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UNIX OPERATING SYSTEM

  • UNIX OS is referred as the "kernel" which is special type of the program that runs directly on the hardware and implements the process model and other system services

  • UNIX provides functionality in these four ways

    • Providing system call API for software interrupts
    • Handling the asynchronous interrupts
    • Unusal conditions in the program like divide by zero, overflowing, etc are handled by the kernel by it's intervention
    • A set of special system process such as swapper and pagedeamon perfrom system wide task such as controlling number of active process in memory pool

  • UNIX provides two modes of execution

    • User mode
      • only normal instructions available for executing on CPU
    • Kernel mode
      • normal + priveleged instrutions available for executing on CPU

  • Why need this seperation of modes ?

    • Protection from accidentally or maliciously corrupting another process or the kernel. The damage done by any process can be localized and usually also have saves the user from getting unsual outcomes.

Interrupts

  • Interrupts basically change the normal sequence of execution inside CPU (fetch decode and execute) and transfer the control of CPU (change of PC register) to service the interrupt.

  • Who services the interrupt, its the kernel ! How ? at the time after bootstrapping is done the kernel call its init process which simply copies its own code to all possible locations where interrupts can occur. Basically manipulating the IVT and creating mapping for the appropriate ISR's

  • The two types of interrupts are given below

    • Hardware Interrupts
      • Hardware devices raise the hardware interrupts. Such type of interrupts are asynchronous in nature i.e. can occur any time and any number of times.
      • Why need this ? Computer is made of many hardware devices which are connected to the CPU motherboard and inorder for the devices to communicate they need to interrupt the normal CPU execution.
    • Software Interrupts
      • There are special type of instructions in assembly language which raise software interrupts ?
      • Why need this ? The user application code just executes in normal mode thus it doesn't get chance to execute priveleged instructions. Now user application code needs way to request kernel who with priveleged instrutions can do specify task.

  • Interrupts can also occur when the system is the kernel mode. In this case the system will remain in the kernel mode even after handler completes.

System Calls - Talking with the kernel

  • Any process cannot directly talk with the kernel, so there is any API provided by the kernel in from of the system calls which help the user application code to avail the services provided by the kernel.

  • System call become very significant becuase they are the ones which help to switch from user mode to the kernel mode for doing a paritcular task. Note for this switch obviously the system call will raise a software interrupt.

Kernel Execution Context

  • Now as system calls help in calling the kernel for providing a service, we say kernel execution happens on behalf of the process context. Process telling kernel to do something

  • However some task are not performed by context of process, but the hardware devices rasing the hardware interrupts such situation the kernel is called on behalf of the system context