resched.c

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#include <xinu.h>

struct defer Defer;

void resched(void)
{
    struct procent *ptold; /* Ptr to table entry for old process    */
    struct procent *ptnew; /* Ptr to table entry for new process    */

    /* If rescheduling is deferred, record attempt and return */

    if (Defer.ndefers > 0)
    {
        Defer.attempt = TRUE;
        return;
    }

    /* Point to process table entry for the current (old) process */

    ptold = &proctab[currpid];

    if (ptold->prstate == PR_CURR)
    { /* Process remains eligible */
        if (ptold->prprio > firstkey(readylist))
        {
            return;
        }

        /* Old process will no longer remain current */

        ptold->prstate = PR_READY;
        insert(currpid, readylist, ptold->prprio);
    }

    /* Force context switch to highest priority ready process */

    currpid = dequeue(readylist);
    ptnew = &proctab[currpid];
    ptnew->prstate = PR_CURR;
    preempt = QUANTUM; /* Reset time slice for process  */
    ctxsw(&ptold->prstkptr, &ptnew->prstkptr);

    /* Old process returns here when resumed */

    return;
}

/*------------------------------------------------------------------------
 *  resched_cntl  -  Control whether rescheduling is deferred or allowed
 *------------------------------------------------------------------------
 */
status resched_cntl(int32 defer)
{
    switch (defer)
    {

    case DEFER_START: /* Handle a deferral request */

        if (Defer.ndefers++ == 0)
        {
            Defer.attempt = FALSE;
        }
        return OK;

    case DEFER_STOP: /* Handle end of deferral */
        if (Defer.ndefers <= 0)
        {
            return SYSERR;
        }
        if ((--Defer.ndefers == 0) && Defer.attempt)
        {
            resched();
        }
        return OK;

    default:
        return SYSERR;
    }
}