UCOS task switching process

One 、UCOS The task obtains the environment recovery process before the processor runs

The task enters the ready state when it is established , Their stacks are initialized by software , Make it similar to the stack after interruption . Each task in memory will create a virtual processor （ Used to back up physical processors ）. The main information stored in the virtual processor is ： The breakpoint address of the program （PC）、 Task stack pointer （SP）、 Program status word register （PSR）、 General purpose register contents , And function call information （ Already exists on stack ）. These contents are usually saved in the task stack . When you need to run a task , Assign the private stack pointer of the task to the stack pointer of the processor SP, Copy the virtual processor of the task to the actual processor .

The processor passes through two pointer registers PC and SP To connect with task code and task stack ,PC It refers to the task code executed ,SP Is the stack that points to the task .

Task stack pointer （SP） Point to the top of the task private stack of the currently running task , Use task control block （TCB） Save task stack pointer （SP）.

 

Two 、UCOS The environment backup process before the processor stops running when the task is deprived  

answer ： When the task is deprived , The transition from running state to thread state , There will be task switching .

When you need to abort the current task , Copy the virtual processor corresponding to the task to memory , At the same time, the task control block （TCB） Save the private stack pointer of the task at this time SP. Then assign the private stack pointer of another task to be run to the stack pointer of the processor SP, Its virtual processor is also copied into the actual processor , The scheduler of the operating system performs these two replication tasks according to certain rules .

When the task level scheduler （OSSched（）) When it is determined that a new high priority task needs to be executed , Will call OSCtxSw（）.

OSCtxSw Do the task switching execution steps ：

1）OSCtxSw（） First, save the status register and program counter of the current task to the stack of the current task . The order in which registers are stacked and when interrupts occur CPU The order of pressing registers onto the stack is the same . Assume here that the status register SR Stack first , The remaining registers are then stacked in turn .

2）OSCtxSw（） take CPU The current value of the stack pointer is saved to the control block of the task that will stop running OS_TCB in , That is to say OSTCBCurPtr->StkPtr = R14.

3）OSCtxSw（） The of the new task OS_TCB The value of the stack pointer of the task saved in the manned CPU Stack pointer register , The value of this pointer was saved the last time the task was switched , It points to the top of the task stack at that time . This step is R14 = OSTCBHighRdyPtr->StkPtrc.

4） Last ,OSCtxSw（） Recover from the new stack CPU Register value . Program counters and status registers usually pop up the stack when executing interrupt return instructions .