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void  OSCtxSw (void)

Description

OSCtxSw() is  is called from the macro macro OS_TASK_SW(), which in turn is called from from OSSched() to perform a task-level context switch. Interrupts are disabled when when OSCtxSw() is  is called.

Prior to calling calling OSCtxSw(), OSTCBCurPtr to  to point at the the OS_TCB of  of the task that is being switched out, and and OSSched() sets  sets OSTCBHighRdyPtr to  to point at the the OS_TCB of  of the task being switched in.

Files

os.h/os_cpu_a.asm

Prototype

Arguments

None

Returned Values

None

Required Configuration

None

Callers

OSSched().

Notes/Warnings

None

Example Usage

The pseudocode for OSCtxSw() follows:

(1) OSCtxSw() must save all of the CPU registers onto the current task’s stack. OSCtxSw() is called from the context of the task being switched out. Therefore, the CPU stack pointer is pointing to the proper stack. The user must save all of the registers in the same order as if an ISR started and all the CPU registers were saved on the stack. The stacking order should therefore match that of OSTaskStkInit().

(2) The current task’s stack pointer is then saved into the current task’s OS_TCB.

(3) Next, OSCtxSw() must call OSTaskSwHook().

(4) OSPrioHighRdy is copied to OSPrioCur.

(5) OSTCBHighRdyPtr is copied to OSTCBCurPtr since the current task is now the task being switched in.

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

(7) All the CPU registers from the new task’s stack are restored.

(8) Finally, OSCtxSw() must execute a return from interrupt instruction.