Lesson 2 (System Calls)

• What are the Operating System Services which are helpful to User:

  • User interface
  • Program execution
  • I/O operations
  • File-system manipulation
  • Communications
  • Error detection
• What are the Operating Systems that ensure efficient operation of system via resource sharing?

  • Resource allocation
  • Accounting
  • Protection and security
• What are system calls?

  • A programming interface to the services provided by the OS
  • Typically written in high-level language (C/C++)
  • Mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use
• What is Application Program Interface (API)?

  API specifies a set of functions that are available to an application programmer, including the parameters that are passed and expected return values
• Why user APIs rather than system calls?

  • It provides an abstraction of the function call behaviour
  • provides interoperability amongst different OS/HW versions (libraries can work on diff architectures)
• System Call Implementation (1)

  • A number is associated with each system call
  • System-call interface maintains a table indexed according to these numbers
  • The system call interface invokes intended system call in OS kernel and returns status of the system call and any return values
• System Call Implementation (2)

  • If >1 application is accessing the API, both application can see the same memory region
  • API is storing the memory, but when in execution and not loaded, it is stored in HD/SSD
  • The caller need know nothing about how the system call is implemented

  
• Three methods to pass parameters to Operating System:

  1. Pass parameters in registers
  2. Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register
  3. Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system
• Characteristic and problem with passing parameters in register

  In some cases, may be more parameters than registers

  
• Parameters stored in a block in memory, and address of block is passed as a parameter in a register:

  • approach taken by Linux and Solaris

  PROs: + Fast and Flexible

  CONs: - slower to access diff parameters in memory compared to accessing parametes in registers

  
• Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system

  • Combines the first 2 methods together (memory + registers)

  pushes paraemters in stack + most function uses short parameters and is put in registers

  
• Loadable Kernel Modules

  • implemented by most modern OS
  • Uses object-oriented approach
  • Each core component is separate
  • Each talks to the others over known interfaces
  • Each is loadable as needed within the kernel

  
• Monolithic Operating Systems

  Consists of 2 seperable parts:

  1. Systems programs
  2. The kernel

  • Consists of everything below the system call interface and above the physical hardware
  • Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level

  
• Mikrokernel System Structure

  • Moves as much from the kernel into user space (The kernel contains strictly the necessary services that the kernel needs to provide.)
  • Communication takes place between user modules using message passing

  
• Pros and Cons of Mikrokernel System

  • Benefits:
  • Easier to extend a microkernel
  • Easier to port the operating system to new architectures
  • More reliable (less code is running in kernel mode)
  • More secure
  • Detriments:
  • Performance ovehead of user space to kernel space communication
• Hybrid Kernels


  Combine a micro-kernel with extended functionality to enhance performance

  
• Virtualisation


  Allows operating systems to run applications within other.

  Emulation used when source CPU type different from target type (i.e. ARM to Intel x86)

  • Generally slowest method but flexible
  • Translation of instruction/architecture if it a program
  • logically builds a CPU within the CPU

  Virtualization – OS natively compiled for CPU, running guest OSes also natively compiled