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; /* 这个全局变量在userinit()函数中赋值 */
  
  int nextpid = 1;
  extern void forkret(void);
  extern void trapret(void);
  
  static void wakeup1(void *chan);
  
  void
  pinit(void)
  {
    /*
      初始化进程全局变量进程锁
    */
    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.
  
  /* 
   * allocproc is to allocate a slot(struct proc) in the process table
   * and to initialize the parts of the process's state required for 
   * its kernel thread to execute.
   * 
   * allocproc is called for each new process 
   *
   * allocproc is written so that it can be used by fork
   * allocproc sets up the new process with a specially prepared kernel stack 
   * and set of kernel registers that cause it to "return" to user space when it first runs
   * 
   * 与linux kernel一样，每个进程有一个相应的内核栈
   */
  static struct proc*
  allocproc(void)
  {
    struct proc *p;
    char *sp;
    /*获取锁*/
    acquire(&ptable.lock);
  
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++)
      if(p->state == UNUSED)
        goto found;
    /*释放锁*/
    release(&ptable.lock);
    return 0;
  
    // ///////////////////
    // 在进程表里找到一个进程
    // ///////////////////
  
  found:
    p->state = EMBRYO; /*EMBRYO:萌芽*/
    p->pid = nextpid++;
    //p->tick_counts = 0;
    //p->alarmticks = -1;
    release(&ptable.lock);
  
    // Allocate kernel stack.
    if((p->kstack = kalloc()) == 0){ /*allocate a kernel stack for the process's kernel thread*/
      p->state = UNUSED;
      return 0;
    }
  
    // 记住sp的结束位置 
    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;
    /*struct context {
      uint edi;
      uint esi;
      uint ebx;
      uint ebp;
      uint eip;
    };*/
    sp -= sizeof *p->context;
    p->context = (struct context*)sp;
    memset(p->context, 0, sizeof *p->context);
  
    /*
     * 由于内核线程开始执行的时候，会将p-context的内容拷贝进寄存器
     * 所以内核线程执行的第一个函数为forkret
     */
    p->context->eip = (uint)forkret;
    /*
    +------------+ <--- old_sp
    | trapframe  | 假设这个进程是从用户态进来的，所以留有trapframe空间
    +------------+ <--- tf/new_sp1, trapframe里的值将会在上层函数userinit中设置好
    |  trapret   |
    +------------+ <--- new_sp2,the address that forkret will return to
    |eip(forkret)|
    +------------+ this setting will cause the kernel thread execute at the start of forkret
    |    ...     |
    +------------+
    |    edi     |
    +------------+ <--- new_sp3(p->context)
    |   empty    |  
    +------------+ <--- p->kstack = kalloc()
  
    */
    return p;
  }
  
  //PAGEBREAK: 32
  // Set up first user process.
  /*
   * userinit() and setupkvm() create an whole address space
   * inituvm() 则负责完成 user part of address space
   */
  void
  userinit(void) /*这个函数只调用一次, 创建的init process是所有进程的父进程*/
  {
    struct proc *p;
    extern char _binary_initcode_start[], _binary_initcode_size[]; /*这是initcode.S的加载位置和大小*/
  
    p = allocproc(); /*这个函数每次创建进程的时候都会调用*/
    
    initproc = p;
  
    // 创建该进程内核空间页表相关的内容
    if((p->pgdir = setupkvm()) == 0) /*create a page table*/
      panic("userinit: out of memory?");
    
    // 创建该进程用户空间页表相关内容
    // 这时已经决定了要载入initcode这个二进制文件了
    // 由于目前xv6不支持文件系统，所以选择将这个二进制文件直接编译进内核，
    // _binary_initcode_start和_binary_initcode_size描述的该二进制文件在内核镜像中的位置
    inituvm(p->pgdir, _binary_initcode_start, (int)_binary_initcode_size);
    
    p->sz = PGSIZE;
    memset(p->tf, 0, sizeof(*p->tf));
  
    /*这里设置的trapframe的内容后面会restore到相应的寄存器中*/
    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; /*this bit set to allow hardware interrupt*/
    
    // 因为在上面的inituvm()函数中, 暂时只分配了一页物理内存, 映射到虚拟内存那也只有一页大小, 
    // 所以暂时有效虚拟地址范围为 0x0～0x1000，即initcode二进制文件存在与这一段空间
    // 这里有点奇怪的是这里process image与用户栈相连？
    p->tf->esp = PGSIZE; 
    p->tf->eip = 0; /*entry point of initcode.S, address 0*/
  
    safestrcpy(p->name, "initcode", sizeof(p->name)); /*p->name mainly for debugging*/
    p->cwd = namei("/"); /*set the process's current working directory*/
  
    // 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; /*RUNNABLE state marks it available for scheduling*/
  
    release(&ptable.lock);
  }
  
  // Grow current process's memory by n bytes.
  // Return 0 on success, -1 on failure.
  /*
   * If n is positive, growproc() allocates one or more physical pages 
   * and maps them at the top of the process's address space.
   * If n is negative, growproc() unmaps one or more pages from the process's
   * address space and free the corresponding physical pages.
   */
  int
  growproc(int n)
  {
    uint sz;
    struct proc *curproc = myproc();
  
    sz = curproc->sz; /*proc->sz is the process's current size*/
    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); /*sets %*/
    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;
    /*让子进程和父进程的trapframe相同，这样子进程回到用户空间后，才会和父进程回到的地方一样*/
    *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 cpu *c = mycpu();
    c->proc = 0;
    
    for(;;){
      // Enable interrupts on this processor.
      sti();
  
      // Loop over process table looking for process to run.
      acquire(&ptable.lock);
      for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
        if(p->state != RUNNABLE) 
          continue;
  
        // ////////////////////////////////
        // 运行刚找到的进程状态为RUNNABLE的进程
        // ////////////////////////////////
  
        // Switch to chosen process.  It is the process's job
        // to release ptable.lock and then reacquire it
        // before jumping back to us.
  
        // 将找到的进程设置为CPU当前执行的进程
        c->proc = p; 
  
        // 这里主要是为了切换到目标进程到页目录
        // 在内核执行的时候切换页表是可以的, 因为在setupkvm()中将所有页表的内核映射都设置相同
        switchuvm(p); /*tell the hardware to start using the target process's page table*/
        p->state = RUNNING;
  
        /* 
         * this perform a context switch to the target process's kernel thread
         * the current context is not a process but rather a special per-cpu scheduler context,
         * so scheduler tells swtch() to save the current hardware registers in 
         * per-cpu storage(cpu->scheduler) rather than in any process's kernel thread context
         */ 
        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);
    }
  }
  
  // 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;
  }
  
  // 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.
  /*
   * the final ret instruction(3078) in swtch(..., ...) 
   * pop the target process's %eip from the kernel stack, finishing the context switch
   * allocproc had previously set initproc's p->context->eip to forkret,
   * so the ret starts executing forkret
   */
  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 with its own kernel stack(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");
    }
  }
  