5. Software

An operating system is the software layer that:

• manages the system resources — the CPU's time, memory, storage and peripherals;
• provides a platform for running application software — programs ask the OS for resources instead of controlling the hardware directly;
• provides an interface so users can interact with the computer hardware (a graphical user interface with windows and icons, or a command-line interface).

• Each user has an account with a username and password (or biometrics), so the OS authenticates who is using the computer.
• Each account has its own files, settings and permissions / access rights; an administrator account can install software and change settings, standard accounts cannot.
• The OS can lock an account after too many wrong passwords, and log who signed in.
• It provides security features such as a firewall, encryption of drives, and installing security updates.

• A file system is the way the OS organises and keeps track of files on storage: it records where each file's data is stored, its name, size, type, dates and permissions.
• Directories (folders) group files in a hierarchy (a tree of folders inside folders), so files can be found by a path such as Documents/School/notes.txt.
• The OS lets users and programs create, open, save, rename, copy, move and delete files and folders, and controls who may read or change each one.

• Allocating memory to processes: the OS decides which part of RAM each running program (process) may use, and frees it when the process ends.
• Loading processes into RAM: a program must be copied from secondary storage into RAM before the CPU can run it.
• Paging: memory is divided into fixed-size blocks called pages (and RAM into page frames of the same size). A process's pages can be placed in any free frames, not side by side; a page table records where each page is.
• Segmentation: memory is divided into variable-size blocks called segments, matching logical parts of a program (e.g. a function, or its data).
• Virtual memory: when RAM is full, pages not needed now are moved to secondary storage and swapped back when needed (see §2.3).
• Memory protection: each process may only use its own memory, so one program cannot read or overwrite another's data (or the OS's) — this stops crashes spreading and protects data.

A process is a program that is running.

• Scheduling: the CPU can only run one process per core at a time, so the OS decides which process runs next and for how long, switching between them very quickly. This makes several programs appear to run at once (multitasking) and gives important processes priority.
• Inter-process communication: the OS provides safe ways for processes to share data and send messages to each other (e.g. copy and paste between programs), while keeping their memory separate.

• The OS manages peripherals (printers, keyboards, cameras, storage devices): it sends data to and receives data from them, and handles queues such as print jobs.
• A device driver is software that lets the OS communicate with a particular hardware device — it translates the OS's general commands into the instructions that device understands.
• When a new device is connected, the OS loads (or downloads) its driver.

An interrupt is a signal sent to the CPU to say that something needs attention.

How an interrupt is handled:

1. The CPU checks for interrupts at the end of each fetch–decode–execute cycle.
2. If there is one, the CPU looks at its priority. If it is more important than the current task, the current task is paused.
3. The contents of the registers (including the PC) for the current process are saved (on a stack), so it can later resume exactly where it stopped.
4. The correct interrupt service routine (ISR) — the code that deals with that type of interrupt — is found and run.
5. When the ISR finishes, the saved register values are restored and the interrupted process carries on.

• High-level languages (e.g. Python, Java) use English-like words and mathematical notation. One statement often does many machine operations. They are machine independent (portable) and must be translated.
• Low-level languages relate closely to the CPU's instruction set: machine code (binary instructions the CPU runs directly) and assembly language (short mnemonics such as LDD, ADD, STO for machine code instructions, translated by an assembler).
• Syntax is the set of rules for how statements must be written in a language (keywords, punctuation, structure). Breaking them is a syntax error and the code will not be translated.

total = price * quantity     # high-level: one line

The same idea in assembly could take several lines: load price, multiply by quantity, store total.

An IDE is one piece of software with the tools needed to write, translate, test and debug programs.