Chapter 2: Operating System Services (cont.)

Touch Screen Interface:

A touch screen interface is a type of computer display screen that allows the user to interact with the computer by touching the screen directly, instead of using a mouse or keyboard. This technology enables the user to perform actions such as tapping, swiping, and zooming by touching the screen with their fingers or a stylus.

Function Call vs Library Function Call vs System Call

Function Calls:

• A function call is an instruction in a program that invokes a specific function or subroutine to perform a certain task.
• A function call consists of the function name followed by a list of arguments in parentheses.
• The function executes the instructions within its body and returns control to the calling code once it is finished.

System Calls:

• System calls are a type of function call that provide an interface for a program to request services from the operating system.
• System calls allow the program to perform tasks such as accessing files, allocating memory, creating processes, and communicating with other programs.

Library Calls:

• Library calls are a type of function call that provide access to a library of pre-written functions.
• A library is a collection of functions and data that can be used by a program to perform various tasks.
• Library calls allow the programmer to make use of these pre-written functions without having to write the code themselves, making it easier to develop programs quickly and efficiently.
  • Linking
    • The linker is a software tool that combines object files and libraries generated by a compiler into a single executable program.
    • It resolves symbolic references between functions and variables and adjusts memory addresses to create the final program

System Calls

• A request to the operating system to perform some activity
• These can be expensive

  • The system needs to perform many things before executing a system calls

    • The OS code takes control o the CPU, privileges are updated

    privilege mode is called kernel mode

Steps in making a system call (See picture numbers and below)

• 1-3: PUSH parameter in stack

• 4-5: Library Call (puts syscall # in CPU register)

• 6: Switch to kernel mode (return address saved on stack)

• 7: Find system call handler

• 8: Run handler (index via table of pointers of syscall handles)

• 9: Return to user mode

• 10: Return to user program (via trap/interrupt)

• 11: Clean up

• Address space

• Parameter (in reverse order)

• System call # in CPU register

• Switch from user mode to kernel mode and back

• Trap (a interrupt) interrupt ⇒ interrupt vector ⇒ interrupt handler (ISR)

• Use of stack (during system call) and clean up (before completion of system call)

System call :

• OS is trusted, user is not
• OS has super privileges, user does not
• Must take measures to prevent abuse

Function Call:

• Same User
• Same ‘domain of trust’

<aside> 💡 Cache is transparent

</aside>

Five Categories of System calls

• Process Management

• File management:

  • open()
  • close()
  • read()

• Device Management:

  • mkdir():
  • rmdir():
  • link()
  • unlink()
  • mount()
  • unmount()

• Information management

  • stat()
  • times(): Process running time
  • getrusage(): Resource usage
  • clock_gettime(): Get system time

• Communication

  • pipe()
  • shmger(): share memory b/t processes
  • mmap(): maps virtual memory
  • socket(): network socket
  • connect(): connect to a remote server
  • accept(): accept remote connection
  • send(): send network messages

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System Call Parameter Passing: Memory, Address, stack

Passing parameters to the OS when making a system call depends on the type of call. For example, opening a file requires the file name, access mode and type. Getting system time requires the format and zone. These parameters must be passed in a data structure such as an array, list or structure.

Operating System Structures

⇒ There are six types of OS structures

<aside> 💡 C library is portable. because this ensures that code written using the library can be easily ported to different systems with a C compiler, regardless of operating system and hardware.

</aside>

graph TD
  GCc --> O/S1 
  O/S1 --> P1
  GCC --> O/S2
  O/S2 --> P2
  GcC --> O/S3 
  O/S3 --> P3

1. Monolithic Structure/Simple Structure:

• A single large executable containing all the functions of the operating system, including process management, memory management, and file system management. (Main will contain minimum code)
• DOS has no modern software engineering techniques
• Allow all program direct access to hardware
• Does not break the system into sub-systems
• Debugging is very tedious.

2. Layered Structure:

• A hierarchical approach in which the operating system is divided into a number of layers, each layer building on the functionality of the lower layer.
• This allows for modular design and easier maintenance of the operating system.
• Break the OS into smaller layers