System Software#

OS Tasks#

Memory management 内存管理

• Memory optimisation 内存优化 - determines how the computer memory is allocated and deallocated.
• Memory organisation - determines how much memory is allocated to an application, including the use of paging and virtual memory systems
• Memory protection - to ensure 2 programs do not try to use same memory space

File management

• Maintain the directory structures
• Provide file naming conventions 提供文件命名规范

Security management

• To ensure the integrity, confidentiality and availability of data
• Use firewall in traffic routing
• Give access right to the users (Prevents unauthorized access, use usernames and passwords)
• Carries out automatic backup

Hardware management

• Communicate with all peripheral devices using device drivers
• Control the access of data between peripherals
• Receives and handles interrupts from hardware devices

Process management

• Enables multiprogramming and multitasking

Utility Softwares#

Disk formatter

• Prepare a hard disk is ready to use (data can be stored on it)
• It will delete any existing data on disk

Virus checker

• Detects and removes harmful virus softwares
• Constantly checks the incoming and outgoing files

Defragmentation software

As an HDD becomes full, blocks used for files will become scattered all over the disk surface.

This slows the data access time, the HDD read-write head requires several movements just to find and retrieve the data making up the required file. (这个问题对SDD影响不大)

It would be advantageous if files could be stored in contiguous sectors.

• rearrange the blocks of data to store files in contiguous sectors wherever possible

Disk contents analysis / repair software

• To check disk drives for empty space and disk usage by reviewing files and file folders.
• Unwanted files and downloads can be removed

File compression - reduces the file size, use the disk space efficiently.

Back-up software

• Makes copies of files on another storage medium

• It should be a regular process

• The ability to restore data

Program Libraries#

A library on a computer where programs and routines are stored which can be freely accessed by other software developers for use in their own programs.

Software under development is often constructed using existing code from program libraries, which …

• reduces the development time
• there is no need to rewrite the existing code in program libraries
• Leads to modular programming

Dynamic link library is the library routine that can be linked to another program only at the run time stage.

The program loads .dll files into the memory when required.

Benefits:

• The executable file is smaller.
• It can save memory and execution time.
• DLL files can change independently.

Example:

1
.
2
├── bink2w32.dll  # Program Library
3
├── Game.cdx
4
├── Game.exe
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├── Launcher.cdx
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├── Launcher.exe  # The executable program itself
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├── steam_api.cdx
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├── steam_api.dlll  # Program Library
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├── steam_emu.ini
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└── USRDIR

Assembler#

The mnemonics used translates into machine opcodes. (把LDD之类的指令转换成0和1组成的machine code)

One-pass Assembler - processes the source code in a single scan

• It defines symbols and literals (各种变量), storing them in the symbol and literal tables

• 在读取assembly code的同时直接生成machine code

• 优点： 速度快，内存占用极小（不需要存储整个中间代码）。

• 缺点： 代码结构复杂，且无法处理复杂的向前引用（比如向前引用一个变量长度未知的数据段），且生成的代码通常需要多次“打补丁”，效率较低。

Two-pass Assembler - reads the source code twice to generate object code (executable program)

• In first pass: scans the code to define symbols, assign memory address and gather information about literals and variables. 主要获取和整理信息

• In second pass: converts symbolic op-codes into their corresponding numeric op-codes

  • resolves symbol values
  • generates data for literals

  And then produces ther final object code

• 优点： 能完美处理所有前向引用，生成的代码更优、更干净，无需运行时打补丁。

• 缺点： 需要把整个源代码或中间表示（Intermediate Code）保存在内存中，且读取磁盘两次，速度较慢，内存占用较大。

假设汇编代码：

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100: JMP NEXT   ; 跳转到后面的标签
2
101: NOP
3
102: NEXT: ADD #1

• One Pass：读到第100行时，不知道 NEXT 是102。于是生成机器码 JMP 000，记录“第100行需要修改”。读到102行时，返回去把 000 改成 102。如果NEXT后面定义了变量占10个字节，单遍汇编器很难计算偏移量，容易出错。
• Two Pass：
  • Pass 1：扫描全部，记录 NEXT = 102。
  • Pass 2：再次读第100行，直接查表得到102，生成 JMP 102。一气呵成，无需修改。

- DeepSeek

Compiler - Two-Step translation#

Aim: To achieve shorter execution time

Java and some other high level language programs may require two-step translation, i.e, they will be partially compiled and partially interpreted

To execute the program, the object code can be interpreted by an interpreter or compiled using a compiler.

Java code first translated to bytecode by Java compiler (Bytecode is the intermediate code in Java)

Bytecode finally interpreted by the Java Virtual Machine to produce machine code

$$\text{Java} \xrightarrow{\text{Java Compiler}} \text{Bytecode} \xrightarrow{\text{Java VM}} \text{Machine Code}$$

视频推荐：从C到01：速通传统编译流程_哔哩哔哩_bilibili

IDE Features#

For coding : context-sensitive prompts.

For error-detection : dynamic syntax checks.

For presentation : prettyprint, expand and collapse code blocks.

For debugging : single stepping, break points, report window.