PMON Limitations
PMON Limitations

When the target system is halted at a breakpoint, it's possible to save
the contents of most general purpose registers. However, in this scheme,
the contents of two general purpose registers must be
sacrificed; two registers are required for use by the PMON code
that extracts the contents of the other general purpose registers. The
registers chosen are k0 and k1 because those are the only two registers
that are guaranteed (by convention) to not contain useful values when
interrupts are enabled.  Users should
keep in mind that the contents of these registers at the time of a
breakpoint are therefore not available. 

It should also be noted that PMON makes use of the CPU's
exception handling mechanism whenever a breakpoint occurs. Therefore,
care must be taken in debugging exception handlers, to ensure that any
necessary machine state is saved before a debug exception may occur. In
particular, the EPC (Exception Program Counter), and SR (Status
Register) must be saved, along with any other registers which may be
used by the handler.

In practice, this means that breakpoints or single-stepping may safely
be used within the body of an exception handler, provided it is written
in such a way that all necessary state is saved as its initial action
on entry. However neither of these features may be used during the
first few instructions of the handler; these must be allowed to
complete the task of saving essential machine state to the stack. 

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