In this lesson, you will learn how to architect a minimalist exception handler that captures a "snapshot" of the processor state the microsecond a fatal fault occurs.
When a system encounters a hard fault or an unhandled exception, your primary goal is to preserve the evidence before the CPU resets or the stack is overwritten. A minimalist handler avoids complex logic—such as printing to a console or complex logging—because the system state is already unstable. Instead, your handler should focus on two things: saving register contents to a designated "crash memory" region and halting execution.
The Anatomy of a Minimalist Snapshot
Most architectures, such as ARM Cortex-M, automatically push a subset of registers (the exception frame) onto the stack when a fault occurs. Your handler needs to intercept this. By writing a naked HardFault_Handler in assembly or C, you can force the processor to point to a stable memory address that persists across reboots.
Consider this minimal C approach using a volatile struct mapped to a known SRAM location:
typedef struct {
uint32_t r0, r1, r2, r3, r12, lr, pc, psr;
} CrashFrame_t;
void __attribute__((naked)) HardFault_Handler(void) {
__asm volatile (
"mov r0, sp \n" // Get the current stack pointer
"bl RecordState \n" // Branch to helper to save r0 contents
"b . \n" // Infinite loop: Halt the processor
);
}
By pointing the stack pointer to a known, non-initialized section of RAM, you ensure that even if the stack is corrupted, the critical registers (like the Program Counter at the time of failure) are preserved.
Why Minimalism Matters
If your crash handler performs memory allocation or complex peripheral initialization, it is highly likely to trigger another fault, effectively wiping the original evidence. A minimalist handler uses only raw memory writes. Once you have captured the pc (Program Counter) and lr (Link Register), you can cross-reference these with your link map post-mortem to determine exactly which function triggered the collapse.
Key Takeaway: Design your crash handlers to be "zero-dependency" operations that prioritize raw register preservation over system recovery, ensuring you always have a forensic snapshot of the point of failure.