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12-6 Delphi Language Guide
An exit procedure can learn the cause of termination by examining the ExitCode
integer variable and the ErrorAddr pointer variable. In case of normal termination,
ExitCode is zero and ErrorAddr is nil. In case of termination through a call to Halt,
ExitCode contains the value passed to Halt and ErrorAddr is nil. In case of termination
due to a runtime error, ExitCode contains the error code and ErrorAddr contains the
address of the invalid statement.
The last exit procedure (the one installed by the runtime library) closes the Input and
Output files. If ErrorAddr is not nil, it outputs a runtime error message. To output
your own runtime error message, install an exit procedure that examines ErrorAddr
and outputs a message if it’s not nil; before returning, set ErrorAddr to nil so that the
error is not reported again by other exit procedures.
Once the runtime library has called all exit procedures, it returns to the operating
system, passing the value stored in ExitCode as a return code.