Preserving the Debugger Operating Environment
4-15
4
Exception Vector Tables
Notice in the preceding example 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 162Bug or it may create a separate exception
vector table of its own. The two following sections detail these two
methods.
Using 162Bug Target Vector Table
The 162Bug initializes and maintains a vector table area for target
programs. A target program is any program started by the bug:
❏ Manually with the GO command
❏ Manually with trace commands (T, TC, or TT)
❏ Automatically with the BO command.
The start address of this target vector table area is the base address
($00) of the debugger memory. This address is loaded into the
target-state VBR at power-up and cold-start reset and can be
observed by using the
RD command to display the target-state
registers immediately after power-up.
The 162Bug initializes the target vector table with the debugger
vectors listed in Table 4-2 on page 4-13 and fills the other vector
locations with the address of a generalized exception handler. 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 4-2 are overwritten, then the accompanying
debugger functions are lost.
The 162Bug 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 that it contains.
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