stm32 里的0xFFFFFFul
表示常量类型的方法，就是放后面的；定义变量放前面。
“u”表示“unsigned”，无符号
“l”表示“long”，长整型
如果不加UL的话那就是默认的int型，UL后缀.也是一种强制转换方式.
“SysTick_LOAD_RELOAD_Pos”代表“0”
“SysTick_LOAD_RELOAD_Msk”代表“(0xFFFFFFul << SysTick_LOAD_RELOAD_Pos)”
typedef struct
{
__IO uint32_t CTRL; /!< Offset: 0x000 (R/W) SysTick Control and Status Register /
__IO uint32_t LOAD; /!< Offset: 0x004 (R/W) SysTick Reload Value Register /
__IO uint32_t VAL; /!< Offset: 0x008 (R/W) SysTick Current Value Register /
__I uint32_t CALIB; /!< Offset: 0x00C (R/ ) SysTick Calibration Register /
} SysTick_Type; //声明一个SysTick_Type型的结构体。
define SysTick ((SysTick_Type ) SysTick_BASE ) /!< SysTick configuration struct */
define SysTick_BASE (SCS_BASE + 0x0010UL) /*!< SysTick Base
Address */
define SCS_BASE (0xE000E000UL) /!< System Control Space Base Address /
CTRL_TICKINT_Set是一个宏定义，定义如下
/* CTRL TICKINT Mask */
define CTRL_TICKINT_Set ((u32)0x00000002)
define CTRL_TICKINT_Reset ((u32)0xFFFFFFFD)
上面代码中定义一个SysTick_Type类型的结构体实例SysTick，而从根本上来说这是一个地址，
就是STM32芯片内部分配给滴答时钟的实际地址0xE000E000UL+0x0010UL。SysTick->CTRL |= CTRL_TICKINT_Set;
就是给地址0xE000E000+0x0010 +0x000赋一个0x00000002的值，对应滴答时钟的CTRL寄存器的第2位置1
。即为开启中断的意思。 在该文中有详细的说明，个人觉得事实也本应是这样的，可是在我的core_cm3.h头文件中却找不到对SysTick寄存器定义相应的地址
/** \brief Structure type to access the System Timer (SysTick).
*/
typedef struct
{
__IO uint32_t CTRL; /!< Offset: 0x000 (R/W) SysTick Control and Status Register /
__IO uint32_t LOAD; /!< Offset: 0x004 (R/W) SysTick Reload Value Register /
__IO uint32_t VAL; /!< Offset: 0x008 (R/W) SysTick Current Value Register /
__I uint32_t CALIB; /!< Offset: 0x00C (R/ ) SysTick Calibration Register /
} SysTick_Type;
/* SysTick Control / Status Register Definitions */
define SysTick_CTRL_COUNTFLAG_Pos 16 /!< SysTick CTRL: COUNTFLAG Position /
define SysTick_CTRL_COUNTFLAG_Msk (1UL << SysTick_CTRL_COUNTFLAG_Pos) /!< SysTick CTRL: COUNTFLAG Mask /
define SysTick_CTRL_CLKSOURCE_Pos 2 /!< SysTick CTRL: CLKSOURCE Position /
define SysTick_CTRL_CLKSOURCE_Msk (1UL << SysTick_CTRL_CLKSOURCE_Pos) /!< SysTick CTRL: CLKSOURCE Mask /
define SysTick_CTRL_TICKINT_Pos 1 /!< SysTick CTRL: TICKINT Position /
define SysTick_CTRL_TICKINT_Msk (1UL << SysTick_CTRL_TICKINT_Pos) /!< SysTick CTRL: TICKINT Mask /
define SysTick_CTRL_ENABLE_Pos 0 /!< SysTick CTRL: ENABLE Position /
define SysTick_CTRL_ENABLE_Msk (1UL << SysTick_CTRL_ENABLE_Pos) /!< SysTick CTRL: ENABLE Mask /
/* SysTick Reload Register Definitions */
define SysTick_LOAD_RELOAD_Pos 0 /!< SysTick LOAD: RELOAD Position /
define SysTick_LOAD_RELOAD_Msk (0xFFFFFFUL << SysTick_LOAD_RELOAD_Pos) /!< SysTick LOAD: RELOAD Mask /
/* SysTick Current Register Definitions */
define SysTick_VAL_CURRENT_Pos 0 /!< SysTick VAL: CURRENT Position /
define SysTick_VAL_CURRENT_Msk (0xFFFFFFUL << SysTick_VAL_CURRENT_Pos) /!< SysTick VAL: CURRENT Mask /
/* SysTick Calibration Register Definitions */
define SysTick_CALIB_NOREF_Pos 31 /!< SysTick CALIB: NOREF Position /
define SysTick_CALIB_NOREF_Msk (1UL << SysTick_CALIB_NOREF_Pos) /!< SysTick CALIB: NOREF Mask /
define SysTick_CALIB_SKEW_Pos 30 /!< SysTick CALIB: SKEW Position /
define SysTick_CALIB_SKEW_Msk (1UL << SysTick_CALIB_SKEW_Pos) /!< SysTick CALIB: SKEW Mask /
define SysTick_CALIB_TENMS_Pos 0 /!< SysTick CALIB: TENMS Position /
define SysTick_CALIB_TENMS_Msk (0xFFFFFFUL << SysTick_VAL_CURRENT_Pos) /!< SysTick CALIB: TENMS Mask /
/@} end of group CMSIS_SysTick /
上面代码中却怎么也找不到SysTick 寄存器地址的定义，黄天不负有心人，最终还是找到了，
原来在正点原子提供的代码中，把所有的寄存器的结构体定义完之后，统一给各个寄存器定义了地址，
该定义在所有结构体定义的后面。代码如下：
/* Memory mapping of Cortex-M3 Hardware */