Digital Systems, Binary, Hexadecimal, ASCII, Storage & Data Representation

Why do computers use binary?

Electronic components have two stable states (on/off), making binary the natural choice for digital systems.

How many unique values can n bits represent?

2ⁿ unique values. E.g. 8 bits = 2⁸ = 256 values.

Convert 8-bit binary 01001010 to denary.

74

Convert denary 200 to 8-bit binary.

11001000

What is two's complement used for?

To represent negative numbers in binary.

How do you convert a positive number to two's complement negative?

Flip all bits of the positive binary number, then add 1.

Convert -35 to 8-bit two's complement.

11011101

What is the range of values in 8-bit two's complement?

-128 to +127

Add binary 01110011 + 00101101.

10100000

What is an overflow error?

When the result of a calculation is too large to fit in the allocated number of bits.

What is the effect of an overflow error?

The result wraps around or becomes incorrect, causing unexpected program behaviour.

What does a logical left shift by 1 do to a binary number?

Shifts all bits left by one place, fills the right with 0.

What does a logical right shift by 1 do to a binary number?

Shifts all bits right by one place, fills the left with 0.

What does an arithmetic right shift do?

Shifts all bits right but fills vacated MSB positions with the original sign bit.

How does an arithmetic right shift differ from a logical right shift?

Arithmetic fills vacated bits with the sign bit; logical always fills with 0.

Why does a right shift reduce precision?

Bits shifted off the right end are permanently lost.

Why is hexadecimal used?

It is more compact than binary — each hex digit represents exactly 4 bits.

Convert binary 11101010 to hexadecimal.

EA

Convert binary 00111111 to hexadecimal.

3F

Convert hex 3F to binary.

00111111

Convert hex EA to binary.

11101010

What is 7-bit ASCII?

A character encoding standard that uses 7 bits to represent 128 characters.

What is one limitation of ASCII?

It can only represent 128 characters.

What was created to address ASCII's limitations?

Unicode — a standard that supports over 100,000 characters.

What is the ASCII decimal code for uppercase 'A'?

65

What is the ASCII decimal code for lowercase 'a'?

97

What is the ASCII decimal code for the digit '0'?

48

What is the von Neumann stored program concept?

Both program instructions and data are stored together in the same memory.

What is the role of the clock in the FDE cycle?

It generates regular pulses that synchronise CPU operations.

What is the role of the address bus?

Carries the memory address of the data or instruction the CPU wants to access.

What is the role of the data bus?

Carries actual data and instructions between the CPU, RAM and I/O devices.

What is the role of the control bus?

Carries control signals to coordinate operations between components.

What happens during the fetch stage of the FDE cycle?

The CPU reads the instruction from RAM at the address in the Program Counter.

What happens during the decode stage of the FDE cycle?

The Control Unit interprets the binary instruction to determine what operation is needed.

What happens during the execute stage of the FDE cycle?

The CPU carries out the instruction.

What is primary storage?

Fast memory directly accessible by the CPU.

What is secondary storage?

Non-volatile long-term storage such as HDD, SSD or optical discs.

Why is primary storage needed if secondary storage exists?

RAM is much faster and directly accessible by the CPU.

Why is secondary storage needed if primary storage exists?

RAM is volatile — data is lost when the computer is switched off.

How does a magnetic HDD store data?

Tiny magnetic particles on spinning platters are polarised to represent 0s and 1s.

What is the main advantage of an HDD?

High storage capacity at a low cost per GB.

What is the main disadvantage of an HDD?

It has moving parts, making it slower than an SSD.

How does an optical disc store data?

A laser reads pits and lands on a reflective spinning surface.

What is a strength of optical storage?

Portable, cheap to produce, and good for distributing software or media.

What is a weakness of optical storage?

Low capacity compared to HDD/SSD and slow data transfer speeds.

How does an SSD store data?

Electrical charge is trapped in floating-gate transistors.

What is the main advantage of an SSD?

Much faster read/write speeds than an HDD.

What is the main disadvantage of an SSD?

More expensive per GB than an HDD.

What is magnetic tape storage best suited for?

Large-scale archival backups.

Calculate the number of bits in a 2.5 GB file.

20,000,000,000 bits

What unit is used to measure data storage that equals 1,024 megabytes?

Gigabyte (GB)

What is the role of an operating system?

To manage hardware resources, files, processes, security and the user interface.

What is scheduling in an OS?

The OS decides which process gets CPU time.

What is paging in an OS?

Dividing a process's memory into fixed-size pages.

What is a device driver?

Software that translates generic OS commands into device-specific instructions.

Why are device drivers needed?

To allow the OS to communicate with a peripheral.

What is a GUI?

Graphical User Interface — uses visual elements like windows, icons and a pointer. Intuitive and easy for general users.

What is a CLI?

Command Line Interface — the user types text commands. Harder to learn but more powerful for scripting and remote administration.

What is the purpose of authentication in an OS?

To verify a user's identity (e.g. via password or biometrics) before granting access to the system.

What are file permissions in an OS?

Rules controlling what an authenticated user can do with files — e.g. read, write or execute access.

What does defragmentation software do?

Reorganises fragmented files on an HDD so data is stored contiguously, improving read/write speed.

What does backup software do?

Creates copies of data so it can be restored after deletion, hardware failure or a cyberattack.

What does encryption software do?

Scrambles data so it can only be read by someone with the correct decryption key.

What is a virus?

Malware that attaches to legitimate files and replicates when the file is run, spreading to other files and systems.

What is a trojan?

Malware disguised as legitimate software. Once installed it creates backdoors or steals data. It does not self-replicate.

What is ransomware?

Malware that encrypts the victim's files and demands payment for the decryption key.

What is spyware?

Malware that secretly monitors user activity and sends information such as keystrokes or passwords to an attacker.

What is a worm?

Malware that self-replicates and spreads across networks without needing to attach itself to a file.

How does anti-malware software work?

It detects, quarantines and removes malware using signature databases and heuristic analysis of suspicious behaviour.

Why is out-of-date anti-malware a risk?

New malware strains won't be in an outdated signature database, so they won't be detected or blocked.

Why is unpatched software a security risk?

Known vulnerabilities are publicly listed. Attackers exploit systems that haven't applied the patch fixing those weaknesses.

What is phishing?

A social engineering attack using fake emails or websites to trick users into revealing passwords or financial details.

What is spear phishing?

A targeted phishing attack aimed at a specific individual or organisation, using personalised details to appear more convincing.

What is pretexting?

A social engineering tactic where an attacker creates a fabricated scenario to manipulate someone into giving information or access.

What is shoulder surfing?

Physically watching someone enter a password or PIN to steal their credentials.

What is baiting?

Leaving infected USB drives or media in public places, hoping someone will plug them in and install the malware.

What is ethical hacking?

Authorised simulated attacks on a system carried out to find vulnerabilities before malicious attackers do.

What is penetration testing?

A method of testing a system's security by simulating real attacks to identify and report weaknesses.

How does a firewall protect a network?

It monitors and filters incoming and outgoing traffic based on predefined rules, blocking unauthorised connections.

What is a physical security measure?

Controls like locks, key cards, CCTV and security guards that prevent unauthorised physical access to hardware.

What is an acceptable use policy?

A document stating the rules for how employees may use IT systems, reducing the risk of misuse.

Why are regular backups important for security?

They allow data to be restored after ransomware, accidental deletion or hardware failure, minimising data loss.

What is an audit trail?

A log of all actions performed on a system, used to detect suspicious activity and investigate security incidents.

Why is code review important for security?

Other developers can identify vulnerabilities such as SQL injection risks or backdoors that the original developer missed.

Why are computers connected in a network?

To share resources such as files, printers and internet access, communicate, and enable collaborative working.

What is a LAN?

Local Area Network — covers a small geographical area like a building or campus, typically owned by one organisation.

What is a WAN?

Wide Area Network — spans large distances such as cities or countries. The internet is the largest WAN.

Why are network protocols needed?

They define agreed rules for how data is formatted, transmitted and received, allowing different devices to communicate.

What is an IP address used for?

To uniquely identify a device on a network so data can be correctly routed to its destination.

What is a property of copper cable?

Carries electrical signals, relatively cheap to install, but susceptible to interference and limited bandwidth over long distances.

What is a property of fibre optic cable?

Carries data as light pulses, giving very high bandwidth, low signal loss over long distances, and no electrical interference.

What is Wi-Fi?

Wireless networking using radio waves with a typical range of up to ~100m. High bandwidth but signal weakens through walls.

What is Bluetooth?

Short-range wireless technology (~10m) for connecting personal devices such as headphones or keyboards.

What is Zigbee?

A low-power, low-data-rate wireless protocol designed for short-range IoT devices such as smart home sensors.

What is NFC?

Near Field Communication — operates over just a few centimetres. Used for contactless payments and device pairing.

What is bandwidth?

The maximum amount of data that can be transmitted per second, measured in bits per second (bps).

What is latency?

The time delay between data being sent and it being received, measured in milliseconds.

Calculate the transmission time for a 400 Mb file at 40 Mbps.

10 seconds.

Calculate the transmission time for a 50 MB file at 40 Mbps.

10 seconds.

What is a bus topology?

All devices connect to a single shared cable (backbone). Cheap but if the backbone fails the whole network goes down.