Daniel Schwierzeck <daniel.schwierz...@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
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