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;

    return 0;

found:
    p->state = EMBRYO;
    p->pid = nextpid++;
    p->queue = 1;
    p->runTime = 0;
    q1[++n1] = p;

    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;
}