Page 73 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes clock interrupt occurs, the interrupt handler checks how much time the current running process
has used. If it has used up its entire time slice, then the CPU scheduling algorithm (in kernel)
picks a different process to run. Each switch of the CPU from one process to another is called
a context switch.
3.2.1 Major Steps of Context Switching

• The values of the CPU registers are saved in the process table of the process that was
running just before the clock interrupt occurred.

• The registers are loaded from the process picked by the CPU scheduler to run next.
In a multiprogrammed uniprocessor computing system, context switches occur frequently
enough that all processes appear to be running concurrently. If a process has more than one
thread, the Operating System can use the context switching technique to schedule the threads
so they appear to execute in parallel. This is the case if threads are implemented at the kernel
level. Threads can also be implemented entirely at the user level in run-time libraries. Since in
this case no thread scheduling is provided by the Operating System, it is the responsibility of
the programmer to yield the CPU frequently enough in each thread so all threads in the process
can make progress.
3.2.2 Action of Kernel to Context Switch among Threads

The threads share a lot of resources with other peer threads belonging to the same process so
a context switch among threads for the same process is easy. It involves switch of register set,
the program counter and the stack. It is relatively easy for the kernel to accomplish this task.

3.2.3 Action of Kernel to Context Switch among Processes
Context switches among processes are expensive. Before a process can be switched its process
control block (PCB) must be saved by the operating system. The PCB consists of the following
information:

• The process state.
• The program counter, PC.
• The values of the different registers.

• The CPU scheduling information for the process.
• Memory management information regarding the process.

• Possible accounting information for this process.
• I/O status information of the process.

When the PCB of the currently executing process is saved the operating system loads the PCB
of the next process that has to be run on CPU. This is a heavy task and it takes a lot of time.

Before a process can be switched its process control block (PCB) must be
saved by the operating system.

3.3 Multi-threading in IDL

ITT-VIS has added support for using threads internally in IDL to accelerate specific numerical
computations on multi-processor systems. Multi-processor capable hardware has finally become

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