Command words and timing

Primary storage: RAM and ROM

Primary storage is memory the CPU can access directly. It includes RAM and ROM.

RAM (random access memory) holds the data and programs currently in use, including parts of the operating system. It can be read and written. It is volatile: everything is lost when the power goes off.

ROM (read-only memory) holds the start-up instructions (the bootloader firmware) that run when you switch on. It can only be read. It is non-volatile: it keeps its contents without power.

Secondary storage

Secondary storage is non-volatile storage that the CPU does not access directly. Data is copied into RAM first.

It is needed to keep files, programs and the operating system when the power is off. It also holds far more than RAM, for less money per gigabyte.

There are three types: magnetic, optical and solid-state.

Magnetic storage: the hard disk drive

A hard disk drive (HDD) has metal or glass platters coated with a magnetic material. They spin very fast, for example 7,200 times a minute.

A read/write head on a moving arm floats a tiny distance above each surface. To write, it magnetises tiny areas in one direction for 1 and the other direction for 0. To read, it senses the direction.

Each platter is divided into tracks (circles) and sectors (slices of each track). Data is found by moving the head to the right track, then waiting for the right sector to spin past.

Optical storage: CD, DVD and Blu-ray

An optical disc has a single spiral track with tiny dips called pits and flat areas called lands.

A laser shines on the spinning disc. Lands reflect the light back strongly; pits scatter it. A sensor reads these changes as 1s and 0s.

To write a recordable disc, a stronger laser changes small areas of a dye layer, so they reflect differently.

Solid-state (flash) storage

Solid-state storage has no moving parts. It stores data in flash memory chips using transistors.

Each cell has a control gate and a floating gate. Electric charge (electrons) can be trapped in the floating gate. Charge trapped, or not, stands for a 0 or a 1, and stays without power.

Flash memory is built using NAND or NOR technology. NAND is denser and cheaper, so it is used in SSDs, SD cards and USB flash drives.

Virtual memory

When RAM is full, the computer can use part of secondary storage as if it were extra RAM. This is virtual memory.

Data is split into fixed-size blocks called pages. Pages that are not needed right now are moved from RAM to virtual memory. When they are needed again, they are moved back into RAM.

It is needed so the computer can run more programs, or larger programs, than RAM can hold. Without it, programs could crash or refuse to open.

Cloud storage

Cloud storage keeps your files on remote servers owned by a company, which you reach over the internet.

The servers are real, physical computers in large data centres. Your data is usually copied onto several servers in case one fails.

Local storage means a drive you own and keep with you, such as a laptop's SSD or a USB drive.

Brute-force attack

A brute-force attack is when a criminal tries every possible password (or key) until one works. Software tries millions of guesses each second.

The aim is to get into an account or system, then steal data or cause harm.

Longer passwords with a mix of character types have far more possible combinations, so they take much longer to crack.

Data interception

Data interception is when data is stolen while it is being sent, for example over Wi-Fi or across the internet.

A criminal can use a packet sniffer, a program that captures and examines data packets as they pass. On an unsecured public Wi-Fi network this is easy.

The aim is to read passwords, card numbers or private messages.

Distributed denial of service (DDoS)

A denial of service (DoS) attack floods a web server with so many requests that it cannot cope. Real users cannot reach the website.

In a distributed DoS (DDoS), the requests come from thousands of computers at once. These are often ordinary devices infected with malware and controlled by the criminal. This group is called a botnet.

The aim is to take a website offline, often to cause loss of sales or to demand money.

Hacking

Hacking is gaining unauthorised access to a computer system. Unauthorised means without permission.

A hacker may steal, change or delete data, or install malware. This is illegal in most countries.

Ethical hacking is different: a company pays an expert to try to break in, so weaknesses can be found and fixed.

Malware: virus, worm and Trojan horse

Malware (malicious software) is any software written to cause harm. Cambridge lists six types. The first three spread or get in; the last three do specific damage.

Malware: spyware, adware and ransomware

These three types are named after what they do once they are on a device.

Phishing and pharming

Phishing is sending a fake email or message that looks like it comes from a trusted organisation, such as a bank. It tries to make the user click a link to a fake website and type in their personal details.

Pharming is when malicious code is installed on the user's computer, or a DNS server is tampered with. It redirects the user to a fake website, even when they type the correct URL.

Both aim to steal login details, card numbers or other personal data.

Social engineering

Social engineering is manipulating people, rather than computers, into breaking security. It uses trust, fear, curiosity or urgency.

Phishing is one kind. Another is a phone call from someone pretending to be the IT department.

Baiting leaves a USB stick labelled "Salaries" in the car park, hoping someone plugs it in. Scareware is a fake pop-up saying "Your PC is infected, call now".

The aim is to get passwords, access to a building or system, or to get malware installed.

Solutions: access levels, anti-malware and authentication

Access levels give each user only the rights they need. A student can read the school timetable but cannot change it. This limits the damage if an account is hacked.

Anti-malware software includes anti-virus and anti-spyware. It scans files and compares them with a database of known malware. It quarantines or deletes anything harmful and checks downloads as they arrive.

Authentication proves a user is who they say they are.

Solutions: updates, checking messages and links

Automating software updates means the OS and apps download and install fixes by themselves. Updates close security holes that hackers and malware use.

Checking the spelling and tone of communications helps spot phishing. Warning signs are spelling mistakes, a general greeting such as "Dear customer", and an urgent or threatening tone.

Checking URLs in links before clicking: hover over the link to see where it really goes. Look for misspellings such as `paypa1.com` or extra words such as `bbva-secure-login.net`.

Solutions: firewalls and privacy settings

A firewall sits between a computer (or network) and the internet. It can be hardware, software or both.

It examines all traffic coming in and going out. It checks each packet against a set of rules, such as allowed IP addresses or ports. Traffic that breaks the rules is blocked.

It can also log traffic and warn the user when a program tries to connect to the internet.

Privacy settings control who can see your information, for example on social media. Setting a profile to "friends only" stops strangers seeing your location and photos, which social engineers use.

Solutions: proxy servers

A proxy server is a server that sits between the user's computer and the web server. Requests go to the proxy first, and the proxy fetches the page on the user's behalf.

It can filter requests, blocking access to certain websites. Schools use this to block unsuitable sites.

It hides the user's IP address, because the web server only sees the proxy's address. This protects the user's identity.

It can cache (store copies of) popular pages, so they load faster the next time.

In front of a web server, it can absorb and filter heavy traffic, helping to protect against DDoS attacks.

Solutions: SSL

SSL (Secure Sockets Layer) is a security protocol that encrypts data sent between a browser and a web server. Its modern version is called TLS, but the exam uses the name SSL.

It is what turns HTTP into HTTPS. It uses an SSL certificate: a digital document that proves the website is who it claims to be.

Matching threats to defences

Exam questions often give a scenario and ask you to name the threat and the best defence. Try the threats toy: read each scenario, name the threat, then pick the defence.

Always explain how the defence stops that threat. "Install a firewall" is a weak answer. "A firewall blocks incoming traffic that does not meet its rules" is stronger.

There is something to try for this step in the IGCSE library.

Stating the purpose

The purpose of an algorithm is the job it does overall. It is the reason the algorithm exists.

To find it, look at the inputs, then look at what is output at the end. The purpose links the two.

Write one sentence: 'The algorithm inputs … and outputs …'.

CONSTANT CorrectPin ← "4821"
DECLARE Guess : STRING
DECLARE Attempts : INTEGER
Attempts ← 0
REPEAT
  OUTPUT "Enter your PIN: "
  INPUT Guess
  Attempts ← Attempts + 1
UNTIL Guess = CorrectPin OR Attempts = 3
IF Guess = CorrectPin
  THEN
    OUTPUT "Card unlocked"
  ELSE
    OUTPUT "Card blocked"
ENDIF

CORRECT_PIN = "4821"
attempts = 0
guess = ""
# Python has no REPEAT ... UNTIL, so the loop condition is reversed
while guess != CORRECT_PIN and attempts < 3:
    guess = input("Enter your PIN: ")
    attempts = attempts + 1
if guess == CORRECT_PIN:
    print("Card unlocked")
else:
    print("Card blocked")

Describing the processes

Sometimes a question asks you to describe the processes. Now you do go through the steps, but in plain words and in order.

Mention the loop, what is checked, and what is changed or output. Use the identifiers from the algorithm.