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Five megabytes that weighed a tonne

• In 1956 IBM shipped the first hard disk drive. It stored 5 megabytes on fifty spinning platters, each 24 inches across, and the unit weighed about a tonne.
• A 2.5-inch drive today holds a million times as much, using exactly the same idea: magnetise tiny regions of a spinning disc and read them back.
• The exam asks you to describe how each principal device works, step by step, in the order the mark scheme lists.
• This lesson takes them one at a time: the three kinds of storage, then the peripherals.

Magnetic hard disk

• A magnetic hard disk 磁盘 (HDD) stores data as magnetised regions on spinning platters coated with magnetic material.
• Each surface is divided into concentric tracks 磁道, and each track into sectors 扇区. A read/write head on an actuator arm moves across the platter to the right track and waits for the sector to pass under it.
• Cheap per gigabyte and large, but the moving parts make it slower, noisier and easier to damage by a knock.

The actuator arm swings the read/write head across the spinning platter

A platter stores data in concentric tracks, each divided into sectors

Solid-state (flash) memory

• A solid-state drive 固态硬盘 (SSD) stores data as electric charge trapped in floating-gate transistors 浮栅晶体管 inside NAND flash chips. No moving parts.
• So it is faster to access, silent, uses less power and survives being dropped. It costs more per gigabyte, and each cell can only be rewritten a limited number of times.
• The same flash memory is in USB sticks and memory cards.

Inside an SSD: rows of flash memory chips and a controller, nothing that moves

Optical disc reader and writer

• An optical disc 光盘 (CD, DVD, Blu-ray) stores data along one long spiral track as pits and lands. A laser shines on the track and a sensor measures how much light is reflected: a change in reflectivity is read as a bit.
• A writer uses a stronger laser to change the surface, burning a dye layer or altering a phase-change layer, so the reflectivity pattern is created.
• Blu-ray uses a shorter-wavelength laser than DVD, so its pits are smaller and it holds more.

A laser reads the pits and lands of the spiral track

Worked example: choosing a storage device

Laser printer

• A laser printer 激光打印机 works in the steps the mark scheme lists. The page data goes to the printer's buffer. The drum 感光鼓 is given a uniform electrostatic charge.
• A laser, reflected off a rotating mirror, scans the image onto the drum and removes the charge where it strikes, so the charge left matches the page.
• Toner 墨粉, a charged powder, is attracted only to the charged parts of the drum. The paper is given the opposite charge and the toner transfers onto it.
• The fuser 定影器, a pair of heated rollers, melts the toner into the paper. The drum is discharged and cleaned for the next page.

A laser printer: charge, laser, toner, transfer, fuse

3D printer

• A 3D printer 3D打印机 builds a solid object layer by layer from a digital model.
• The model is designed in CAD software or scanned; slicing software cuts it into thin horizontal layers and produces the instructions for each.
• An FDM printer melts plastic filament 塑料丝 and lays it down through a moving nozzle; a resin printer cures liquid resin with UV light; a powder printer fuses powder with a laser. Each layer bonds to the one below.
• Uses: prototypes, custom medical parts such as prosthetics, spare parts made on demand.

An FDM 3D printer melts filament and builds the object one layer at a time

Microphone and speakers

• In a microphone 麦克风, sound waves make a diaphragm 膜片 vibrate. The diaphragm moves a coil in a magnetic field (or changes the gap of a capacitor), producing a varying electrical signal.
• That signal is analogue, so an ADC 模数转换器 (analogue-to-digital converter) samples it and turns it into binary before it can be stored.
• A speaker runs the process backwards: a DAC produces a varying current, the current in a coil makes it move in a magnetic field, and the attached cone pushes the air to make sound.

Inside a microphone: sound vibrates the diaphragm and coil to produce a current

A speaker is a microphone in reverse: current moves the coil, the cone moves the air

Touchscreen

• A touchscreen 触摸屏 is both an input device and an output device.
• Capacitive 电容式: the glass carries a layer that stores charge. A finger touching it changes the charge at that point, and sensors at the corners work out the coordinates. Accurate, bright, supports multi-touch, but needs a conductive touch.
• Resistive 电阻式: two flexible layers separated by a small gap. Pressing them together completes a circuit at the touch point and the voltage gives the coordinates. Works with a stylus or a gloved hand, but is dimmer and cannot do multi-touch.

Capacitive touchscreens respond to the charge of a fingertip

Virtual reality headset

• A virtual reality headset 虚拟现实头显 shows each eye a slightly different image on its own display (or half of one screen), so the brain sees depth.
• A gyroscope and an accelerometer sense how the head turns and tilts; the images are redrawn many times a second to match, so the scene appears fixed while the user looks around.
• Headphones supply sound that changes with head position, and external cameras or infrared sensors may track where the user is standing.

A VR headset: two images, motion sensors and sound that follows the head

Worked example: "describe the principal operation"

Marks that slip away

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