RT-Thread 读后感5 ——初始化线程栈

线程第一次运行的时候，加载到CPU寄存器的参数就放在线程栈里面，改函数 rt_hw_stack_init（）在cpuport.c中实现。具体代

码如下：

/*  初始化线程栈  */
rt_uint8_t *rt_hw_stack_init(void      *tentry,
	                           void      *parameter,
														 rt_uint8_t   *stack_addr)
{
      struct stack_frame *stack_frame;
	    rt_uint8_t  *stk;
	    unsigned long  i;
	
	    /* 获取栈顶指针 rt_hw_stack_init 在调用时，传给stack_addr的是（栈顶指针-4）*/
	    stk = stack_addr + sizeof(rt_uint32_t);
	    
	    /*让stk指针向下8字节对齐*/
	    stk = (rt_uint8_t *)RT_ALIGN_DOWN((rt_uint32_t)stk,8);
	
	    /*stk指针继续向下移动sizeof(struct stack_frame)个偏移*/
	    stk -=sizeof(struct stack_frame);
	    
	    /* 将stk指针强制转换为 stack_frame类型后存到stack_frame*/
	    stack_frame =(struct stack_frame *)stk;
	
	    /*以stack_frame 为起始地址，将栈空间里面的sizeof(struct stack_frame)个内存初始化为0xdeadbeef */
	    for(i=0; i<sizeof(struct stack_frame)/sizeof(rt_uint32_t); i++)
	    {
		    ((rt_uint32_t *)stack_frame)[i] = 0xdeadbeef;
		  }
	    
			/*初始化异常发生时自动保存的寄存器*/
			stack_frame->exception_stack_frame.r0 =(unsigned long)parameter;
			stack_frame->exception_stack_frame.r1  = 0;                        /* r1 */
      stack_frame->exception_stack_frame.r2  = 0;                        /* r2 */
      stack_frame->exception_stack_frame.r3  = 0;                        /* r3 */
      stack_frame->exception_stack_frame.r12 = 0;                        /* r12 */
      stack_frame->exception_stack_frame.lr  = 0;                        /* lr */
      stack_frame->exception_stack_frame.pc  = (unsigned long)tentry;    /* entry point, pc */
      stack_frame->exception_stack_frame.psr = 0x01000000L;              /* PSR */
      
			/* 返回线程指针 */
			return stk;
}

代码 stk = (rt_uint8_t *)RT_ALIGN_DOWN((rt_uint32_t)stk,8);   让stk这个指针向下8个字节对齐，确保stk是8字节对齐的地址。

代码 stk -=sizeof(struct stack_frame);     stk 指针继续向下移动sizeof（struct stack_frame）个偏移，即16字的大小。

代码 stack_frame =(struct stack_frame *)stk;   将stk指针强制转换为stack-frame类型后存到指针变量stack_frame中。

改函数最终会返回线程栈顶指针stk， 

在线程栈初始化函数中，定义了struct  stack_frame 类型的结构体指针stack_frame，改结构体类型具体如下：

struct exception_stack_frame
{
   /*异常发生时自动加载CPU寄存器的内容*/
	 rt_uint32_t  r0;
	 rt_uint32_t  r1;
	 rt_uint32_t  r2;
	 rt_uint32_t  r3;
	 rt_uint32_t  r12;
	 rt_uint32_t  lr;
   rt_uint32_t  pc;
	 rt_uint32_t  psr;
};


struct stack_frame
{
   /*异常发生时需要手动加载CPU寄存器的内容*/
	 rt_uint32_t   r4;
	 rt_uint32_t   r5;
	 rt_uint32_t   r6;
	 rt_uint32_t   r7;
	 rt_uint32_t   r8;
	 rt_uint32_t   r9;
   rt_uint32_t   r10;
	 rt_uint32_t   r11;
	 struct exception_stack_frame   exception_stack_frame;
};