The only hardware resources used by the debugger are the EIA-232-D ports, which are initialized to interface to the debug terminal. If these ports are reprogrammed, the terminal characteristics must be modified to suit, or the ports should be restored to the debugger-set characteristics prior to reinvoking the debugger. Exception Vectors Used by 177Bug The exception vectors used by the debugger are listed in Table B-3. These vectors must reside at the specified offsets in the target program's vector table for the associated debugger facilities (breakpoints, trace mode, etc.) to operate. When the debugger handles one of the exceptions listed in Table B-3, the target stack pointer is left pointing past the bottom of the exception stack frame created; that is, it reflects the system stack pointer values just before the exception occurred. In this way, the operation of the debugger facility (through an exception) is transparent to users.
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.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.
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