In this lesson, you will learn how to preserve and restore the precise state of your CPU registers when a fault occurs, ensuring that your system can resume or recover safely.
The Anatomy of a Processor Context
When a fault—such as a division by zero, memory access violation, or hardware watchdog trigger—occurs, the CPU stops its current execution flow immediately. To recover, we cannot simply jump back to the start of the program; we must return to the exact point of interruption with all registers (the "context") in their original state.
The processor context consists of the Program Counter (PC), Stack Pointer (SP), Status Register (PSR), and general-purpose registers (R0-R12). If these are not saved, the "resumed" code will likely crash or behave unpredictably because the CPU’s working state has been corrupted.
Saving and Restoring Context
In most embedded architectures, the hardware performs a partial "stacking" of registers automatically when an exception is triggered. However, the software handler is responsible for pushing the remaining volatile registers onto the stack to create a "snapshot" of the system state.
Consider this conceptual assembly-level flow for an exception handler:
; 1. Preserve current state
PUSH {R0-R12, LR} ; Manually save the remaining registers
; 2. Execute recovery logic
BL fault_recovery_logic
; 3. Restore preserved state
POP {R0-R12, LR} ; Bring registers back to their state at fault
; 4. Return from exception
BX LR ; CPU restores hardware-stacked registers
By pushing these registers onto the stack, you create a memory-resident record of the faulting condition. This allows your recovery logic to examine the values of R0-R12 to debug the cause of the crash, or simply restore them to allow the system to resume execution without the main loop "knowing" a fault even occurred. If the fault is unrecoverable, this context is often dumped to non-volatile memory (like Flash or EEPROM) for post-mortem analysis, providing the documentation required for safety-critical design rationale.
Key Takeaway: Managing processor context requires manually capturing the CPU’s volatile register state during the transition to an exception handler to ensure the system can accurately resume or perform a meaningful post-mortem analysis.