Computer Hardware and Logic Circuits: Question 4

Worked solution

Part (a): Magnetic hard disk drives

An HDD stores data on one or more rigid circular platters that spin at high speed, coated in a thin layer of magnetic material. A read/write head, positioned just above the platter’s surface, magnetises tiny individual areas of that surface in one of two directions as the platter spins beneath it, one direction represents binary 1, the other represents binary 0. To read data back, the same head detects the existing direction of magnetisation of each area without changing it. [2 marks]: [1] for data being stored as magnetised areas on a spinning platter, [1] for the read/write head magnetising (write) or detecting (read) the direction/polarity of magnetisation.

Part (b): Optical discs

An optical disc, such as a DVD, stores data as a long spiral track of microscopic pits (tiny indentations) and lands (the flat, unindented areas between pits) pressed or burned into a reflective layer. As the disc spins, a laser is shone onto this track. A land reflects the laser strongly back to a light sensor, while a pit scatters the light so much less is reflected. The sensor detects this changing pattern of strong and weak reflection, which is converted into a sequence of binary 0s and 1s. [2 marks]: [1] for data being stored as a pattern of pits and lands on a spiral track, [1] for a laser and light sensor reading the pattern via differences in reflected light.

Part (c): Solid-state storage

Solid-state storage, such as an SSD or a USB flash drive, has no moving mechanical parts. Instead, data is stored electronically in a large array of individual memory cells, each built around a floating-gate transistor. Writing to a cell traps a small electric charge on its floating gate (or removes it); the presence or absence of this trapped charge is read back as a binary 1 or 0. Because nothing has to physically move to locate and read a cell, solid-state storage can access data very quickly. [2 marks]: [1] for data being stored electronically with no moving parts, [1] for the specific mechanism of trapped electric charge in floating-gate transistor cells representing a bit.

Part (d): Choosing a suitable off-line archive medium

The archive copy must be written once and then kept off-line, disconnected from the computer, so the chosen medium must be easy to physically remove and store elsewhere, and must not need continuous power to keep its data safe.

A fixed internal magnetic HDD does not suit this requirement well, since it is not designed to be regularly disconnected and stored away from the computer.

A write-once optical disc (such as a DVD-R) is well suited: it is small, portable and easily removed from the computer once burned; it needs no power at all to retain its data while in storage; and because it can only be written to once, the finished archive cannot later be accidentally overwritten, edited or corrupted by malware, an important safeguard for a backup copy.

Removable solid-state storage, such as a portable SSD or USB flash drive, is also a reasonable choice for the same “removable and needs no power” reasons, and would additionally be faster and more physically robust if the archive ever needs to be restored, though, unlike a write-once disc, it can typically be overwritten or deleted by mistake.

Final answers

• (a) Data is stored as magnetised areas on a spinning platter; a read/write head magnetises or detects their polarity.
• (b) Data is stored as a pattern of pits and lands on a spiral track; a laser and light sensor read differences in reflection.
• (c) Data is stored electronically as trapped electric charge in floating-gate transistor cells; there are no moving parts.
• (d) A write-once optical disc (or removable solid-state storage). Removable, needs no power to retain data, and (for a write-once disc) cannot be accidentally overwritten once archived.