Software

A computer system is made of two parts that work together.

• Hardware 硬件 is the physical 物理的 parts of the computer — the parts you can touch. Examples: the CPU, memory, keyboard, screen and hard disk.
• Software 软件 is the programs 程序 that control the computer and tell the hardware what to do.

Hardware cannot do anything useful on its own. Software needs hardware to run on. Both are needed.

Software is split into two types.

System software

System software 系统软件 controls the computer itself and gives a base for other programs to run on. Examples:

• operating system 操作系统 (see below);
• compiler 编译器 and linker 链接器 (tools that turn programs into a form the computer can run);
• device driver 设备驱动程序 — software that lets the operating system control a piece of hardware;
• utility software 实用程序 — small tools that look after the computer (for example anti-virus, backup, disk clean-up).

Below the operating system sits firmware 固件 – software stored permanently on ROM or flash that runs the moment the computer is switched on. The BIOS and the bootloader 引导程序 are firmware: they test the hardware and then load the operating system. So the layers stack up hardware → firmware → operating system → application software, each running on the layer below.

Application software

Application software 应用软件 lets the user do a useful task. Examples:

• word processing 文字处理 software (writing documents);
• spreadsheet 电子表格 software (working with numbers in a grid);
• database management system 数据库管理系统 (storing and searching data);
• web browser 网页浏览器 (viewing web pages).

An operating system (OS) is the main system software. It controls the whole computer and lets the hardware, the application software and the user work together. Without an OS the computer is very hard to use.

The OS has many roles:

• managing files — saving, opening, naming, moving and deleting files;
• handling interrupts (see below);
• providing a user interface 用户界面 so the user can control the computer;
• managing memory — deciding what is kept in RAM and where;
• managing peripherals 外围设备 and their drivers — controlling devices like printers;
• managing multitasking 多任务处理 — letting several programs run at once;
• providing a platform 平台 for running application software;
• managing security — user accounts, passwords and access rights.

An interrupt 中断 is a signal sent to the processor 处理器 to tell it that something needs attention now.

An interrupt can come from three sources:

• hardware — for example a key is pressed, or a printer runs out of paper;
• software — for example a program tries to divide by zero;
• the user — for example the user presses a "cancel" key.

When an interrupt arrives, the processor stops what it is doing, saves its place, and services (deals with) the interrupt. When that is done, it goes back to what it was doing before. This lets the computer react quickly to important events.

All computers process data using the central processing unit 中央处理器 (CPU). To make the CPU do work, people write programs in a programming language 编程语言. There are two kinds.

High-level languages

A high-level language 高级语言 is written in words that look a bit like English (for example Python). It is:

• easy for people to read, write and fix;
• independent of the computer — the same program can run on different types of computer.

Low-level languages

A low-level language 低级语言 is close to what the hardware actually uses. It includes:

• machine code 机器码 — instructions written in binary (0s and 1s), which the CPU runs directly;
• assembly language 汇编语言 — machine code written with short word codes (mnemonics) instead of binary.

A low-level language relates to the specific 特定的 type of CPU, so a program written for one CPU may not run on another. It is used when the programmer needs full control of the hardware or very fast, small code.

The CPU can only run machine code. A high-level program must first be changed into machine code. The software that does this is a translator 翻译程序. There are three types: a compiler and an interpreter (for high-level code), and an assembler (for low-level code).

Compiler

A compiler translates the whole program into machine code before it is run. After translating, the machine code can be run many times without translating again.

• translation happens once, in full;
• it reports all the errors together, at the end of translating;
• the finished program runs fast and does not need the compiler to run.

Interpreter

An interpreter 解释器 translates and runs the program one line at a time.

• it stops at the first error it finds, which helps when testing;
• the program runs more slowly, because it is translated each time it runs;
• the interpreter must be present every time the program runs.

Use a compiler when you want to share a fast, finished program. Use an interpreter when you are writing and testing a program and want to find errors easily.

Assembler

An assembler 汇编器 translates a low-level assembly-language program (written in mnemonics such as LDA, ADD) into machine code. Each mnemonic becomes one machine instruction – a direct one-to-one translation, unlike a compiler or interpreter, which work on high-level code.

Worked example. A team is writing a program. While developing it they want to find and fix errors quickly; when they ship it they want customers to run it fast without seeing the source code. Which translator suits each stage? While developing, use an interpreter: it translates and runs one line at a time and stops at the first error, so a mistake is easy to locate. To ship, use a compiler: it translates the whole program once into machine code, which then runs quickly with no translator present, and the customer receives only the executable rather than the source. The two are not rivals - name the stage and give the reason, because "a compiler is faster" on its own earns nothing.

An integrated development environment 集成开发环境 (IDE) is software that helps you write programs. It puts many useful tools in one place:

• a code editor 代码编辑器 — for typing in your program;
• a run time environment 运行时环境 — for running the program to test it;
• a translator — a compiler or interpreter to turn your code into machine code;
• error diagnostics 错误诊断 — messages that help you find and understand mistakes;
• auto-completion 自动补全 and auto-correction 自动纠错 — the IDE finishes or fixes code as you type;
• prettyprinting 美化打印 — laying out the code neatly with colour and indentation so it is easy to read.

• Hardware is the physical parts; software is the programs. System software runs the computer (OS, drivers, utilities); application software does a task for the user.
• A compiler translates the whole program at once (the finished program runs fast, and all errors are reported at the end); an interpreter translates line by line (it stops at the first error, runs slower, and is needed every time).
• High-level languages are close to English and run on any computer; low-level languages (assembly, machine code) are tied to one type of CPU.
• An interrupt is a signal that makes the CPU stop, save its place, service the event, then carry on where it left off.
• Learn the operating system's roles: managing files, memory, peripherals, multitasking, security and the user interface.