Every RTOS for a PIC which does not have a software-addressable stack generally requires that all but one of the tasks must have its work divided into uninterruptable pieces which begin and end at the top stack level; the "task-yield" operation does not use a function call, but rather a sequence like
// This code is part of task C (assume for this example, there are tasks
// called A, B, C
movlw JumpC4 & 255
goto TASK_SWITCH_FROM_C
TargetC4:
Elsewhere in the code would be some code like:
TASK_SWITCH_FROM_A:
movwf nextJumpA // Save state of task C
// Now dispatch next instruction for task A
movlw TaskB_Table >> 8
movwf PCLATH
movf nextJumpB,w
movwf PCL
TASK_SWITCH_FROM_B:
movwf nextJumpB // Save state of task C
// Now dispatch next instruction for task A
movlw TaskC_Table >> 8
movwf PCLATH
movf nextJumpC,w
movwf PCL
TASK_SWITCH_FROM_C:
movwf nextJumpC // Save state of task C
// Now dispatch next instruction for task A
movlw TaskA_Table >> 8
movwf PCLATH
movf nextJumpA,w
movwf PCL
At the end of the code, for each task, there would be a jump table; each table would have to fit within a 256-word page (and could thus have a maximum of 256 jumps)
TaskC_Table:
JumpC0 : goto TargetC0
JumpC1 : goto TargetC1
JumpC2 : goto TargetC2
JumpC3 : goto TargetC3
JumpC4 : goto TargetC4
...etc.
Effectively, the movlw
at the start of the task-switch sequence loads the W register with the LSB of the address of the instruction at JumpC4
. The code at TASK_SWITCH_FROM_C
would stash that value someplace, and then dispatch the code for task A. Later, after TASK_SWITCH_FROM_B
is executed, the stored JumpC4
address would be reloaded into W and the system would jump to the instruction pointed to thereby. That instruction would be a goto TargetC4
, which would in turn resume execution at the instruction following the task-switch sequence. Note that task switching doesn't use the stack at all.
If one wanted to do a task switch within a called function, it might be possible to do so if that function's call and return were handled in a manner similar to the above (one would probably have to wrap the function call in a special macro to force the proper code to be generated). Note that the compiler itself wouldn't be capable of generating code like the above. Instead, macros in the source code would generate directives in the assembly-language file. A program supplied by the RTOS vendor would read the assembly-language file, look for those directives, and generate the appropriate vectoring code.
I assume you are using code composer and you should(Keil uvision is a other good option). When you must select in the Flash button's arrow the "debug" option(little insect icon).
Than when you are going to Flash your MCU a interface will open showing the register and where you can track variable values.
Regards, MathieuL.
Best Answer
Yes, a debugger can tell you at any point in time how much memory is being used by the stack. Just check the current value of the stack pointer.
Another thing you can try is to fill up all of the unused RAM area with some obvious bogus value, like 0xAAAA. Run your code for a while and then check the RAM to see how many of the bogus values were overwritten...that's your maximum stack size.
The amount of memory used by static variables should be shown in some kind of log or report from the linker, as this is determined at compile time.