/*
* This routine is called to reschedule the CPU.
* if the calling process is not in RUN state,
* arrangements for it to restart must have
* been made elsewhere, usually by calling via sleep.
*/
swtch()
{
static struct proc *p;
register i, n;
register struct proc *rp;
if(p == NULL)
p = &proc[0];
/*
* Remember stack of caller
*/
savu(u.u_rsav);
/*
* Switch to scheduler's stack
*/
retu(proc[0].p_addr);
loop:
runrun = 0;
rp = p;
p = NULL;
n = 128;
/*
* Search for highest-priority runnable process
*/
i = NPROC;
do {
rp++;
if(rp >= &proc[NPROC])
rp = &proc[0];
if(rp->p_stat==SRUN && (rp->p_flag&SLOAD)!=0) {
if(rp->p_pri < n) {
p = rp;
n = rp->p_pri;
}
}
} while(--i);
/*
* If no process is runnable, idle.
*/
if(p == NULL) {
p = rp;
idle();
goto loop;
}
rp = p;
curpri = n;
/* Switch to stack of the new process and set up
* his segmentation registers.
*/
retu(rp->p_addr);
sureg();
/*
* If the new process paused because it was
* swapped out, set the stack level to the last call
* to savu(u_ssav). This means that the return
* which is executed immediately after the call to aretu
* actually returns from the last routine which did
* the savu.
*
* You are not expected to understand this.
*/
if(rp->p_flag&SSWAP) {
rp->p_flag =& ~SSWAP;
aretu(u.u_ssav);
}
/* The value returned here has many subtle implications.
* See the newproc comments.
*/
return(1);
}
/*
* This routine is called to reschedule the CPU.
* if the calling process is not in RUN state,
* arrangements for it to restart must have
* been made elsewhere, usually by calling via sleep.
*/
swtch()
{
// static variable p points to the last process that swtch selected.
static struct proc *p;
register i, n;
register struct proc *rp;
// if this is the first time switch is called, then p points to the system process.
if(p == NULL)
p = &proc[0];
/*
* Remember stack of caller
*/
savu(u.u_rsav);
/*
* Switch to scheduler's stack
*/
// proc[0] is system process that is used for scheduling. It is initialized
// when system boots.
retu(proc[0].p_addr);
loop:
// runrun is set when there is a process that has the higher priority
// than currently running process.
runrun = 0;
// rp points to the current process so that the swtch selects a process
// that appears to after current process if there are more than two processes
// that have the highest priority.
rp = p;
// after finishing the loop below, p will point the highest-priority
// runnable process.
p = NULL;
// 128 is the lowest priority. All processes in the system can have
// priority smaller than 128.
n = 128;
/*
* Search for highest-priority runnable process
*/
i = NPROC;
do {
rp++;
// for circular search
if(rp >= &proc[NPROC])
rp = &proc[0];
// finds runnable and in-memory process
if(rp->p_stat==SRUN && (rp->p_flag&SLOAD)!=0) {
// compares it with the candidate. if new one wins updates candidate.
if(rp->p_pri < n) {
p = rp;
n = rp->p_pri;
}
}
} while(--i);
/*
* If no process is runnable, idle.
*/
if(p == NULL) {
p = rp;
idle();
goto loop;
}
rp = p;
// curpri is global variable that represents the priority of running process.
curpri = n;
/*
* Switch to stack of the new process and set up
* his segmentation registers.
*/
retu(rp->p_addr);
sureg();
/*
* If the new process paused because it was
* swapped out, set the stack level to the last call
* to savu(u_ssav). This means that the return
* which is executed immediately after the call to aretu
* actually returns from the last routine which did
* the savu.
*
* You are not expected to understand this.
*/
// TODO: what does it mean?
// when the swapped-out data comes back into the memory again?
if(rp->p_flag&SSWAP) {
rp->p_flag =& ~SSWAP;
aretu(u.u_ssav);
}
/*
* The value returned here has many subtle implications.
* See the newproc comments.
*/
return(1);
}
