Context Switch Routine

Write the context switch for a cooperative RTOS on Cortex-M: save the callee-saved registers of the outgoing task, swap stack pointers, and restore the incoming task.

Constraints

• Runs in PendSV so the hardware has already stacked r0 to r3, r12, LR, PC and xPSR.
• Save and restore r4 to r11 manually.
• The task stack pointer is PSP.
• The current TCB pointer is a global whose first member is the saved SP.
• Must return via EXC_RETURN, not BX LR to a normal address.
• Do not touch MSP.