Pumpkin, Inc.

This is a call graph for the test_salvo avr-fcc 3.4.6 project. The test_salvo project
was written to illustrate a problem with using the OS_Replace() macro.

OSDispatch() pushes r28,r29 onto stack, then adjusts stack frame and jumps to task. This works when using OS_Yield(), or any of the OS_Wait type user services because in the end, they all call OSCtxSw() which cleans up the stack, including removing r28,r29. This is not happening when using OS_Replace().

With OS_Replace(), the contents of registers r28 and r29 that are left on the stack by OSDispatch() are used as a return address by the task being OS_Replaced.

The WinAVR (Salvo Pro) test_salvo.zip project can be found here

The callgraph.txt file can be found here

Tom

[This message has been edited by zoomcityzoom (edited September 13, 2006).]

[This message has been edited by zoomcityzoom (edited September 13, 2006).]

Thanks for the workaround. OS_Replace() works on the AVR with the ICCAVR compiler. I think, because to the hardware stack being used only for return addresses.

You can imagine my confusion when I switched from ICCAVR to WinAVR! I'm in it this far, I'm going to see if I can come up with a solution.

Yes, it's broken for several ports. I think it only worked for the PICC/PICC-18 ports, but I'm not sure. Am investigating.

Here's what one user did recently:

code:

---

I worked around replacing
               OS_Replace(MODEM_TaskConnect,1);
with
               OSCreateTask(MODEM_TaskConnect, ANOTHER TCB, 1);
               OS_Destroy();
 
It costs me another TCB but later I can reuse the destroyed TCBs with OSCreateTask ()

---

------------------

I have a new OS_Replace() macro and an updated version of portgccavr.S that now work together properly.

Now, OSDispatch() pushes r29, r29 on to the stack, saves the SP at OSframeP+0, retrieves and jumps to the task entry/re-entry address. Since we agree that a task's local variables/parameters are not preserved, OSDispatch() does not have to mess with the frame at all. Here's what the stack looks like before and after OSDispatch().

code:

---

; ==== Stacks in OSSched(), just prior to call to OSDispatch()
;
;                 [  rtn address low :         main()] 0x10ff
;                 [ rtn address high :         main()] 0x10fe
;            [SP] [                                  ] 0x10fd
;
;
;
; ==== Stacks upon entering OSDispatch()
;
;                 [  rtn address low :         main()] 0x10ff
;                 [ rtn address high :         main()] 0x10fe
;                 [              R28 :      OSSched()] 0x10fd
;                 [  rtn address low :      OSSched()] 0x10fc
;                 [ rtn address high :      OSSched()] 0x10fb
;            [SP] [                                  ] 0x10fa
;
;
; ==== Stacks once inside task. Note, this shows that the
;      task has 4 bytes of local data. These will be lost
;      after first call to OSCtxSw().
;
;
;                 [  rtn address low :         main()] 0x10ff
;                 [ rtn address high :         main()] 0x10fe
;                 [              R28 :      OSSched()] 0x10fd
;                 [  rtn address low :      OSSched()] 0x10fc
;                 [ rtn address high :      OSSched()] 0x10fb
;                 [              R28 :   OSDispatch()] 0x10fa
;                 [              R29 :   OSDispatch()] 0x10f9
;                 [    task's local data & params    ] 0x10f8
;                 [    task's local data & params    ] 0x10f7
;                 [    task's local data & params    ] 0x10f6
;                 [    task's local data & params    ] 0x10f5
;            [SP] [                                  ] 0x10f4
;

---

The revised OSCtxSw() does OSEnterCritical(), pops the task's return address off the stack, saving this in OScTcbP->tFP as the re-entry address, restores SP from the value saved by OSDispatch(), pops r29 and r28, followed by OSLeaveCritical() and returns to OSSched(). This approach effectively destroys a task's local variables/parameters (as explained in the manual.) Here's what the stacks look like before, during, and after OSCtxSw().

code:

---

; Notes: This example illustrates 4-bytes of stack used for task's
;        local data and why static vars should be used within tasks.
;
;
; ====  After entering OSCtxSw() and OSEnterCritical():
;
;                 [  rtn address low :         main()] 0x10ff
;                 [ rtn address high :         main()] 0x10fe
;                 [              R28 :      OSSched()] 0x10fd
;                 [  rtn address low :      OSSched()] 0x10fc
;                 [ rtn address high :      OSSched()] 0x10fb
;                 [              R28 :   OSDispatch()] 0x10fa
;                 [              R29 :   OSDispatch()] 0x10f9
;                 [    task's local data & params    ] 0x10f8
;                 [    task's local data & params    ] 0x10f7
;                 [    task's local data & params    ] 0x10f6
;                 [    task's local data & params    ] 0x10f5
;                 [  rtn address low :         task()] 0x10f4
;                 [ rtn address high :         task()] 0x10f3
;            [SP] [                                  ] 0x10f2
;
;
; ====  After popping the stack's return address. Saved at
;       TcbP->tFP for use as entry/resume address by OSDispatch()
;
;
;                 [  rtn address low :         main()] 0x10ff
;                 [ rtn address high :         main()] 0x10fe
;                 [              R28 :      OSSched()] 0x10fd
;                 [  rtn address low :      OSSched()] 0x10fc
;                 [ rtn address high :      OSSched()] 0x10fb
;                 [              R28 :   OSDispatch()] 0x10fa
;                 [              R29 :   OSDispatch()] 0x10f9
;                 [    task's local data & params    ] 0x10f8
;                 [    task's local data & params    ] 0x10f7
;                 [    task's local data & params    ] 0x10f6
;                 [    task's local data & params    ] 0x10f5
;            [SP] [  rtn address low :         task()] 0x10f4
;                 [ rtn address high :         task()] 0x10f3
;                 [                                  ] 0x10f2
;
;
;
; ====  After restoring SP to what was saved by OSDispatch()
;
;
;                 [  rtn address low :         main()] 0x10ff
;                 [ rtn address high :         main()] 0x10fe
;                 [              R28 :      OSSched()] 0x10fd
;                 [  rtn address low :      OSSched()] 0x10fc
;                 [ rtn address high :      OSSched()] 0x10fb
;                 [              R28 :   OSDispatch()] 0x10fa
;                 [              R29 :   OSDispatch()] 0x10f9
;            [SP] [    task's local data & params    ] 0x10f8
;                 [    task's local data & params    ] 0x10f7
;                 [    task's local data & params    ] 0x10f6
;                 [    task's local data & params    ] 0x10f5
;                 [  rtn address low :         task()] 0x10f4
;                 [ rtn address high :         task()] 0x10f3
;                 [                                  ] 0x10f2
;
;
; ====  Stacks just before return to OSSched()
;
;
;                 [  rtn address low :         main()] 0x10ff
;                 [ rtn address high :         main()] 0x10fe
;                 [              R28 :      OSSched()] 0x10fd
;                 [  rtn address low :      OSSched()] 0x10fc
;                 [ rtn address high :      OSSched()] 0x10fb
;            [SP] [              R28 :   OSDispatch()] 0x10fa
;                 [              R29 :   OSDispatch()] 0x10f9
;                 [    task's local data & params    ] 0x10f8
;                 [    task's local data & params    ] 0x10f7
;                 [    task's local data & params    ] 0x10f6
;                 [    task's local data & params    ] 0x10f5
;                 [  rtn address low :         task()] 0x10f4
;                 [ rtn address high :         task()] 0x10f3
;                 [                                  ] 0x10f2
;

---

Here's the new OS_Replace() macro. It works, but I'm afraid I'm a newbie when it comes to inline assembly for avr-gcc.

code:

---

#define OS_Replace(tFP, prio)									
do {															
	__asm__ __volatile__( "in __tmp_reg__, __SREG__
	"		
						  "cli
	"								
						  "lds __zero_reg__, OSframeP+0
	"	
						  "out __SP_L__, __zero_reg__
	"		
						  "lds __zero_reg__, OSframeP+1
	"	
						  "out __SP_H__, __zero_reg__
	"		
						  "out __SREG__, __tmp_reg__
	"		
						  "clr __zero_reg__" :  :  );				
	OSCreateTask(tFP, OScTcbP, prio);							
	__asm__ __volatile__( "pop r29
	"					
						  "pop r28
	"					
						  "ret
	" : :  );						
} while (0)

---

@aek: I'll email you the portgccavr_new.S source.

-Tom

EDIT: In the original macro, I forgot to zero r1 after its use and to pop the top two bytes into r29, r28. This is now fixed.

[This message has been edited by zoomcityzoom (edited September 16, 2006).]