Fundamentals Overview IDE (Editor) Syntax Variables Data Types Declarations Scope Operators

Standard Functions Arithmetic Trigonometry Strings Arrays Date/Time Flow Control Procedures Printing Files Folders

Additional Capabilities Keyboard Sound Graphics Concepts Screen (text) Screen (drawing) Gaming (sprites) Event Trapping Errors System Calls

Exception Functions Summary
When QBasic detects an error it stops execution unless the programmer has placed specific response commands in the program.

The following functions are available to monitor program execution and to respond to any errors (exceptions) that occur.

Trace Execution	tron, troff
Device Identification	erdev, erdev$
Error Information	erl, err
Error Simulation	error
Error Trapping	on error...goto, resume

Exception Function Reference
Here's a quick reference of the available exception functions, in alphabetical order.

• erdev - returns error code from the last device that generated a critical error

      result% = erdev
      

• erdev$ - returns the name of the device that generated the error

      result$ = erdev$
      

• erl - returns line number where most error occurred

      result% = erl
      

  Note: if lines are not numbered, erl returns the closest line number before the line where the error occurred.

• err - returns most recent run-time error

      result% = err
      

• error - simulates an occurrence of an error (QBasic or user-defined)

      error 11          # divide by zero error
      error 57          # device i/o error
      

• on error..goto - enables error tracking and direct program to branch to an error handling routine or to simply resume execution with the next statement

      On Error GoTo Handler    # goes to error-handling routine
      On Error Resume Next     # continues on line following the error
      

  If On Error is not used, any error will end the QBasic program.

• resume -

      resume line    # continues at "line"
      resume next    # continues at statement following the 
                     # one that caused the error
      

• troff - disables tracing of program statements

      troff
      

• tron - enables tracing of program statements

      tron
      

  When tracing is on, QBasic highlights each line of the programs as it executes. If the line affects the screen, the screen is also flashed to show the result.

Run-Time Error Codes
Here is a list of the QBasic run-time error codes, as supplied by err.

     1       NEXT without FOR           ¦  37      Argument-count mismatch
     2       Syntax error               ¦  38      Array not defined
     3       RETURN without GOSUB       ¦  40      Variable required
     4       Out of DATA                ¦  50      FIELD overflow
     5       Illegal function call      ¦  51      Internal error
     6       Overflow                   ¦  52      Bad file name or number
     7       Out of memory              ¦  53      File not found
     8       Label not defined          ¦  54      Bad file mode
     9       Subscript out of range     ¦  55      File already open
     10      Duplicate definition       ¦  56      FIELD statement active
     11      Division by zero           ¦  57      Device I/O error
     12      Illegal in direct mode     ¦  58      File already exists
     13      Type mismatch              ¦  59      Bad record length
     14      Out of string space        ¦  61      Disk full
     16      String formula too complex ¦  62      Input past end of file
     17      Cannot continue            ¦  63      Bad record number
     18      Function not defined       ¦  64      Bad file name
     19      No RESUME                  ¦  67      Too many files
     20      RESUME without error       ¦  68      Device unavailable
     24      Device timeout             ¦  69      Comm-buffer overflow
     25      Device fault               ¦  70      Permission denied
     26      FOR without NEXT           ¦  71      Disk not ready
     27      Out of paper               ¦  72      Disk-media error
     29      WHILE without WEND         ¦  73      Feature unavailable
     30      WEND without WHILE         ¦  74      Rename across disks
     33      Duplicate label            ¦  75      Path/File access error
     35      Subprogram not defined     ¦  76      Path not found

If you have any suggestions for additions to these tutorials, please let me know.