systems.

Write it exactly as it is — key words: microprocessor, larger system (built into the device), one specific task.

10. Quick Recap — Full Chapter Cheat-Sheet