Motorola MVME177, Installation And Use Manual

Page: 119 / 171

Preserving the Debugger Operating Environment
B-39
B
Notice that the value of the target stack pointer register (A7) has not
changed even though a trace exception has taken place. Your
program may either use the exception vector table provided by
177Bug or it may create a separate exception vector table of its own.
The two following sections detail these two methods.
Using 177Bug Target Vector Table
The 177Bug initializes and maintains a vector table area for target
programs. A target program is any program started by the bug:
❏
Manually with GO command
❏
Manually with TR type command
❏
Automatically with the BO command
The start address of this target vector table area is the base address
of the debugger memory. This address loads into the target-state
VBR at power-up or cold-start reset and can be observed by using
the
RD command to display the target-state registers immediately
after power-up.
The 177Bug initializes the target vector table with the debugger
vectors listed in Table B-3 and fills the other vector locations with
the address of a generalized exception handler (refer to the
177Bug
Generalized Exception Handler section in this chapter). The target
program may take over as many vectors as desired by simply
writing its own exception vectors into the table. If the vector
locations listed in Table B-3 are overwritten, the accompanying
debugger functions are lost.
The 177Bug maintains a separate vector table for its own use. In
general, you do not have to be aware of the existence of the
debugger vector table. It is completely transparent and you should
never make any modifications to the vectors contained in it.