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The CLS statement clears various parts of the display screen.
 
CLS [{0 | 1 | 2}]
 
Usage Notes
    ■ The CLS statement can be used to:
        - Clear the entire screen (except the bottom line of text), the text
          viewport, or the graphics viewport
        - Regenerate the bottom line of text
        - Return the cursor to the home position (the upper-left
          corner of the screen).
    ■ The behavior of a particular CLS statement depends on:
        - The CLS statement argument: 0, 1, 2, or none.
        - The current screen mode.
        - Whether or not a VIEW statement has been executed which has
          changed the graphics viewport from the default of the entire
          screen.
        - Whether or not a VIEW PRINT statement has been executed which
          established a user-defined text viewport.
        - Whether or not a KEY ON statement has been executed, which
          established the bottom text line as a list of function-key
          assignments. The bottom line on the screen may be 25, 30, 43,
          or 60, depending upon the current screen mode.
    ■ The following fundamentals about the graphics and text viewports
      determine how the CLS statement works:
        - SCREEN mode 0 has only a text viewport. All other screen modes
          have both a text and graphics viewport.
        - The default graphics viewport is the entire screen.
        - The default text viewport is the entire screen, except for the
          bottom line of text.
        - After a user-defined graphics viewport (or "clipping region")
          has been established by execution of a VIEW statement, it is
          reset to the default by execution of any of these statements:
            VIEW without arguments;
            CLEAR;
            any SCREEN statement that changes the screen resolution.
        - After a user-defined text viewport has been established by
          execution of a VIEW PRINT statement, it is reset to the default
          by execution of any of these statements:
            VIEW PRINT without arguments,
            any text-oriented statement, such as WIDTH, that changes the
            number of text columns or text lines on the screen.
        - If the KEY ON statement has been executed, the CLS statement
          regenerates the bottom screen line as the function key assignment
          list or as a line of blank characters.
    ■ Following is a table that summarizes how the CLS statement works:
 
      Statement    Condition             Operations Performed
      ═════════    ════════════════════  ═══════════════════════════════════
      CLS 0        Any screen mode,      1. Clear entire screen of all text
                   any combination of       and graphics, except bottom text
                   user-defined             line.
                   viewports             2. Clear graphics viewport.
                                         3. Regenerate bottom text line.
                                         4. Return the cursor to home.
 
      CLS 1        Graphics screen mode  1. Clear default graphics viewport.
                   with no user-defined  2. Do not clear text viewport.
                   graphics viewport     3. Regenerate bottom text line.
                                         4. Return the cursor to home.
 
      CLS 1        Graphics screen mode  1. Clear user-defined graphics
                   with user-defined        viewport.
                   graphics viewport     2. Do not clear text viewport
                                         3. Do not regenerate bottom
                                            text line.
                                         4. Do not return the cursor to home.
 
      CLS 1        In SCREEN mode 0      Has no effect.
                   (text mode)
 
      CLS 2        Any screen mode       1. Clear the text viewport.
                                         2. Do not clear the graphics
                                            viewport.
                                         3. Do not regenerate the bottom
                                            text line.
                   Note: The bottom line of text will not be cleared
                         even if it is included in the user-defined
                         text viewport.
 
      CLS          Graphics-screen mode  1. Clear default graphics viewport.
                   with no user-defined  2. Regenerate bottom text line.
                                         3. Return the cursor to home.
 
      CLS          Graphics-screen mode  1. Clear user-defined graphics
                   with user-defined        viewport.
                   graphics viewport     2. Do not regenerate bottom text
                                            line.
                                         3. Do not return the cursor to home.
 
      CLS          In SCREEN mode 0      1. Clear text viewport.
                   (text mode)           2. Regenerate bottom text line.
                                         3. Return the cursor to home.