Debugging with GT.M

This information is from the GT.M Programmer's Manual. I'll just summarize a few points.

• $ECODE

   This is a list of past errors, separated by commas. 
   Subsequent errors are added to the end of the list.
   Example:
     GTM>w $ECODE
     ,M6,Z150373850, 
   Errors beginning with: "M" are standard errors
                          "U" are user errors
                          "Z" are GT.M errors

• $ZSTATUS

   This is detail about the most recent error.
   Example:
     GTM>w $ZSTATUS
     150373850,SubName+3^MyFunct,%GTM-E-UNDEF, Undefined local variable: Jxack

• $ZMESSAGE(x)

   This will give a text message for a given error number.
   Example: To get details about the error number, 150373850, given above...
     GTM>w $ZMESSAGE(150373850)
     %GTM-E-UNDEF, Undefined local variable: !AD

• ZPRINT x (e.g. ZPRINT SubName+3^MyFunct)

   This will display the line that cause the program exception.
   The 'x' may be obtained from $ZSTATUS, or from $ZPOS (see below)

• $ZPOSITION (OR $ZPOS)

   Shows the current position of the current line of execution 
  (or the last line exececuted in case of an error)

• ZWRITE

   Shows the entire variable table, displaying all defined variables.

• ZBREAK address

   Toggles a break point at address
   It is possible to have the breakpoint execute code, and then resume.  See manual.

• ZSTEP x

   Will execute the program one line at a time.  I think this would be typically
   used after a ZBREAK drops command back to GT.M> prompt.
     ZSTEP INTO -- start execution at current execution point, and stops at
                   next line (even if that line is in a subroutine).
     ZSTEP [OVER] -- start execution at current exec. point, and stop at the 
                   next line at the same level on the invocation stack.
     ZSTEP OUTOF -- ??

• ZCONTINUE

   Continue execution from break point.