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