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  4.3.2 Screen Mode
  To invoke screen mode, press PF3. In screen mode by default
  the debugger splits the screen into three displays called SRC,
  OUT, and PROMPT.
  --SRC: module SAMPLE$MAIN -scroll-source--------------------------
          1: 10 !SAMPLE
          2:
          3: EXTERNAL SUB TRIPLE &
          4: ,PRINT_SUB
          5:
          6: WHEN ERROR USE HANDLER_1
      -> 7: CALL TRIPLE
          8: CALL PRINT_SUB
          9:
  - OUT -output---------------------------------------------
  stepped to SAMPLE$MAIN\%LINE 7
  - PROMPT -error-program-prompt----------------------------
  DBG> STEP
  DBG>

  The SRC display, at the top of the screen, shows the source
  code of the module (compilation unit) that is currently exe-
  cuting. An arrow in the left column points to the next line to
  be executed, which corresponds to the current location of the
  program counter (PC). The line numbers, which are assigned
  by the compiler, match those in a listing file.


                                            Note

        BASIC line numbers are treated as text by the de-
        bugger. In this chapter, line numbers refer to the
        sequential line numbers generated by the compiler.
        When a program includes or appends code from an-
        other file, the included lines of code are also numbered
        in sequence by the compiler. These line numbers are
        on the extreme left of a listing file. An explanation of
        the listing file format is in Chapter 3.

  The PROMPT display, at the bottom of the screen, shows the
  debugger prompt (DBG>), your input, debugger diagnostic
  messages, and program output. In the example, the debugger
  commands that have been issued are shown.

  The OUT display, in the center of the screen, captures the
  debugger's output in response to the commands that you
  issue.

  The SRC and OUT displays are scrollable so that you can see
  whatever information may scroll beyond the display win-
  dow's edge. Press KP8 to scroll up and KP2 to scroll down.
  Press KP3 to change the display to be scrolled (by default, the
  SRC display is scrolled). Scrolling a display does not affect
  program execution.

  If the debugger cannot locate source lines for the currently
  executing module, it tries to display source lines in the next
  module down on the call stack for which source lines are
  available and issues the following message:
  %DEBUG-I-SOURCESCOPE, Source lines not available for .0\%PC.
              Displaying source in a caller of the current routine.

  Source lines may not be available for the following reasons:

  .
        The PC is within a system routine, or a shareable image
        routine for which no source code is available.
  .
        The PC is within a routine that was compiled with-
        out the /DEBUG compiler command qualifier (or with
        /NODEBUG).
  .
        The source file was moved to a different directory after it
        was compiled (the location of source files is embedded in
        the object modules). Use the SET SOURCE command to
        direct the debugger to the new location.