Laptop User Manual

Preserving the Debugger Operating Environment
MVME166IG/D2 4-15
4
The following is an example of an exception handler which can pass an
exception along to the debugger:
*
*** EXCEPT - Exception handler ****
*
EXCEPT SUBQ.L #4,A7 Save space in stack for a PC value.
LINK A6,#0 Frame pointer for accessing PC space.
MOVEM.L A0-A5/D0-D7,-(SP) Save registers.
:
: decide here if your code handles exception, if so, branch...
:
MOVE.L BUFVBR,A0 Pass exception to debugger; Get saved VBR.
MOVE.W 14(A6),D0 Get the vector offset from stack frame.
AND.W #$0FFF,D0 Mask off the format information.
MOVE.L (A0,D0.W),4(A6) Store address of debugger exc handler.
MOVEM.L (SP)+,A0-A5/D0-D7 Restore registers.
UNLK A6
RTS Put addr of exc handler into PC and go.
166Bug Generalized Exception Handler
The 166Bug has a generalized exception handler which it uses to handle all of
the exceptions not listed in Table 4-2. For all these exceptions, the target stack
pointer is left pointing to the top of the exception stack frame created. In this
way, if an unexpected exception occurs during execution of your code, you are
presented with the exception stack frame to help determine the cause of the
exception. The following example illustrates this:
Example: Bus error at address $F00000. It is assumed for this example
that an access of memory location $F00000 initiates bus error
exception processing.