Description

OSIntCtxSw() is called from OSIntExit() to perform a context switch when all nested interrupts have returned.

Interrupts are disabled when OSIntCtxSw() is called.

OSTCBCurPtr points at the OS_TCB of the task that is switched out when OSIntCtxSw() is called and OSIntExit() sets OSTCBHighRdyPtr to point at the OS_TCB of the task that is switched in.

Files

os.h/os_cpu_a.asm

Prototype

void  OSIntCtxSw (void)

Arguments

None

Returned Values

None

Required Configuration

None

Callers

OSIntExit().

Notes/Warnings

None

Example Usage

The pseudocode for OSIntCtxSw() is shown below. Notice that the code does only half of what OSCtxSw() did. The reason is that OSIntCtxSw() is called from an ISR and it is assumed that all of the CPU registers of the interrupted task were saved at the beginning of the ISR. OSIntCtxSw() therefore must only restore the context of the new, high-priority task.

          void  OSIntCtxSw (void)
          {
              OSTaskSwHook();                                 (1)
              OSPrioCur           = OSPrioHighRdy;            (2)
              OSTCBCurPtr         = OSTCBHighRdyPtr;          (3)
              SP                  = OSTCBHighRdyPtr->StkPtr;  (4)
              Restore all CPU registers;                      (5)
              Return from interrupt;                          (6)
          }


(1) OSIntCtxSw() must call OSTaskSwHook().

(2) OSPrioHighRdy needs to be copied to OSPrioCur.

(3) OSTCBHighRdyPtr needs to be copied to OSTCBCurPtr because the current task will now be the new task.

(4) The stack pointer of the new task is restored from the OS_TCB of the new task.

(5) All the CPU registers need to be restored from the new task’s stack.

(6) A return from interrupt instruction must be executed.