Module 5: Context Switch Module (Scheduler Module)

Yields control from the current process and schedules the next process for execution.

Switch Context¶

Get the pid of the current process from System Status Table;

Push the BP register of the current process to the top of it's kernel stack. 
/* The ExpL application does not push the Base Pointer register (BP). See ExpL calling conventions. 
Hence it is saved to the stop of the Kernel Stack */

Save the SP%512, PTBR and PTLR to the Kernel SP, PTBR and PTLR fields of the 
Process Table entry of the current process;  

if (PAGING_STATUS in the System Status Table is not 0) /* Paging is ongoing */
    If the paging process is blocked     /* the paging process is executing a disk operation */
        Choose Idle Process for scheduling.
    else
        Choose the Swapper Daemon to be scheduled.
else
{
        Find the next non swapped process to schedule using the Round Robin scheduling technique, 
        excluding the Swapper Daemon;
        /* Check the SWAP_FLAG in the process table */
            If no process (that is not swapped out) is in  READY or CREATED state, select the Idle process;
}

Set the PTBR and PTLR registers to the corresponding values in the process table entry
of the new process;

Set the new PID in the System Status Table;

if (the new Process is in CREATED state){       /* The process has just been forked from a parent process */

        Set SP to the value of UserSP field in the Process table entry of the new process;
    Set BP to the value stored at the beginning of the kernel stack.    
    /* BP value of the process is saved to the beginning of the kernel stack by Fork() system call at process creation. */

        Set the state of the new process as (RUNNING, - );

    Set the MODE_FLAG in the process table entry 0.
        Use ireturn statement to transfer control back to user mode;
}

Set the state of the new process as (RUNNING, - );

Read the KPTR field and the UArea Page number from the Process table entry of the
new process;

Set SP to UArea_Page * 512 + KPTR;

Restore the BP register of the new process from the top of it's kernel stack.

return;