Task Management and Lab 5

ELET 7404 Embedded & Real Time
Operating Systems
Task Management
and Lab5
Fall 2007

Task Code Rules
• Must be an infinite loop
• for(;;) { }
• while(1) { }
• Must have its OWN STACK space setup in the
beginning of code by the use of OS_STK
• Must call a service at the bottom of the infinite loop

Task Structure
Void YourTask (void *pdata)
{
for (;;) {
/* USER CODE */
Call one of uC/OS-II’s services:
OSFlagPend();
OSMboxPend();
OSQPend();
OSSemPend();
OSTaskSuspend(OS_PRIO_SELF);
OSTimeDly();
OSTimeDlyHMSM();
}
/* USER CODE */
}

Creating a Task
OSTaskCreate ( )
OSTaskCreateExt ( )

Idle Task
Always created
Created by OSInit()
Always the lowest priority, OS_LOWEST_PRIO
The idle task can never be deleted by application
software

OSTaskCreate()
Requires four arguments:
• Task : pointer to the task code
• Pdata: pointer to an argument that is passed to
your task when it starts executing
• Ptos: pointer to the top of the stack that is assigned
to the task
• Prio: is the desired task priority

OSTaskCreate(): Example
OSTaskCreate(TestStart, , (void *)0, &TestStartStk[0], 0);
/* Here the stack grows from low to high memory */
OSTaskCreate(TestStart, , (void *)0,
&TestStartStk[TASK_STK_SIZE
TASK_STK_SIZE-1], 0);
/* Here the stack grows from high to low memory */

OSTaskCreateExt()
• Offers more flexibility but has more overhead
• Requires 9 arguments
• First four, same as OSTaskCreate()
• 5 TH argument: Id – unique identifier
• 6 TH argument: Pbos – pointer to tasks bottom of stack
TH argument: Stk_size – specifies stack size in number
of elements.
• 7 TH
TH argument: Pext – pointer to a user-supplied supplied data
area that can be used to extend the OS_TCB of the task
• 8 TH
TH argument: Opt – specifies whether stack checking is
allowed, whether the stack will be cleared, and whether
floating point operations are performed by the task,
among others
• 9 TH
TCB -Task Control Block

Note about the TCB (Task Control Block)
When a task is created, it is assigned a Task Control Block
(OS_TCB) . This TCB is used to maintain the state of a task
when it is preempted.
The TCB resides in RAM
Note about the opt argument
In order to perform stack checking on a task, the
OS_TASK_OPT_STK_CHK flag in the opt argument must
be set
Also the stack must contain zeros, then use the function
OS_TASK_OPT_STK_CLR

OSTaskCreateExt( ( ): Example
OSTaskCreateExt(TaskStart, , (void *)0, ptos, , TASK_START_PRIO,
TASK_START_ID, pbos, , size, (void *)0,
OS_TASK_OPT_STK_CHK | OS_TASK_OPT_STK_CLR);
Note for lab 5:
OSTaskCreateExt(STKWritTask, , (void *)0, (OS_STK *)&STKWritTaskStk[0],
5, 5, (OS_STK *)&STKWritTaskStk[100-1], 1], 100, (void *)0,
OS_TASK_OPT_STK_CHK | OS_TASK_OPT_STK_CLR);

Notes about Lab 5: check stack space on a task
To check space on a task you should:
• Set OS_TASK_CREATE_EXT to 1 in OS_CFG.H
• Set the opt argument in OSTaskCreateExt( ) to
OS_TASK_OPT_STK_CHK | OS_TASK_OPT_STK_CLR

Notes about Lab 5: check stack space on a task
Stack checking allows to reduce the amount of RAM
needed by the application by not overallocating stack space
OSTaskStkChk( ) provides this information
• To perform stack checking, microC requires that the
stack be filled with zeros when the stack is created
• Also micro C needs to know the location of the bottom
of the stack and the size of the stack you assigned to
the task
• These two values are stored in the task’s OS_TCB
when the task is created but only if the task was created
with OSTaskCreateExt( )

Functions in OS_TASK.C
Tasks
• Change Priority of a Task [ OSPrioChange() ]
• Create a Task [ OSTaskCreate() ]
• Create a Task (Extended Version) [ OSTaskCreateExt() ]
• Delete a Task [ OSTaskDel() ]
• Request that a Task Delete Itself [ OSTaskDelReq() ]
• Resume a Suspended Task [ OSTaskResume () ]
• Stack Checking [ OSTaskStkChk() ]
• Suspend a Task [ OSTaskSuspend() ]
• Query a Task [ OSTaskQuery() ]

Task
• Can be created prior to multitasking or created by another
task
• At least one task must be created before multitasking
begins
• Task Counter
•OSTaskCtr
• Counter is incremented in OSTaskCreate() and
OSTaskCreateExt() and decremented in
OSTaskDel()
- To display the #tasks running:
printf("#Tasks:%2u\n", OSTaskCtr);

Task Stacks
• Each task must have its own stack space
• Stack must be declared as being of type OS_STK
• Stack must consist of contiguous memory locations
• Allocate stack space statically (compile time) or
dynamically (run-time)

Static stack
Static OS_STK MyTaskStack[stack_size];
OR
OS_STK MyTaskStack[stack_size];
• Stack can grow from low to high memory or
vice versa

Configuration File
• You will have to modify a configuration file
to enable or disable features of
MicroC/OS
/OS-II
• OS_CFG.H
• Initially this file is named, os_cfg_r.h while it
isn’t t being used
• Rename os_cfg.r.h to OS_CFG.H to get started

Time Management
• Time Management
• Found in OS_TIME.C
• OSTimeDly()
• Enter a delay in clock ticks
• OSTimeDlyHMSM()
• Enter a delay in hours, minutes, seconds,
milliseconds
• OSTimeDlyResume()
• Resumes a task that has been delayed by
OSTimeDly() or OSTimeDlyHMSM()
• OSTimeGet()
• Gets the current value of the system clock
• OSTimeSet()
• Sets the desired value of the system clock in ticks

Kernel Core
•OS_CORE.c
• Priority Resolution Table
• Initialization [ OSInit() ]
• Interrupt Subroutine enter and exit
• Scheduler (enable and disable)
• Start Multitasking [ OSStart() ]
• System Tick
• Get version [ OSVersion() ]
• Statistics Task
• Create Idle Task
• Idle Task

About LAB 5:
Stack Space with MicroC/OS-II
• Displaying the actual stack space for a new task
that displays its own stack space
• Create a new task with the lowest priority that
displays the used stack space for this new task

void STKWritTask()
{
char x[2];
INT32U stk_size;
OS_STK_DATA stk_data;
for(;;)
{
OSTimeDly(8000);
// period= 8000mS
// LAB5: Using service to check stack space on this task+ stk_data.OSFree = total
OSTaskStkChk(OS_STAT_PRIO, &stk_data); stk_size = stk_data.OSUsed ;
itoa(stk_size,x);
DispClrScr();
DispStr(0,x);
}
}