CPX10Operating Systems, Utility Software and Robust Software

The purpose of an operating system (OS) is to control the computer’s hardware and software. Windows, macOS, Linux, Android and iOS are operating systems.

• Every application runs on top of the OS. When an app saves a file or prints, it asks the OS to do it.
• Pearson’s spec names four functions: file management, process management, peripheral management and user management. Memory management (and virtual memory) is part of process management.
• The OS also provides a user interface, so people can give the computer instructions.
• Command-line interface (CLI): the user types text commands. Powerful and quick for experts, uses few resources, and commands can be saved and repeated. But commands must be learned exactly.
• Graphical user interface (GUI): windows, icons, menus and a pointer. Easy for beginners to learn, but it needs more memory and processing power.
• Natural language interface: the user speaks or types ordinary sentences, like asking a voice assistant a question. Hands-free and needs no training, but it can misunderstand accents, background noise or unclear requests.

The operating system sits between the user and applications above and the hardware below.

Operating system (OS)

Software whose purpose is to control the computer’s hardware and software.

User interface

The part of the OS that lets a person give the computer instructions and see the results: CLI, GUI or natural language.

File management is the process of organising, storing and retrieving files.

• Files are organised in a hierarchical (tree) structure of directories (folders) and sub-folders.
• The tree starts at the root directory. Every folder and file sits somewhere below the root.
• Each file has a unique path from the root, such as C:\Users\Sam\Documents\essay.docx. (Windows uses \ between names; macOS and Linux use /.)
• Two files can have the same name if they are in different folders, because their paths are still different.
• The OS provides common file-handling functions: save, open, rename, delete, plus copy and move.
• The OS keeps track of where each file is physically stored on the drive, so the user only has to know its name and folder.

A folder tree. The path to a file lists every folder from the root down to it.

File management

The process of organising, storing and retrieving files.

Root directory

The top folder of the hierarchy; every other folder and file is below it.

Path

The route from the root through each folder to a file. It identifies the file uniquely.

When a program is loaded into memory so it can be executed, it becomes a process. Many processes are active at once. Process management shares the finite resources (the CPU and main memory) between them.

• Only one process at a time has exclusive use of the CPU (on a single processor).
• Processes wait their turn in a queue.
• A scheduling algorithm decides the order, so processes can be prioritised: an urgent process may be moved ahead.
• Each process gets the CPU for a short time slice. If it hasn’t finished when its time slice ends, it goes to the back of the queue.
• Time slices are so short that all the programs seem to run at the same time.
• You don’t need to know any particular scheduling algorithm by name.

Each process uses the CPU for one time slice. Unfinished processes rejoin the back of the queue.

Process

A program that has been loaded into memory so it can be executed.

Scheduling algorithm

The method the OS uses to decide which process gets the CPU next, and to prioritise processes.

Time slice

The short, fixed amount of CPU time a process gets before the next process has its turn.

• The OS shares main memory (RAM) between processes, giving each process its own section of RAM.
• Keeping sections separate stops one process overwriting another’s instructions or data.
• When RAM is short, the OS uses virtual memory: an area of secondary storage (hard drive or SSD) set aside to act as extra memory.
• RAM is divided into pages. A paging algorithm moves pages of processes that aren’t being used right now out of RAM into virtual memory.
• When one of those processes needs the CPU again, its pages are swapped back into RAM (and something else may be swapped out).
• Secondary storage is much slower than RAM, so a computer that swaps a lot (because it has too little RAM for what is running) slows down.

Pages of inactive processes are swapped out to virtual memory on secondary storage, and swapped back when needed.

Virtual memory

An area of secondary storage that the OS uses as extra main memory when RAM is full.

Paging

Dividing memory into pages and swapping pages of processes between RAM and virtual memory as they are needed.

Peripheral management means controlling the devices connected to the computer, such as the keyboard, mouse, monitor, printer and scanner.

• Every model of device works differently. The OS uses a device driver for each one: software that translates the OS’s general instructions into the commands that particular device understands (and the device’s signals back again).
• Because of drivers, an application can just ask the OS to “print this”, without knowing which printer is attached.
• Devices are much slower than the CPU. The CPU doesn’t wait for them: it carries on running other processes.
• When a device needs attention (a key is pressed, the printer is ready for more data, the printer has run out of paper), it sends an interrupt: a signal to the OS that it needs immediate attention.
• The OS then pauses the current process, deals with the device (using its driver), and goes back to what it was doing.
• Data waiting to go to a slow device can be held in a queue in memory (for example, a queue of print jobs).

The OS talks to each device through its driver. The device sends an interrupt when it needs attention.

Peripheral

A device connected to the computer, such as a keyboard, monitor or printer.

Device driver

Software the OS uses to communicate with a particular peripheral.

Interrupt

A signal from a device to the OS that it needs immediate attention.

On a computer with several users, the OS controls who can use it and what each person can do.

• Adding and deleting users: an administrator creates an account for each person and removes it when they leave.
• Authenticating users: checking each user’s identity when they log in, for example with a username and password.
• Granting access rights to applications, data and systems: each user (or group of users) is given access only to what they need.
• Permissions control what users can do with a file or folder, such as read only, read and write (change), or no access.
• An administrator account has extra privileges, such as installing software and managing other users’ accounts.
• This protects data: users can’t open or change files that aren’t theirs, even by accident.

User management

Adding and deleting users, authenticating them, granting access rights and using permissions to control what they can do.

Access rights

What applications, data and systems a user is allowed to use.

Permissions

Settings that control what a user can do with a file or folder, e.g. read only or read and write.

Utility software is a collection of tools that help maintain the functionality of a computer. Pearson’s spec names five.

• File repair: repairs or recovers data from damaged or corrupted files, for example a document that won’t open after a power cut or a disk fault.
• Backup: automates making copies of files, so data can be recovered if something goes wrong (deleted by mistake, drive failure, ransomware). It can run on a schedule.
• Data compression: reduces the size of files so they take up less storage space and take less time to transfer. (Lossy and lossless compression are in Storage units, file sizes and compression.)
• Disk defragmentation: reorganises the data on a magnetic hard drive so that the parts of each file are stored contiguously (next to each other).
• Anti-malware: detects, quarantines and removes malware. (How it detects malware is in Cybersecurity.)
• You may be asked to choose the right utility for a task. Match the problem to the tool.

Utility software

A collection of tools that help maintain the functionality of a computer.

File repair software

Repairs or recovers data from damaged or corrupted files.

Data compression software

Reduces file sizes so files use less storage space and take less time to transfer.

Anti-malware

Detects, quarantines and removes malware.

• Full backup: copies every file each time. Simple to restore (one backup), but slow to make and needs lots of storage.
• Incremental backup: after one full backup, each backup copies only the files that have changed since the last backup. Quick and small, but a restore needs the last full backup plus every incremental backup since, in order.
• As a hard drive fills and files are deleted and saved, gaps appear. New files get split into pieces in different gaps: they are fragmented.
• To read a fragmented file, the drive’s read/write head has to move to several places, which takes time.
• Defragmentation moves the pieces so each file is in one continuous run, and gathers free space together. Files then open more quickly.
• Solid state drives have no moving read/write head, so they don’t need defragmenting.

A week of backups. Restoring on Thursday needs Sunday’s full backup and every incremental after it.

Before and after defragmentation: each file’s blocks end up next to each other, with the free space together at the end.

Full backup

A copy of every file, made each time the backup runs.

Incremental backup

A copy of only the files that have changed since the last backup.

Fragmentation

When the parts of a file are stored in separate places on a magnetic disk.

Defragmentation

Reorganising a magnetic hard drive so that the parts of each file are stored contiguously.

Robust software keeps working correctly when it meets unexpected input or conditions, and is free from vulnerabilities that criminals could exploit.

• A vulnerability is a weakness that an attacker could use to get in, steal data or stop the software working.
• Programming bugs: errors in the code, for example a crash when a field is left empty.
• Poor or weak design: security wasn’t thought about at the start.
• Insufficient testing: problems are only found after release.
• Poor programming practice, such as using third-party code libraries without checking them.
• Inadequate validation of input: lets in data the program can’t handle, or that tricks it.
• Weak authentication: short or default passwords, no limit on login attempts.
• Unencrypted sensitive data: if the data is stolen, it can be read straight away.

Robust software

Software that works correctly even with unexpected input or conditions and is free from vulnerabilities.

Vulnerability

A weakness in software that could be exploited by a criminal.

Two methods help developers find vulnerabilities and keep software robust.

• An audit trail is a record of every change made to the code: who made it, when, and what was changed.
• Audit trails give accountability: if a change introduces a vulnerability, the team can see who made it and when, and ask why.
• Because each change is tracked, it is easier to go back to an earlier version that worked.
• A code review is when code is checked for errors and vulnerabilities before it is released. It can be done by a programmer or by an automated tool.
• Programmer review: an experienced person reads the code. They can understand what it is meant to do and spot design flaws. But it takes a long time, needs someone experienced, and people make mistakes or miss things.
• Automated review: software scans the code. It is fast, can check every line of a large program consistently, and can run every time code changes. But it only finds the kinds of problem it has been programmed to look for.

Audit trail

A record of changes made to the code: who made each change, when, and what changed.

Code review

Checking code for errors and vulnerabilities, by a programmer or an automated tool.