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Chapter 5
As a reminder, the location of the .StkLimitPtr is determined by the “ stk_limit ”
argument passed to OSTaskCreate() , when the task is created as shown below:
OS_TCB
MtTaskTCB;
CPU_STK MyTaskStk[1000];
OSTaskCreate(&MyTaskTCB,
“MyTaskName”,
MyTask,
&MyTaskArg,
MyPrio,
&MyTaskStk[0], /* Stack base address */
100 , /* Set .StkLimitPtr to trigger exception at stack usage > 90% */
1000, /* Total stack size (in CPU_STK elements) */
MyTaskQSize,
MyTaskTimeQuanta,
(void *)0,
MY_TASK_OPT,
&err);
Of course, the value of .StkLimitPtr used by the CPU’s stack overflow detection hardware
needs to be changed whenever μC/OS-III performs a context switch. This can be tricky
because the value of this register may need to be changed so that it first points to NULL ,
then change the CPU’s stack pointer, and finally set the value of the stack checking register
to the value saved in the TCB’s .StkLimitPtr . Why? Because if the sequence is not
followed, the exception could be generated as soon as the stack pointer or the stack
overflow detection register is changed. One can avoid this problem by first changing the
stack overflow detection register to point to a location that ensures the stack pointer is
never invalid (thus the NULL as described above). Note that I assumed here that the stack
grows from high memory to low memory but the concept works in a similar fashion if the
stack grows in the opposite direction.
3) Software-based stack overflow detection
Whenever μC/OS-III switches from one task to another, it calls a “hook” function
( OSTaskSwHook() ), which allows the μC/OS-III port programmer to extend the capabilities
of the context switch function. So, if the processor doesn’t have hardware stack pointer
overflow detection, it’s still possible to “simulate” this feature by adding code in the context
switch hook function and, perform the overflow detection in software. Specifically, before a
task is switched in, the code should ensure that the stack pointer to load into the CPU does
not exceed the “limit” placed in .StkLimitPtr . Because the software implementation
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