user manual

Preserving the Debugger Operating Environment

B-37

❏ Allocates space for the system stack

❏ Initializes the system stack pointer to the top of this area

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.