オペレーティングシステムとユーティリティソフトウェア

1202 alarm

• Three minutes before the first Moon landing, the Apollo 11 guidance computer flashed a code the crew had never seen in training: 1202. A misconfigured radar was flooding it with work it could not finish.
• The computer's tiny operating system 操作系统 did what an operating system is for. It ranked every job by priority, dropped the ones that did not matter, and kept the descent calculations running. The landing continued.
• Without that scheduler, the computer would have frozen and the landing been aborted. Every phone and laptop today makes the same decisions thousands of times a second.
• This lesson is why a computer needs an OS, the five management tasks the syllabus names, and the utility programs bundled with it.

Why a computer needs an operating system

• Hardware alone can only fetch and execute instructions. It knows nothing about files, programs, networks or users.
• The OS provides an interface between the user and the hardware, and hides the complexity of the hardware from the user and from application programs.
• It manages the hardware resources, processor time, memory, storage and input/output, and shares them between programs; it loads application software into memory and runs it; and it provides security and utilities.
• Without one, every program would need its own device drivers and only one program could safely run at a time.

One layer between every program and the hardware

Memory management

• Memory management 内存管理 allocates memory to each program when it is loaded and frees it when the program ends.
• It keeps every program's memory separate, memory protection 内存保护, so one program cannot overwrite another's data.
• It swaps pages between RAM and disk, virtual memory 虚拟内存 through paging 分页, so more programs can be open than physical memory allows.

Each process gets its own fenced block

Process management

• A running program is a process 进程. Process management 进程管理 creates and ends processes.
• It decides which process gets the CPU next, scheduling 调度, and for how long, a time slice 时间片, then saves the process's state and switches to the next.
• It resolves conflicts when two processes want the same resource, and can end one that stops responding.

Worked example: how the OS makes multitasking possible

Hardware management and interrupts

• The OS talks to each device through its device driver 设备驱动, queues and buffers data going to slow devices such as a printer, and shares one device between several programs.
• A hardware interrupt 硬件中断 comes from a device: a key pressed, a printer out of paper, a disk transfer finished, a power failure. A software interrupt 软件中断 comes from a program: division by zero, an invalid instruction, a request for an OS service.
• The interrupt handler 中断处理程序 saves the state of the current process, deals with the interrupt, then restores the process and continues.

File management and security management

• File management: creates, names, copies, moves and deletes files and folders; keeps the directory 目录 structure and a record of where each file sits on the disk; allocates disk space; enforces access rights 访问权限 for each user.
• Security management: user accounts and passwords, access rights, encryption of stored data, a firewall, automatic security updates, and a log of who did what.
• A "describe" question wants what the OS actually does under each heading, one action per mark.

Utility software

• Utility programs 实用程序 are system software that maintain, repair or optimise the computer rather than doing a user's task.
• Disk formatter: prepares a disk for use by setting up its file system, deleting any existing data. Virus checker: compares files against a database of known signatures 签名, watches behaviour, quarantines what it finds; must be updated.
• Defragmentation software: gathers the scattered blocks of each file together. Disk contents analysis and repair: shows what uses the space; finds and fixes bad sectors and file-system errors.
• File compression: shrinks files for storage and transfer. Back-up software: copies files to another medium on a schedule; a full copy, then incremental backups 增量备份 of what changed.

Each one does a single maintenance job

Worked example: how defragmentation improves performance

Worked example: one line from each utility to its purpose

Marks that slip away

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