[OS] Operating System(2-1): Service, Interface, System calls, Link/Loaders

🍀 운영체제 전공 수업 정리

Operating System Services

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📚 OS Provide an environment for the execution of programs and services to programs and users

• Helpful Services to Users
  1. UI(User Interface)
     • Almost all OSes provide a user interface
     • Types of UI:
     1. Command-Line interface(CLI)
     2. Graphics User interface(GUI)
     3. touch-screen
     4. Batch: 일괄 처리 방식
  2. Program execution
     • OS must be able to load programs into memory, run them, and terminate them
     • termination can be normal or abnormal(indicating error)
  3. I/O operations
     • Running programs may require I/O operations(Ex: File access, I/O device control)
  4. File-system manipulation
     • OS provides file management services for programs
     • Key function:
     • Reading/Writing files and directories
     • Creating and deleting files
     • Searching and listing file information
     • Managing file permissions
  5. Communication
     • Processes exchange information either on the same computer or over a network
     • Communication methods:
     1. Shared Memory: Multiple processes share a common memory space to communicate
     2. Message Passing: The OS moves packets to facilitate communication(ex: email)
  6. Error detection
     • The OS must continuously monitor and detect errors in the system
     • OS 역할:
     • 오류 감지 후 적절한 조치 수행
     • 오류가 발생해도 시스템이 올바르게 유지되도록 보장
     • 디버깅 기능 제공: 사용자 및 개발자의 문제 해결 능력 향상
  7. Resource allocation
     • 여러 사용자 혹은 여러 개의 많은 프로그램이나 장치를 동시에 돌릴 때, 시스템 리소스는 한정적임
     • 그래서 시스템 리소스를 적절하게 할당해야함
     • Key resources managed by the OS:
     • CPU cycle: CPU를 최대한 배분해서 GPU의 활용률을 높여야함
     • main memory
     • file storage
     • I/O device
  8. Logging
     • Tracks which users consume how many resources
     • Helps optimize system performance and security
  9. Protection and security
     • 멀티유저 시스템 및 네트워크 환경에서 정보 소유자가 데이터 사용을 제어할 수 있도록 보장
     • 동시에 실행되는 프로세스들이 서로 간섭하지 않도록 보호
     • protection과 security는 다른 개념
     • protection: Ensures controlled access to system resources
     • security:
       • Protects the system from unauthorized access(requires user authentication)
       • Defends I/O devices from unauthorized access attempts
       • 

         View of Operating_System_Service

User and Operating System-Interface

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CLI(Command Line Interface)

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📚 CLI or Command interpreter: 사용자가 직접 명령을 입력하여 운영체제와 상호작용할 수 있도록 하는 인터페이스

• 커널 내부에서 구현될 수도 있고, 시스템 프로그램으로 구현될 수도 있음.
• CLI는 여러 형태로 구현될 수 있으며, 이를 Shells이라고 함.
• Types of CLI Commands:
  • Built-in Commands(내장 명령어)
    • Commands that are part of the shell itself(Ex: cd, echo, exit)
  • Externel programs(외부 프로그램 호출)
    • The shell executes external programs(Ex: ls, grep, vim)
    • Adding new features does not require modifying the shell

GUI(Graphical User Interface)

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📚 GUI: A desktop metaphor interface, designed for user-friendly interaction

• 보통 마우스, 키보드, 모니터를 사용하여 조작
• GUI Components:
  • Icons: Represent files, programs, and actions
  • Mouse interactions: Provide Options, execute functions, open folders, etc
• Invented at Xerox PARC
• Modern systems offer **both CLI and GUI**
• Example:
  • Microsoft Windows → Primarily GUI, but includes CLI
  • Apple Mac OS X → Uses Aqua GUI, but supports CLI with UNIX shell
  • Unix and Linux → Primarily CLI, but support GUI options like CDE, KDE, and GNOME
• 주요 GUI 환경:
  1. KDE(K Desktop Environment) → Linux 기반의 데스크톱 환경.
  2. GNOME(Gnu Network Object Model Environment) → Linux 및 Unix 시스템에서 사용되는 GUI 환경, 리눅스의 Graphical User Interface는 GNOME
  3. CDE(Common Desktop Environment) → LUnix 시스템에서 사용되는 GUI 환경

Touchscreen Interface

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• Touchscreen devices require new interfaces
  • Mouse not possible or not desired
  • Actions and selection based on gestures
  • Virtual keyboard for text entry

System Calls(⭐)

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📚 System Calls:

• **Programming interface** to access services provided by the OS
• Typically written in high-level languages like C or C++
• Programs do not use direct system calls
• Instead, they access system services through high-level API(Application Programming Interface)
• Common API Types: Win32 API(for Windows), POSIX API(for UNIX, Linux, macOS 등의 POSIX-baseed system), JAVA API(for Java Virtual Machine(JVM))

System call sequence to copy the contents of one file to another file

Example of Standard API read()

System Call Implementation

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• 각 시스템 호출은 특정 숫자와 연관됨
• System-call interface maintains a table indexed by these number
• Role of system call interface
  • System call interface invokes(호출) the intended system call in the OS kernel
  • Returns the status of the system call and any return values
• Caller Does Not Need to Know Implementation
  • The caller need know nothing about how the system call is implemented
  • It only needs to follow the API and understand the expected results
  • Most OS interface details are hidden from the programmer by API(information encapsulation/정보 은닉화)
  • System calls are managed by run-time support library(컴파일러와 함께 제공되는 라이브러리)

open() 시스템 콜을 요청하면 번호에 따라서 table에 mapping을 한다
그리고 그 인덱스에 있는 주소값을 따라 코드를 실행하고 리턴을 해준다

System Call Parameter Passing

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• System Calls often require more information

• Exact type and number of parameters vary depending on OS and system call

• Three Methods for Passing Parameters to the OS
  • (1) Registers
    • register에 parameter를 저장하여 전달, 가장 간단하고 빠름(CPU가 가장 빨리 접근 가능하기 때문)
    • table이나 block 같은 큰 파라미터를 전달하기에는 register 용량의 한계가 있음
  • (2) Address of Block/Table
    • Parameters are stored in a block(or table) in memory
    • The **address** of the block is passed in a register (Call by reference of C/C++)
    • Used by Linux and Solaris
  • (3) Stack
    • Parameters are pushed onto the stack by the program
    • The OS pops them from the stack to retrieve the values
  • Block and stack methods do not limit the number or length of parameters

1번째 라인은 register, 2번째는 x주소의 table을 passing한 라인

Types of System Calls

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Process control

• create/terminate process
• end, abort
• load, execute
• get/set process attributes
• wait for time
• wait event, signal event
• allocate and free memory
• Dump memory error
• Debugger for determining bugs, single step execution
• Locks for managing access to shared data b/w processes
  • 다중 프로세스 환경에서 공유 데이터에 대한 동시 접근을 방지(Lock)
  • 여러 process가 같은 데이터에 read/write를 한다면 데이터가 엉키기때문

File management

• Device management의 추상화(상위 개념)
• Create/delete file
• open/close file
• read, write, reposition
• get and set file attributes

Device management

• request/release device
• read, write, reposition
• get and set device attributes
• logically attach or detach devices

Information maintenance

• get/set time or date
• get/set system data
• get and set process, file, or device attributes

Communications

• create, delete communication connection
• send, receive messages
  • message passing model: to send messages to a host name or process name(Ex: client → server)
  • Shared-memory model: Allows processes to create and access shared memory
• transfer status information
• attach and detach remote(원격) devices

Protection

• Control access to resources
• get and set permissions
• allow and deny user access
• 

printf를 쓰게 되면 I/O가 발생되어 OS service를 받게 됨 printf의 write라는 system call을 사용함 (직접 write를 쓰려면 fd, buf등을 직접 지정해야하기 때문에 번거로움)

System Services(=System Utilities)

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📚OS는 System Programs(=System Utilities)을 통해 다양한 서비스를 제공하여 편리한 실행 환경(convenient environment)을 조성함

• 대부분의 유저들은 OS를 system call이 아닌 system programs로 정의된다.

1. File manipulation
   • Create, delete, copy, rename, print, dump, and list files
2. Status Information
   • Retrieve system info(date, time, memory, disk space, users)
   • Performance monitoring, logging, debugging
   • Uses registry to store and retrieve configuration information(설정 정보)
3. File Modification
   • Provide Text editors to create and modify files
   • Commands to search and transform text content
4. Programming-Language Support
   • Includes compilers, assemblers, debuggers, interpreters
5. Program Loading and Execution
   • Uses loaders, linkage editors, and debugging systems
6. Communications
   • Provides virtual connections among processes, users, and systems
   • Enables messaging, remote login, and file transfers
7. Background Services
   • 시스템 부팅 시 자동으로 실행됨
   • 일부 서비스는 시스템 시작 후 종료되며, 일부는 계속 실행됨
   • Includes disk checking, process scheduling, error logging, and printing
   • Runs in user context, not kernel context(커널 exec → background service exec)
   • Known as services, subsystems, daemons
8. Application Programs
   • User-executed programs that are not part of the OS
   • Launched via command-line, mouse clicks, or other user interactions

Linkers and Loaders (⭐)

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Linkers

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• Source code is compiled into relocatable object files
  • relocatable object file: 절대 주소값이 아니라 상대 주소/심볼링 주소를 사용해서 어느 메모리에나 재배치가 가능한 object file
• Linker combines multiple object files into a single executable file
• Also includes libraries(‘s code) if necessary

Loaders

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• Executable file is stored in secondary storage
• 실행을 위해 반드시 loader가 memory에 적재해야 함

• memory에 적재하기 위해 relocation과정에서 program의 final addresses를 assign하고, 그 address에 맞게 code와 data를 수정함

• Dynamically Linked Libraries(DLLs)
  • 프로그램들은 거의 다 표준 라이브러리를 사용함
  • 프로그램마다 표준 라이브러리를 가져오는 비효율을 없애기 위해 라이브러리를 share하기로 함
  • Modern OSs don’t embed libraries into executables
  • DLLs를 사용하여 **필요할 때만 적재**하고, 여러 프로그램에서 **shared**
• The OS knows how to load and execute programs based on standard formats(표준 실행 파일 형식)

link and loader’s role

✅ Why Applications Are Operating System Specific

• A program compiled on one OS usually does not run on another
• Each OS has unique system calls and file formats 그럼 OS 관계없이 돌아가는 프로그램은 없는가?
• Multi-Operating System Apps
  • interpreted languages(e.g., Python, Ruby)
  • VM-based languages(e.g., Java)
  • standard languages(e.g., C) and compile separately for each OS
• Application Binary Interface(ABI)
  • API처럼 CPU 및 OS 환경에서 binary code가 interact하는 방식을 정의