任务特性

在RTOS中，一个实时应用可以作为一个独立的任务，支持抢占，支持优先级，每个任务都有自己的堆栈，当任务切换时将上下文环境保存在堆栈中，再次调用任务时，取出上下文信息，继续运行。

任务优先级

每一个任务都可以分配一个从0~(configMAX_PRIORITIES-1)的优先级，configMAX_PRIORITIES在FreeRTOSConfig中有定义

如果所使用的硬件平台支持类似计算前导零这样的指令（Cortex-M处理器支持）。并且configUSE_PORT_OPTIMISED_TASK_SELECTION设置为1，那么configMAX_PRIORITIES不能超过32。

其他情况下configMAX_PRIORITIES可以设置成任意值，但是考虑到RAM的消耗，configMAX_PRIORITIES最好设置为一个满足应用的最小值。
优先级数字越低表示任务的优先级越低，0的优先级最低，configMAX_PRIORITIES-1的优先级最高，空闲任务的优先级最低，为0。当宏configUSE_TIME_SLICING=1时，多个任务可以共用一个优先级，数量不限。此时处于就绪态的优先级相同的任务就会使用时间片轮转调度器获取运行时间。

任务控制块

FreeRTOS每个任务都有一些属性需要存储，FreeRTOS把这些属性集合到一起一起用一个结构体表示，这个结构体叫做任务控制块：TCB_t，创建任务的时候自动给每个任务分配一个任务控制块，此结构体在task.c中定义：如下：

typedef struct tskTaskControlBlock
{volatile StackType_t	*pxTopOfStack;	/*< Points to the location of the last item placed on the tasks stack.  THIS MUST BE THE FIRST MEMBER OF THE TCB STRUCT. */#if ( portUSING_MPU_WRAPPERS == 1 )xMPU_SETTINGS	xMPUSettings;		/*< The MPU settings are defined as part of the port layer.  THIS MUST BE THE SECOND MEMBER OF THE TCB STRUCT. */#endifListItem_t			xStateListItem;	/*< The list that the state list item of a task is reference from denotes the state of that task (Ready, Blocked, Suspended ). */ListItem_t			xEventListItem;		/*< Used to reference a task from an event list. */UBaseType_t			uxPriority;			/*< The priority of the task.  0 is the lowest priority. */StackType_t			*pxStack;			/*< Points to the start of the stack. */char				pcTaskName[ configMAX_TASK_NAME_LEN ];/*< Descriptive name given to the task when created.  Facilitates debugging only. */ /*lint !e971 Unqualified char types are allowed for strings and single characters only. */#if ( portSTACK_GROWTH > 0 )StackType_t		*pxEndOfStack;		/*< Points to the end of the stack on architectures where the stack grows up from low memory. */#endif#if ( portCRITICAL_NESTING_IN_TCB == 1 )UBaseType_t		uxCriticalNesting;	/*< Holds the critical section nesting depth for ports that do not maintain their own count in the port layer. */#endif#if ( configUSE_TRACE_FACILITY == 1 )UBaseType_t		uxTCBNumber;		/*< Stores a number that increments each time a TCB is created.  It allows debuggers to determine when a task has been deleted and then recreated. */UBaseType_t		uxTaskNumber;		/*< Stores a number specifically for use by third party trace code. */#endif#if ( configUSE_MUTEXES == 1 )UBaseType_t		uxBasePriority;		/*< The priority last assigned to the task - used by the priority inheritance mechanism. */UBaseType_t		uxMutexesHeld;#endif#if ( configUSE_APPLICATION_TASK_TAG == 1 )TaskHookFunction_t pxTaskTag;#endif#if( configNUM_THREAD_LOCAL_STORAGE_POINTERS > 0 )void *pvThreadLocalStoragePointers[ configNUM_THREAD_LOCAL_STORAGE_POINTERS ];#endif#if( configGENERATE_RUN_TIME_STATS == 1 )uint32_t		ulRunTimeCounter;	/*< Stores the amount of time the task has spent in the Running state. */#endif#if ( configUSE_NEWLIB_REENTRANT == 1 )/* Allocate a Newlib reent structure that is specific to this task.Note Newlib support has been included by popular demand, but is notused by the FreeRTOS maintainers themselves.  FreeRTOS is notresponsible for resulting newlib operation.  User must be familiar withnewlib and must provide system-wide implementations of the necessarystubs. Be warned that (at the time of writing) the current newlib designimplements a system-wide malloc() that must be provided with locks. */struct	_reent xNewLib_reent;#endif#if( configUSE_TASK_NOTIFICATIONS == 1 )volatile uint32_t ulNotifiedValue;volatile uint8_t ucNotifyState;#endif/* See the comments above the definition oftskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE. */#if( tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE != 0 )uint8_t	ucStaticallyAllocated; 		/*< Set to pdTRUE if the task is a statically allocated to ensure no attempt is made to free the memory. */#endif#if( INCLUDE_xTaskAbortDelay == 1 )uint8_t ucDelayAborted;#endif} tskTCB;/* The old tskTCB name is maintained above then typedefed to the new TCB_t name
below to enable the use of older kernel aware debuggers. */
typedef tskTCB TCB_t;

任务堆栈

FreeRTOS之所以能正确恢复一个任务的运行就是有任务堆栈，创建任务的时候需要给任务指定堆栈，如果使用xTaskCreate创建任务的话，任务堆栈就会由函数xTaskCreate自动创建。如果使用xTaskCreateStatic创建任务，就需要我们自行定义任务堆栈。
任务堆栈的数据类型是StackType_t，StackType_t本质上是uint32_t类型，在portmacro.h中有定义，所以StackType_t是4字节，实际堆栈大小是我们定义的4倍。