A method to trace a variable or other expression through a computer
program is disclosed. A user determines the variable and the conditions
upon which activity of the variable will be monitored. As a result of the
invention, every time that variable is referenced in a memory operation
or other activity by the program and the conditions set forth by the user
are satisfied, the state of that variable is saved as a snapshot without
interrupting or stopping execution of the program. The snapshots are
accumulated in a history table. The history table can be retrieved and
the state of the variable in any given snapshot can be restored. Other
variables and expressions can be attached to the trigger variable and the
states of these other variables at the time of the activity of the
trigger variable may also be saved in the snapshot. The method may be
incorporated into a program as a tracing device or a program product
separate from the logical processing device executing the program.