1. Chapter 13: Porting μC/OS-II

2. Outline Requirements Hardware Software Tasks of Porting µC/OS-II
OS_CPU_C.H
OS_CPU_C.C
OS_CPU_A.ASM
Testing a port

3. μC/OS-II hardware/software architecture
Application Code (test.c)
Processor independent implementations
Scheduling policy
Event flags
Semaphores
Mailboxes
Event queues
Task management
Time management
Memory management
Application Specific Configurations
OS_CFG.H
Max # of tasks
Max Queue length …
μC/OS-II port for processor specific codes
Software
Hardware
CPU
Timer

4. Requirements C compiler Interrupt support and a timer
Interrupt can be disabled and enabled by C
Support hardware stack
The processor can load and store the stack pointer and other registers in memory (stack pointer is stored in TCB)

5. Porting Tasks of µC/OS-II
Setting the value of 2 #define constants (OS_CPU.H)
OS_CRITICAL_METHOD
OS_STK_GROWTH
Declaring 11 data types (OS_CPU.H)
OS_STK
Declaring 1~3 #define macros (OS_CPU.H)
Critical section
Writing 10 simple functions in C (OS_CPU_C.C)
Hooks
OSTaskStkInit
Writing 4 assembly language functions (OS_CPU_A.ASM)
Context switch (starting, ctx by ISR, ctx by AP)
TickISR procedures

6. Development Tools
A C compiler that generates reentrant code (each function has its stack space)
A linker which is used to combine object files
Resolve references within these modules
A locator which allow you to place the code and data anywhere in the memory map of the target processor

7. OS_CPU.H Compiler-Specific Data Type
BOOLEAN, INT8U, INT8S…
The data type of a task’s stack and the status register
typedef unsigned int OS_STK
typedef unsigned short OS_CPU_SR

8. OS_CPU.H Critical Method Type 1: enable and disable directly
Type 2: save the interrupt status onto the stack
Type 3: save the interrupt status into a local variables

9. OS_CPU.H #if OS_CRITICAL_METHOD == 1
#define OS_ENTER_CRITICAL() asm CLI
#define OS_EXIT_CRITICAL() asm STI
#endif
#if OS_CRITICAL_METHOD == 2
#define OS_ENTER_CRITICAL() asm {PUSHF; CLI}
#define OS_EXIT_CRITICAL() asm POPF
#if OS_CRITICAL_METHOD == 3
#define OS_ENTER_CRITICAL() (cpu_sr = OSCPUSaveSR())
#define OS_EXIT_CRITICAL() (OSCPURestoreSR(cpu_sr))
X86 port

10. OS_CPU.H OS_TASK_SW() OS_TASK_SW() is called from user program.
a macro that is invoked when µC/OS-II switches from a low-priority task to the highest-priority task.
OS_TASK_SW() is called from user program.
OSIntExit() is called from ISR
In this procedure (OS_Task_SW())…
Throw a software trap
The interrupt handler should vector to the assembly language function OSCtxSw()
X86 port
#define OS_TASK_SW() asm INT uCOS

11. OS_CPU.H
typedef unsigned short OS_CPU_SR /*define sie of CPU status register*/

12. OS_CPU_C.C OSTaskStkInit() OSTaskCreateHook() OSTaskDelHook()
OSTaskSwHook()
OSTaskIdleHook()
OSTaskStatHook()
OSTimeTickHook()
OSInitHookBegin()

13. OSTaskStkInit()
This function is called by OSTaskCreate() and OSTAskCreateExt() to initialize the stack frame of a task as an interrupt has just occurred and all the processor registers have been pushed onto that stack.

14. tcb = getHPT() asm {sp = tcb->sp} asm {popa} asm {iret} main()
fun1()
os_start()
main1()
main2()
main3()
code
(Task1)
code
(Task2)
code
(Task3)
Stack
(main)
xxx
ooo
zzz
kkk
ggg
fff
qqqqq
Regs
Regs
Regs
PSW PC
PSW PC
PSW PC
Stack
(Task1)
Ret. adrs
pdata
Stack
(Task3)
Ret. adrs
pdata
Stack
(Task3)
Ret. adrs
pdata
startHighRdy
tcb = getHPT()
asm {sp = tcb->sp}
asm {popa}
asm {iret} sp pc

15. tcb = getHPT() asm {sp = tcb->sp} asm {popa} asm {iret} main1() xxx
code
(Task1)
code
(Task2)
code
(Task3)
Regs
Regs
local
variables
PSW PC
PSW PC
Ret. adrs
pdata
Stack
(Task3)
Ret. adrs
pdata
Stack
(Task3)
Ret. adrs
pdata
tcb = getHPT()
asm {sp = tcb->sp}
asm {popa}
asm {iret}
startHighRdy
Regs sp pc
PSW PC

16. Context switch TCB (task2) TCB (task1) Stack (task1) Stack (task2)
main2() {
xxx
yyy
zzz }
myFunction() {
OSMBoxPost() }
OSTCBStkPtr
OSTCBStkPtr
Regs
(1) Mode Change
(function call)
PSW PC
Regs
PSW PC
OSMboxPost() {
//...
OS_Sched()
OSPrioHighRdy= //Get pointer to HPT ready to run
INT 0x80 //OS_TASK_SW }
Ret. adrs
pdata
Ret. adrs
pdata
(2) Exception
(software interrupt)
(3)
PUSH PSW
PUSH PC+4
(4)
POP PSW
POP PC+4
//(4) ISR (interrupt service routine)
PUSHA
OSTCBCur->OSTCBStkPtr = SP
SP = OSTCBHighRdy->OSTCBStkPtr
POPA
IRET

17. Pseudo code for OSTaskStkInit()
registers
PSW & PC
Ret. addr
pdata

18. OSTaskStkInit()
Under μC/OS-II, a task looks like a C function with one argument.
Push the argument onto the stack
Pass the argument in one or more registers

19. OSTaskStkInit()- pdata passed to the stack
Regs
PSW & PC
Ret. adrs
pdata

20. BC45
5dc6
0022
0080
5d7A
0031

21. Ret. ADRS bp

22. OSCTXSW

23. OSTaskStkInit()- pdata passed to the stack

24. OSTaskStkInit() – passed in register
Because the compiler passed arguments to a function in registers, we need to find out which register is used to store pData

25. Process Termination

26. Stack layout @OSTaskDelSelf void OSTaskDelSelf() {
OSTaskDel(OS_PRIO_SELF); }

27. Stack layout
@OSTaskDel
OSTaskDel(OS_PRIO_SELF) {
/*…*/ }

28. Stack layout
@OSTaskDel
0xFF
OSTaskDel(OS_PRIO_SELF) {
/*…*/ }

29. Hook Functions If (OS_CPU_HOOK_EN == 1) If (OS_CPU_HOOK_EN == 0)
Hook functions are in OS_CPU_C.C
If (OS_CPU_HOOK_EN == 0)
In other files

30. OSTaskCreateHook() & OSTaskInitHook()
These functions are called when…
After: OS setting up most of OS_TCB
Before the OS_TCB is linked to the active task chain and before the task is made ready to run
Interrupt has been enabled
OSTaskInitHook is called immediately before OSTaskCreateHook

31. OSTaskDelHook() Called by OSTaskDel()
It is called before unlinking the task from OS’s internal linked list of active tasks.
This function is called with interrupt disabled

32. OSTaskSwHook() Called by OSCtxSw and OSIntCtxSw
Two variables is meaningful
OSTCBCur //old task
OSTCBHighRdy //new task
This function is called with interrupt disabled

33. OSTaskStatHook()
This function is called once every second by OSTaskStat()

34. OSTimeTickHook() This function is called by OSTimeTick()
OSTimeTick() is called before the tick start to process in order to give your port or application first claim to the tick.

35. OSTaskIdleHook()
We can bring the processor to the power saving mode by placing “stop” instruction here.
void OSTaskIdleHook(void) {
asm(“STOP”); }

36. OSInitHookBegin()/ OSInitHookEnd()
OSInitHookBegin() is called immediately upon entering OSInit().
OSInitHookEnd() is called at the end of OSInit().

37. OS_CPU_A.ASM
OSStartHighRdy()
OSCtxSw()
OSIntCtxSw()
OSTickISR()

38. OSStartHighRdy()
This function is called by OSStart() to start the highest priority task ready-to-run.
OSStartHighRdy() assumes that OSTCBHighRdy() points to the TCB of the task with the highest priority.
OSTCBHighRdy() is set by OSStart()
The function only does half a context switch
Restoring the registers of the highest priority task
Not saving the register of the previous task

39. Pseudocode Call OSTaskSwHook Set OSRunning = true
Get the stack pointer
stack pointer = OSTCBHighRdy -> OSTCBStkPtr;
Restore all processor registers from the task’s stack
Execute “Return from interrupt”

40. OSCtxSw()
A task level context switch is accomplished by issuing a software interrupt instruction.
The sequence of events that leads µC/OS-II to vector to OSCtxSw() is as follows:
The current task calls a system call which causes a higher priority task ready to run. At the end of the service call, the OS calls OSSched().
OSSched() loads the address of the highest priority task into OSTCBHighRdy and then executes the software interrupt or trap instruction by invoking the macro OS_TASK_SW()
#define OS_TASK_SW() asm INT uCOS

41. Pseudocode The machine has saved the return address and status word
void OSCtxSw(void) {
Save processor registers;
Save the current task’s stack pointer into\\
the current task’s OS_TCB:
OSTCBCur->OSTCBStkPtr = Stack pointer;
Call user definable OSTaskSwHook();
OSTCBCur = OSTCBHighRdy;
OSPrioCur = OSPrioHighRdy;
Get the stack pointer of the task to resume:
Stack pointer = OSTCBHighRdy->OSTCBStkPtr;
Restore all processor registers from the new task’s stack;
Execute a return from interrupt instruction; }

42. OSTickISR()
You MUST enable ticker interrupts AFTER multitasking has started, i.e. after calling OSStart().
You should initialize and tick interrupts in the first task that executes following a call to OSStart().

43. Pseudocode void OSTickISR(void) { Save processor registers;
Call OSIntEnter() or increment OSIntNesting;
if (OSIntNesting == 1)
OSTCBCur->OSTCBStkPtr = stack pointer
Call OSTimeTick();
Call OSIntExit();
Restore processor registers;
Execute a return from interrupt instruction; }

44. OSIntExit()
void OSIntExit (void) { if (OSIntNesting == 0) { if (OSLockNesting == 0) { OSIntExitY = OSUnMapTbl[OSRdyGrp]; OSPrioHighRdy = (INT8U)((OSIntExitY << 3) + OSUnMapTbl[OSRdyTbl[OSIntExitY]]); if (OSPrioHighRdy != OSPrioCur) { OSTCBHighRdy = OSTCBPrioTbl[OSPrioHighRdy]; OSIntCtxSw(); } OS_EXIT_CRITICAL();

45. OSIntCtxSw()
This function is called by OSIntExit() to perform context switch from ISR.
Because it is called from ISR, all registers are properly saved.

46. Pseudocode void OSIntCtxSw(void) { Call user-definable OSTaskSwHook();
OSTCBCur = OSTCBHighRdy;
OSPrioCur = OSPrioHighRdy;
Get the stack pointer of the task to resume:
Stack pointer = OSTCBHighRdy->OSTCBStkPtr;
Restore all processor registers from the new task’s stack;
Execute a “return from interrupt” instruction; }

47. Pseudocode – an Old Version
void OSIntCtxSw(void) {
Adjust the stack pointer to remove calls to:
OSIntExit(),
OSIntCtxSw() and possibly the push of the processor status word;
Save the current task’s stack pointer into the current task’s OS_TCB:
OSTCBCur->OSTCBStkPtr = Stack pointer;
Call user definable OSTaskSwHook();
OSTCBCur = OSTCBHighRdy;
OSPrioCur = OSPrioHighRdy;
Get the stack pointer of the task to resume:
Stack pointer = OSTCBHighRdy->OSTCBStkPtr;
Restore all processor registers from the new task’s stack;
Execute a return from interrupt instruction; }

49. Testing of a Port
Steps
Ensure that the code compiles, assembles and links
Verify OSTaskStkInit and OSStartHighRdy
Verify OSCtxSw
Verify OSIntCtxSw and OSTickISR

50. Code Compiling, Assembling and Linking
Ensure that the code compiles, assembles and links
void main() {
OSInit();
OSStart(); }

51. Verifying of OSTaskStkInit and OSStartHighRdy
Disable the statistic task
OS_TASK_STAT_EN = 0
Step over the function OSInit() and then step into the for OSStart()
Step into OSStartHighRdy
Switch to the “Idle” task?
The register order  OSTaskStkInit
“Return” to the idle thread  OSStartHighRdy

52. Verifying of OSCtxSw()
void main() { OSInit(); OSTaskCreate(TestTask, NULL, stack, 0); OSStart(); } Void TestTask(void *pData) { while(1) { OSTimeDly(1);

53. Verifying of OSCtxSw()
OSTimeDly()  OS_Sched()  OSCtxSw ()
When the return from interrupt is executed, the you should be in OS_TaskIdle.

54. Verifying of OSIntCtxSw() and OSTickISR()
Installing of a timer interrupt handler
Call OSTimeDly(1) to result in a context switch to the idle thread by using OSIntCtxSw() and OSTickISR()

55. Verifying of OSIntCtxSw() and OSTickISR()
void main(void) { OSInit(); install the clock tick ISR; OSTaskCreate(TestTask); OSStart(); } Void TestTask(void *pData) { disable interrupt initialize the clock tick interrupt; enable interrupt while (1) { OSTimeDly(1); printf(“%d”, OSTimeGet());