Kernel Structure• This chapter describes some of the structural

aspects of μC/OS-II. • How μC/OS-II handles access to critical sections

of code,• What a task is, and how μC/OS-II knows about

your tasks,• How tasks are scheduled,• How μC/OS-II can determine how much of CPU

your application is using,• How do to write Interrupt Service Routines (ISRs),• What a clock tick is and how μC/OS-II handles it,• How to initialize μC/OS-II and,• How to start multitasking.

OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL(),

• μC/OS-II like all real-time kernels need to disable interrupts in order to access critical sections of code, and re-enable interrupts when done.

• This allows μC/OS-II to protect critical code from being entered simultaneously from either multiple tasks or ISRs.

• To hide the implementation method chosen by the compiler manufacturer, μC/OS-II defines two macros to disable and enable interrupts:

• OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL• Because these macros are processor specific, they are

found in a file called OS_CPU.H. • Each processor port will thus have its own OS_CPU.H file.

Tasks

• A task is active entity which could do some computations.

• A task is typically an infinite loop function

• Alternatively, the task can delete itself upon completion

• Also, if the task calls OSTaskDel(), the task code doesn’t return back to anything.

Task States

Task Control Blocks (OS_TCBs)• When a task is created, it is assigned a Task Control Block,

OS_TCB .• A task control block is a data structure that is used by μC/OS-

II to maintain the state of a task when it is preempted. • When the task regains control of the CPU the task control

block allows the task to resume execution exactly where it left off.

• All OS_TCBs reside in RAM.• An OS_TCB is initialized when a task is created

Ready List• Each task is assigned a unique priority level between 0

and OS_LOWEST_PRIO• Task priority OS_LOWEST_PRIO is always assigned to

the idle task when μC/OS-II is initialized.• Each task that is ready to run is placed in a ready list

consisting of two variables, OSRdyGrp and OSRdyTbl[].• Each bit in OSRdyGrp is used to indicate whenever any

task in a group is ready to run. • When a task is ready to run it also sets its

corresponding bit in the ready table, OSRdyTbl[].• To determine which priority (and thus which task) will

run next, the scheduler determines the lowest priority number that has its bit set in OSRdyTbl[].

Ready List

Task Scheduling• μC/OS-II always executes the highest priority task ready to

run. • The determination of which task has the highest priority and

thus, which task will be next to run is determined by the scheduler.

• Task level scheduling is performed by OSSched().

Locking and Unlocking the Scheduler• The OSSchedLock() function is used to prevent task rescheduling until its

counterpart, OSSchedUnlock(), is called. • The task that calls OSSchedLock() keeps control of the CPU even though other

higher priority tasks are ready to run.

Idle Task• μC/OS-II alwa ys creates a task the Idle Task which is executed

when none of the other tasks is ready to run.• The idle task (OSTaskIdle()) is always set to the lowest

priority, i.e. OS_LOWEST_PRIO. • OSTaskIdle() does nothing but increment a 32 -bit counter

called OSIdleCtr. OSIdleCtr is used by the statistics task to determine how much CPU time (in percentage) is actually being consumed by the application software.

Statistics Task

• μC/OS-II contains a task that provides run-time statistics. This task is called OSTaskStat()

• Created if you set the configuration constant OS_TASK_STAT_EN to 1.

• When enabled, OSTaskStat() executes every second and computes the percentage of CPU usage.

• In other words, OSTaskStat() will tell you how much of the CPU time is used by your application, in percentage.

• This value is placed in the variable OSCPUUsage which is a signed 8-bit integer.

μC/OS-II Initialization

Data structures after calling OSInit()

Starting μC/OS-II• You start multitasking by calling OSStart() . Before you start

μC/OS-II, however, you MUST create at least one of your application tasks as shown in listing

Initializing and Starting μC/OS-II.

Starting multitasking.

Task Management

• Creating a Task, OSTaskCreate()• OSTaskCreate (void (*task)(void *pd), void

*pdata, OS_STK *ptos, INT8U prio)

Stack Checking, OSTaskStkChk()• It is sometimes necessary to determine how

much stack space a task actually uses.

Deleting a Task, OSTaskDel()• It is sometimes necessary to delete a task.

Deleting a task means that the task will be returned to the DORMANT state

• And does not mean that the code for the task will be deleted.

• The task code is simply no longer scheduled by μC/OS -II.

• You delete a task by calling OSTaskDel()• Requesting a task to delete itself,

OSTaskDelReq()

Changing a Task’s Priority, OSTaskChangePrio()

• When you create a task, you assign the task a priority. At run-time, you can change the priority of any task by calling OSTaskChangePrio().

• In other words, μC/OS-II allows you to change priorities dynamically.

Suspending a Task, OSTaskSuspend()

• It is sometimes useful to explicitly suspend the execution of a task.

• This is accomplished with the OSTaskSuspend() function call.

• A suspended task can only be resumed by calling the OSTaskResume() function call.

Getting Information about a Task, OSTaskQuery()

• Your application can obtain information about itself or other application tasks by calling OSTaskQuery().

• In fact, OSTaskQuery() obtains a copy of the contents of the desired task’s OS_TCB.

Time Management

• This chapter will describe five services that deal with time issues:

1) OSTimeDly(),2) OSTimeDlyHMSM(),3) OSTimeDlyResume(),4) OSTimeGet() and,5) OSTimeSet().The functions described in this chapter are found in

the file OS_TIME.C.