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With the exception of the first 1024-byte vector table area, you must
be extremely careful not to use the above-mentioned memory areas
for other purposes. You should refer to the Memory Requirements
section in Chapter 3 to determine how to dictate the location of the
reserved memory areas. If, for example, your program
inadvertently wrote over the static variable area containing the
serial communication parameters, these parameters would be lost,
resulting in a loss of communication with the system console
terminal. If your program corrupts the system stack, then an
incorrect value may be loaded into the processor Program Counter
(PC), causing a system crash.
Hardware Functions
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.
Preserving the Debugger Operating Environment
Allocates space for the system stack
Initializes the system stack pointer to the top of this area
B
B-37
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