Saving and Restoring Registers Correctly During Context Switches on Cortex-M0

When a context switch occurs on a Cortex-M0 processor, the state of the current thread must be saved so that it can be restored when the thread is switched back in. This includes saving the values of all registers that are modified by the thread. The processor must also load the register values for the new thread that is being switched in. Doing this properly is essential for correct multitasking operation.

Registers to Save and Restore

The most important registers to save and restore during a context switch on Cortex-M0 are:

• General purpose registers R0-R12
• Link register (LR)
• Program counter (PC)
• Processor status registers xPSR

These contain key data like the stack pointer, program counter, and status flags that enable the thread to resume where it left off. The LR and PC registers are especially important for returning to the correct instruction sequence.

Saving Process State

When a context switch occurs, the kernel or RTOS must store the values of the registers listed above for the current thread somewhere in memory. This usually occurs in the thread’s stack area, but other memory locations could be used if needed.

A typical sequence would be:

1. Get current stack pointer value from SP register
2. Decrement stack pointer to allocate space to save registers
3. Store LR, PC, and xPSR to the stack
4. Store R0-R12 to the stack
5. Update stack pointer for next thread

This preserves the entire state of the CPU so that the thread can later be resumed. The specific assembly instructions used depend on the toolchain and assembler directives. But PUSH and POP instructions are commonly used to efficiently save and restore multiple registers to the stack.

Restoring Process State

When the context switch swaps the old thread back in, the opposite procedure must occur:

1. Get stack pointer for the old thread
2. Pop the stored R0-R12 from the stack into the registers
3. Pop the stored xPSR, PC and LR from the stack back into those registers
4. Increment stack pointer back to previous state

This restores the exact CPU state the thread had before it was swapped out. When the thread resumes, it will pick up right where it left off.

What Can Go Wrong

Mistakes in saving and restoring registers during context switches can lead to bizarre crashes, hangs, or data corruption. Some common issues include:

• Forgetting to save and restore a register the thread uses, losing its value
• Overwriting registers from one thread with data for another
• Allowing interrupts/exceptions between saving and restoring state
• Stack overflow from improper stack pointer management
• Restoring the wrong stack pointer for the thread
• Incorrect order pushing/popping registers to/from the stack

To avoid these kinds of problems, it is crucial to very carefully manage the stack pointer, handle registers in the right order, and disable interrupts during critical points in context switching.

Tips for Correct Context Switching

Follow these tips when writing context switch code for Cortex-M0:

• Use PUSH and POP instructions to save/restore registers
• Always save xPSR last and restore it first
• Disable interrupts before saving state and re-enable after restoring
• Double check stack pointer increments/decrements
• Check assembly output to verify register order
• Validate stack does not collide with other memory areas
• Test context switching thoroughly for each and every thread
• Consider using a debugger/simulator to step through switch code

Context Switch Example

Here is some sample Cortex-M0 assembly code that performs a context switch: PUSH {R4-R11} ; Save R4-R11 MOV R0, SP ; Save SP SUB SP, SP, #52 ; Adjust SP for saving remaining regs MRS R1, PSR ; Save PSR PUSH {R0-R3} ; Save R0-R3 PUSH {R1} ; Save PSR MOV R1, LR ; Save LR PUSH {R1} MOV R1, PC ; Save PC PUSH {R1} POP {R1} ; Restore PC MOV PC, R1 POP {R1} ; Restore LR MOV LR, R1 POP {R1} ; Restore PSR MSR PSR, R1 POP {R0-R3} ; Restore R0-R3 MOV SP, R0 ; Restore SP POP {R4-R11} ; Restore R4-R11

This shows the stack-based approach to saving and restoring the key registers R0-R12, LR, PC, and PSR. Interrupts would need to be disabled around the PUSH and POP instructions to prevent corruption of state.

Conclusion

Managing register state properly during context switches is crucial for correct multitasking on Cortex-M0 processors. All registers modified by a thread must be saved before switching out and restored when switching back in later. Following best practices for stack pointer management, register order, and interrupt disabling can help avoid common issues. With careful programming of context switch routines, robust concurrent operation can be achieved on Cortex-M0.