text: #include "types.h"
  #include "defs.h"
  #include "param.h"
  #include "memlayout.h"
  #include "mmu.h"
  #include "x86.h"
  #include "proc.h"
  #include "spinlock.h"
  
  struct {
    struct spinlock lock;
    struct proc proc[NPROC];
  } ptable;
  
  static struct proc *initproc;
  
  int nextpid = 1;
  extern void forkret(void);
  extern void trapret(void);
  
  static void wakeup1(void *chan);
  
  void pinit(void) {
    initlock(&ptable.lock, "ptable");
  }
  
  // Must be called with interrupts disabled
  int cpuid() {
    return mycpu()-cpus;
  }
  
  // Must be called with interrupts disabled to avoid the caller being
  // rescheduled between reading lapicid and running through the loop.
  struct cpu* mycpu(void) {
    int apicid, i;
    
    if(readeflags()&FL_IF)
      panic("mycpu called with interrupts enabled\n");
    
    apicid = lapicid();
    // APIC IDs are not guaranteed to be contiguous. Maybe we should have
    // a reverse map, or reserve a register to store &cpus[i].
    for (i = 0; i < ncpu; ++i) {
      if (cpus[i].apicid == apicid)
        return &cpus[i];
    }
    panic("unknown apicid\n");
  }
  
  // Disable interrupts so that we are not rescheduled
  // while reading proc from the cpu structure
  struct proc* myproc(void) {
    struct cpu *c;
    struct proc *p;
    pushcli();
    c = mycpu();
    p = c->proc;
    popcli();
    return p;
  }
  
  //PAGEBREAK: 32
  // Look in the process table for an UNUSED proc.
  // If found, change state to EMBRYO and initialize
  // state required to run in the kernel.
  // Otherwise return 0.
  static struct proc* allocproc(void) {
    struct proc *p;
    char *sp;
  
    acquire(&ptable.lock);
  
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++)
      if(p->state == UNUSED)
        goto found;
  
    release(&ptable.lock);
    return 0;
  
  found: 
    p->state = EMBRYO;
    p->pid = nextpid++;
    p->priority = 10; // Default priority for processes
  
    release(&ptable.lock);
  
    // Allocate kernel stack.
    if((p->kstack = kalloc()) == 0){
      p->state = UNUSED;
      return 0;
    }
    sp = p->kstack + KSTACKSIZE;
  
    // Leave room for trap frame.
    sp -= sizeof *p->tf;
    p->tf = (struct trapframe*)sp;
  
    // Set up new context to start executing at forkret,
    // which returns to trapret.
    sp -= 4;
    *(uint*)sp = (uint)trapret;
  
    sp -= sizeof *p->context;
    p->context = (struct context*)sp;
    memset(p->context, 0, sizeof *p->context);
    p->context->eip = (uint)forkret;
  
    return p;
  }
  
  //PAGEBREAK: 32
  // Set up first user process.
  void userinit(void) {
    struct proc *p;
    extern char _binary_initcode_start[], _binary_initcode_size[];
  
    p = allocproc();
    
    initproc = p;
    if((p->pgdir = setupkvm()) == 0)
      panic("userinit: out of memory?");
    inituvm(p->pgdir, _binary_initcode_start, (int)_binary_initcode_size);
    p->sz = PGSIZE;
    memset(p->tf, 0, sizeof(*p->tf));
    p->tf->cs = (SEG_UCODE << 3) | DPL_USER;
    p->tf->ds = (SEG_UDATA << 3) | DPL_USER;
    p->tf->es = p->tf->ds;
    p->tf->ss = p->tf->ds;
    p->tf->eflags = FL_IF;
    p->tf->esp = PGSIZE;
    p->tf->eip = 0;  // beginning of initcode.S
  
    safestrcpy(p->name, "initcode", sizeof(p->name));
    p->cwd = namei("/");
  
    // this assignment to p->state lets other cores
    // run this process. the acquire forces the above
    // writes to be visible, and the lock is also needed
    // because the assignment might not be atomic.
    acquire(&ptable.lock);
  
    p->state = RUNNABLE;
  
    release(&ptable.lock);
  }
  
  // Grow current process's memory by n bytes.
  // Return 0 on success, -1 on failure.
  int growproc(int n) {
    uint sz;
    struct proc *curproc = myproc();
  
    sz = curproc->sz;
    if(n > 0){
      if((sz = allocuvm(curproc->pgdir, sz, sz + n)) == 0)
        return -1;
    } else if(n < 0){
      if((sz = deallocuvm(curproc->pgdir, sz, sz + n)) == 0)
        return -1;
    }
    curproc->sz = sz;
    switchuvm(curproc);
    return 0;
  }
  
  // Create a new process copying p as the parent.
  // Sets up stack to return as if from system call.
  // Caller must set state of returned proc to RUNNABLE.
  int fork(void) {
    int i, pid;
    struct proc *np;
    struct proc *curproc = myproc();
  
    // Allocate process.
    if((np = allocproc()) == 0){
      return -1;
    }
  
    // Copy process state from proc.
    if((np->pgdir = copyuvm(curproc->pgdir, curproc->sz)) == 0){
      kfree(np->kstack);
      np->kstack = 0;
      np->state = UNUSED;
      return -1;
    }
    np->sz = curproc->sz;
    np->parent = curproc;
    *np->tf = *curproc->tf;
  
    // Clear %eax so that fork returns 0 in the child.
    np->tf->eax = 0;
  
    for(i = 0; i < NOFILE; i++)
      if(curproc->ofile[i])
        np->ofile[i] = filedup(curproc->ofile[i]);
    np->cwd = idup(curproc->cwd);
  
    safestrcpy(np->name, curproc->name, sizeof(curproc->name));
  
    pid = np->pid;
  
    acquire(&ptable.lock);
  
    np->state = RUNNABLE;
  
    release(&ptable.lock);
  
    return pid;
  }
  
  // Exit the current process.  Does not return.
  // An exited process remains in the zombie state
  // until its parent calls wait() to find out it exited.
  void exit(void) {
    struct proc *curproc = myproc();
    struct proc *p;
    int fd;
  
    if(curproc == initproc)
      panic("init exiting");
  
    // Close all open files.
    for(fd = 0; fd < NOFILE; fd++){
      if(curproc->ofile[fd]){
        fileclose(curproc->ofile[fd]);
        curproc->ofile[fd] = 0;
      }
    }
  
    begin_op();
    iput(curproc->cwd);
    end_op();
    curproc->cwd = 0;
  
    acquire(&ptable.lock);
  
    // Parent might be sleeping in wait().
    wakeup1(curproc->parent);
  
    // Pass abandoned children to init.
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
      if(p->parent == curproc){
        p->parent = initproc;
        if(p->state == ZOMBIE)
          wakeup1(initproc);
      }
    }
  
    // Jump into the scheduler, never to return.
    curproc->state = ZOMBIE;
    sched();
    panic("zombie exit");
  }
  
  // Wait for a child process to exit and return its pid.
  // Return -1 if this process has no children.
  int wait(void) {
    struct proc *p;
    int havekids, pid;
    struct proc *curproc = myproc();
    
    acquire(&ptable.lock);
    for(;;){
      // Scan through table looking for exited children.
      havekids = 0;
      for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
        if(p->parent != curproc)
          continue;
        havekids = 1;
        if(p->state == ZOMBIE){
          // Found one.
          pid = p->pid;
          kfree(p->kstack);
          p->kstack = 0;
          freevm(p->pgdir);
          p->pid = 0;
          p->parent = 0;
          p->name[0] = 0;
          p->killed = 0;
          p->state = UNUSED;
          release(&ptable.lock);
          return pid;
        }
      }
  
      // No point waiting if we don't have any children.
      if(!havekids || curproc->killed){
        release(&ptable.lock);
        return -1;
      }
  
      // Wait for children to exit.  (See wakeup1 call in proc_exit.)
      sleep(curproc, &ptable.lock);  //DOC: wait-sleep
    }
  }
  
  //PAGEBREAK: 42
  // Per-CPU process scheduler.
  // Each CPU calls scheduler() after setting itself up.
  // Scheduler never returns.  It loops, doing:
  //  - choose a process to run
  //  - swtch to start running that process
  //  - eventually that process transfers control
  //      via swtch back to the scheduler.
  void scheduler(void) {
    struct proc *p;
    struct proc *p1;
    struct cpu *c = mycpu();
    c->proc = 0;
    
    for(;;){
      // Enable interrupts on this processor.
      sti();
  
      // Points to high priority process
      struct proc *highP = 0;
  
      // Loop over process table to looking for process to run
      acquire(&ptable.lock);
      for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
        if(p->state != RUNNABLE)
          continue;
  
        highP = p;
  
        // Choose process with highest priority
        for(p1 = ptable.proc; p1 < &ptable.proc[NPROC]; p1++) {
  	if(p1->state != RUNNABLE)
  	  continue;
  	if(highP->priority > p1->priority) // Larger value is lower priority
  	  highP = p1;
        }
        // Switch to chosen process.  It is the process's job
        // to release ptable.lock and then reacquire it
        // before jumping back to us.
        p = highP; //Update p to process with highest priority
        c->proc = p;
        switchuvm(p);
        p->state = RUNNING;
  
        swtch(&(c->scheduler), p->context);
        switchkvm();
  
        // Process is done running for now.
        // It should have changed its p->state before coming back.
        c->proc = 0;
      }
      release(&ptable.lock);
    } // for(;;)
  } // scheduler()
  
  // Enter scheduler.  Must hold only ptable.lock
  // and have changed proc->state. Saves and restores
  // intena because intena is a property of this
  // kernel thread, not this CPU. It should
  // be proc->intena and proc->ncli, but that would
  // break in the few places where a lock is held but
  // there's no process.
  void sched(void) {
    int intena;
    struct proc *p = myproc();
  
    if(!holding(&ptable.lock))
      panic("sched ptable.lock");
    if(mycpu()->ncli != 1)
      panic("sched locks");
    if(p->state == RUNNING)
      panic("sched running");
    if(readeflags()&FL_IF)
      panic("sched interruptible");
    intena = mycpu()->intena;
    swtch(&p->context, mycpu()->scheduler);
    mycpu()->intena = intena;
  }
  
  // Function to set priority of a process
  int setPriority(int pid, int priority) {
    struct proc *p;
    
    // Priority value must be 0-31
    if(priority < 0)
      priority = 0;
    if(priority > 31)
      priority = 31;
  
    acquire(&ptable.lock);
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++) {
      // Process with given pid is found
      if(p->pid == pid) {
        // Set priority to given priority
        p->priority = priority;
        break;
      }
    }
    release(&ptable.lock);
    return pid;
  }
  
  
  // Give up the CPU for one scheduling round.
  void yield(void) {
    acquire(&ptable.lock);  //DOC: yieldlock
    myproc()->state = RUNNABLE;
    sched();
    release(&ptable.lock);
  }
  
  // A fork child's very first scheduling by scheduler()
  // will swtch here.  "Return" to user space.
  void forkret(void) {
    static int first = 1;
    // Still holding ptable.lock from scheduler.
    release(&ptable.lock);
  
    if (first) {
      // Some initialization functions must be run in the context
      // of a regular process (e.g., they call sleep), and thus cannot
      // be run from main().
      first = 0;
      iinit(ROOTDEV);
      initlog(ROOTDEV);
    }
  
    // Return to "caller", actually trapret (see allocproc).
  }
  
  // Atomically release lock and sleep on chan.
  // Reacquires lock when awakened.
  void sleep(void *chan, struct spinlock *lk) {
    struct proc *p = myproc();
    
    if(p == 0)
      panic("sleep");
  
    if(lk == 0)
      panic("sleep without lk");
  
    // Must acquire ptable.lock in order to
    // change p->state and then call sched.
    // Once we hold ptable.lock, we can be
    // guaranteed that we won't miss any wakeup
    // (wakeup runs with ptable.lock locked),
    // so it's okay to release lk.
    if(lk != &ptable.lock){  //DOC: sleeplock0
      acquire(&ptable.lock);  //DOC: sleeplock1
      release(lk);
    }
    // Go to sleep.
    p->chan = chan;
    p->state = SLEEPING;
  
    sched();
  
    // Tidy up.
    p->chan = 0;
  
    // Reacquire original lock.
    if(lk != &ptable.lock){  //DOC: sleeplock2
      release(&ptable.lock);
      acquire(lk);
    }
  }
  
  //PAGEBREAK!
  // Wake up all processes sleeping on chan.
  // The ptable lock must be held.
  static void wakeup1(void *chan) {
    struct proc *p;
  
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++)
      if(p->state == SLEEPING && p->chan == chan)
        p->state = RUNNABLE;
  }
  
  // Wake up all processes sleeping on chan.
  void wakeup(void *chan) {
    acquire(&ptable.lock);
    wakeup1(chan);
    release(&ptable.lock);
  }
  
  // Kill the process with the given pid.
  // Process won't exit until it returns
  // to user space (see trap in trap.c).
  int kill(int pid) {
    struct proc *p;
  
    acquire(&ptable.lock);
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
      if(p->pid == pid){
        p->killed = 1;
        // Wake process from sleep if necessary.
        if(p->state == SLEEPING)
          p->state = RUNNABLE;
        release(&ptable.lock);
        return 0;
      }
    }
    release(&ptable.lock);
    return -1;
  }
  
  //PAGEBREAK: 36
  // Print a process listing to console.  For debugging.
  // Runs when user types ^P on console.
  // No lock to avoid wedging a stuck machine further.
  void procdump(void) {
    static char *states[] = {
    [UNUSED]    "unused",
    [EMBRYO]    "embryo",
    [SLEEPING]  "sleep ",
    [RUNNABLE]  "runble",
    [RUNNING]   "run   ",
    [ZOMBIE]    "zombie"
    };
    int i;
    struct proc *p;
    char *state;
    uint pc[10];
  
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
      if(p->state == UNUSED)
        continue;
      if(p->state >= 0 && p->state < NELEM(states) && states[p->state])
        state = states[p->state];
      else
        state = "???";
      cprintf("%d %s %s", p->pid, state, p->name);
      if(p->state == SLEEPING){
        getcallerpcs((uint*)p->context->ebp+2, pc);
        for(i=0; i<10 && pc[i] != 0; i++)
          cprintf(" %p", pc[i]);
      }
      cprintf("\n");
    }
  }
  