Daniel Schwierzeck daniel.schwierzeck@gmail.com writes:
When booting from ROM, early exceptions can't be handled properly. Instead of busy-looping give the developer the possibilty to examine the situation. Thus issue a SDBBP instruction to transfer control to hardware debugger if one is attached.
You could make the SDBBP into a UHI operation that can be read by a debugger as an unhandled exception rather than an unexpected breakpoint (assuming said debugger knows about UHI). The fragment I use in lightweight boot code is:
move k0, t9 # Preserve t9 move k1, a0 # Preserve a0 li $25, 15 # UHI exception operation li $4, 0 # Use hard register context sdbbp 1 # Invoke UHI operation
You lose k0/k1 in this which may be undesirable but it might be a reasonable trade off. This shouldn't go in the debug vector of course!
Matthew