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.