Linux0.11 系统调用进程创建与执行（九）（上）

struct task_struct {
  // ...
  
    /* 本任务的局部表描述符。 0 空， 1 代码段 cs， 2 数据段和堆栈段 ds&ss */
  struct desc_struct ldt[3];
    /* 本进程的任务状态段信息结构 */
  struct tss_struct tss;
};

#define INIT_TASK \
/* state etc */ { 0,15,15, \
/* signals */ 0,{{},},0, \
/* ec,brk... */ 0,0,0,0,0,0, \
/* pid etc.. */ 0,-1,0,0,0, \
/* uid etc */ 0,0,0,0,0,0, \
/* alarm */ 0,0,0,0,0,0, \
/* math */  0, \
/* fs info */ -1,0022,NULL,NULL,NULL,0, \
/* filp */  {NULL,}, \
  { \
    {0,0}, \
/* ldt */ {0x9f,0xc0fa00}, \
    {0x9f,0xc0f200}, \
  }, \
/*tss*/ {0,PAGE_SIZE+(long)&init_task,0x10,0,0,0,0,(long)&pg_dir,\
   0,0,0,0,0,0,0,0, \
   0,0,0x17,0x17,0x17,0x17,0x17,0x17, \
   _LDT(0),0x80000000, \
    {} \
  }, \
}

void main(void) /* This really IS void, no error here. */
{               /* The startup routine assumes (well, ...) this */
                /*
                 * Interrupts are still disabled. Do necessary setups, then
                 * enable them
                 */

  ROOT_DEV = ORIG_ROOT_DEV;
  drive_info = DRIVE_INFO;
  memory_end = (1 << 20) + (EXT_MEM_K << 10);
  memory_end &= 0xfffff000;
  if (memory_end > 16 * 1024 * 1024)
    memory_end = 16 * 1024 * 1024;
  if (memory_end > 12 * 1024 * 1024)
    buffer_memory_end = 4 * 1024 * 1024;
  else if (memory_end > 6 * 1024 * 1024)
    buffer_memory_end = 2 * 1024 * 1024;
  else
    buffer_memory_end = 1 * 1024 * 1024;
  main_memory_start = buffer_memory_end;
#ifdef RAMDISK
  main_memory_start += rd_init(main_memory_start, RAMDISK * 1024);
#endif
  mem_init(main_memory_start, memory_end);
  trap_init();
  blk_dev_init();
  chr_dev_init();
  tty_init();
  time_init();
  sched_init();
  buffer_init(buffer_memory_end);
  hd_init();
  floppy_init();
  sti();
  move_to_user_mode();
  if (!fork()) { /* we count on this going ok */
    init();
  }
  /*
   *   NOTE!!   For any other task 'pause()' would mean we have to get a
   * signal to awaken, but task0 is the sole exception (see 'schedule()')
   * as task 0 gets activated at every idle moment (when no other tasks
   * can run). For task0 'pause()' just means we go check if some other
   * task can run, and if not we return here.
   */
  for (;;)
    pause();
}

#define move_to_user_mode() \   // 模仿中断硬件压栈，顺序是 ss、esp、eflags、cs、eip
__asm__ ("movl %%esp,%%eax\n\t" \ // 保存堆栈指针 esp 到 eax 寄存器中
  "pushl $0x17\n\t" \       // 堆栈段选择符 ss 入栈， 0x17 即二进制的 10111（特权级3、LDT、数据段）
  "pushl %%eax\n\t" \       // 堆栈指针 esp 入栈
  "pushfl\n\t" \          // 标志进村器 eflags 入栈
  "pushl $0x0f\n\t" \       // Task0 代码段选择符 cs 入栈， 0x0f 即 111 （特权级3、LDT、代码段）
  "pushl $1f\n\t" \       // 将下面标号 1 的偏移地址 eip 入栈
  "iret\n" \            // 出栈恢复现场，翻转特权级从 0 到 3，中断返回，会跳转到下面标号 1处
  "1:\tmovl $0x17,%%eax\n\t" \  // 此时开始执行进程 0 。
  "movw %%ax,%%ds\n\t" \      // 初始化段寄存器指向 task0 的局部表的数据段
  "movw %%ax,%%es\n\t" \      // 下面的代码使 es、fs、gs 与 ds 一致
  "movw %%ax,%%fs\n\t" \
  "movw %%ax,%%gs" \
  :::"ax")

typedef struct desc_struct {
  unsigned long a,b;
} desc_table[256];