System Software

Why a computer needs an OS

Hardware on its own can only fetch and run instructions — it knows nothing about files, programs, networks or users. The operating system 操作系统 (OS) is the software layer that:

• manages the hardware (processor 处理器, memory, I/O, storage) for the running programs.
• provides services (file system, network, user accounts) through a clear interface, so programs need not talk to the hardware directly.
• provides a user interface (command line, GUI, touch).
• lets several programs share the hardware safely — each gets fair CPU time and is kept out of the others' memory.

Without an OS, every program would need its own drivers, and only one program could safely run at a time.

A desktop operating system manages the screen, files and programs for the user

A smartphone runs a mobile operating system such as Android

Key management tasks

• process management 进程管理 — load programs, schedule them on the CPU, switch between them, and kill misbehaving ones. (A running program is a process 进程.)
• memory management 内存管理 — give memory to processes, keep them apart, and use virtual memory 虚拟内存 / paging 分页 so the working set can exceed physical RAM 随机存取存储器.
• file management — organise files and folders on secondary storage 辅助存储器, control permissions, prevent write corruption.
• device management (hardware management) — handle I/O and peripherals through device drivers 设备驱动, buffer data, manage interrupts, and give a uniform interface.
• security management — accounts, permissions, firewall, encryption.
• user interface and networking.

The main jobs the operating system manages

Memory protection keeps each application in its own block of memory

Utility software

Most OSes include utility programs 实用程序 to maintain the system:

Utility programs: antivirus, backup, compression and defragmenter

• disk formatter — a file / disk management tool that prepares a disk (sets up its file system); related tools copy, move and delete files.
• defragmentation software (disk defragmenter 碎片整理) — moves the pieces of fragmented files together on a hard disk to cut seek time (not useful on SSDs).
• disk contents analysis / disk repair software — check disk integrity and fix file-system errors and bad sectors.
• back-up software (backup 备份) — copies user data elsewhere so it can be recovered.
• virus checker (antivirus 杀毒软件) — scans for malware and quarantines threats.
• firewall 防火墙 — filters network traffic by rules.
• file compression (compression 压缩) / archiving; system monitor; updates.

Bundling these with the OS saves the user installing each one.

A program library 程序库 is pre-written code (subroutines 子程序, classes, modules) that programs reuse instead of writing it themselves — e.g. a maths library, a network library, a graphics library.

A new program reusing ready-made routines from libraries

Benefits: saves time (off-the-shelf code), reliable (well-tested, widely used), and standardised (consistent behaviour).

• a static library 静态库 is copied into the executable at compile time (stands alone, but larger and needs rebuilding to update).
• a dynamic library 动态库 (DLL, Dynamic Link Library; .so) is loaded at run time (smaller executables, shared by many programs, updated once for all).

Static: the library is copied into the executable. Dynamic: a shared library file is loaded at run time

You write source code; the computer runs machine code 机器码. A translator 翻译器 converts between them.

Assembler

An assembler 汇编器 translates assembly language 汇编语言 into machine code, one instruction per instruction. Used for low-level code (embedded systems, drivers).

Compiler

A compiler 编译器 translates a high-level program into machine code once, before it runs.

• it reports all errors at compile time; once clean, it produces a stand-alone executable 可执行文件 that runs without the compiler installed and can be run many times.
• generally faster at run time (no translation while running), but tied to one CPU/OS — recompile for each platform.

Interpreter

An interpreter 解释器 translates and runs a high-level program one line at a time, producing no executable.

• it reports an error when it reaches that line, then stops; you can fix it and continue — good for development.
• the interpreter must be installed to run the program; generally slower (each run re-translates), but easy to port across platforms.

A compiler translates once into a standalone program; an interpreter translates line by line, every run

Choosing between them

Hybrid: Java

Java is compiled into bytecode 字节码 (a platform-independent intermediate form), which a virtual machine 虚拟机 (the JVM) then interprets — or uses just-in-time compilation 即时编译 to turn hot parts into native code. So errors are caught early, the bytecode runs anywhere with a JVM ("write once, run anywhere"), and long-running programs reach near-native speed. C# and Python use similar designs.

Java compiles to portable bytecode that any JVM runs — write once, run anywhere

Worked example. Java source is compiled to bytecode, which a JVM then interprets. Why use both, instead of compiling straight to machine code? A compiler produces machine code for one processor and operating system, so a program compiled on one machine will not run on another. Java's compiler instead targets a virtual machine, so the bytecode it produces is identical everywhere; each platform then supplies its own JVM to interpret that bytecode into its own native instructions. One compiled file therefore runs anywhere a JVM exists - "write once, run anywhere". The price is speed: interpreting bytecode is slower than running native code, which is why a real JVM also uses JIT compilation to turn frequently-run bytecode into native code while the program runs. Name both sides - the marks are for portability bought at the cost of speed.

An integrated development environment 集成开发环境 (IDE) brings the tools to write, test and debug code into one application:

An IDE bundles the editor, a Run button and a debugger

• source code editor with syntax highlighting 语法高亮 (keywords, strings, comments in different colours), auto-indent and bracket matching.
• auto-complete 自动补全 — suggests names and shows function parameters as you type.
• translator integration — compile/run with one keystroke; errors shown inline.
• debugger 调试器 — set breakpoints 断点 to pause, step through line by line, and inspect variables.
• version control 版本控制 integration (git), project management, a help system, refactoring 重构 tools (safe renaming), and unit test 单元测试 integration.

An IDE speeds development by putting writing → running → debugging → fixing behind one interface. Common IDEs: Visual Studio, PyCharm, Eclipse, VS Code.

A debugger: set a breakpoint, run, then pause to inspect variables and step through the code

• List the OS's jobs (memory, process, file, device and security management) — "manages resources" alone is too vague.
• Compare compiler vs interpreter vs assembler: what each translates and when errors are reported.
• Explain what an IDE provides (editor, debugger, error diagnostics, auto-complete).