Question

When a user program needs the operating system to perform an input operation on its behalf such as reading a fixed number of bytes from a file into a string variable, it needs to issue a system call. Explain the sequence of events that take place when a user program issues a system call, until the time the request from the user program is satisfied, under the assumption that the computer has dual mode operation.

Answer 1

Computer programs requests system service to perform certain operations. Here, to read a fixed number of bytes from a file into a string variable (input operation), the computer needs permission to perform input operation . User program needs the help of system calls to communicate with the operating system. Process creation , File access , Protection & security , memory management etc are some of the services provided by system calls .

The computer has dual mode operation , The two modes of operation are :

1. User Mode
2. Kernel Mode

When the user program is running its in user mode . To read the fixed number of bytes from the file it issues a system call, i.e it requests for the service which allows the program to perform input operation. When doing this there will be a transition from user mode to kernel mode.Before switching the mode bit is set to 1 .[Refer the diagram below]

Kernel mode is used to handle interrupts , manage input/output operations. System calls are executed in Kernel mode . After executing the system call i.e after performing the input operations, the mode bit is set to 0 before the transition from Kernel mode to User mode.

Diagram of sequence of events :

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