The TSetScreenMode structure.

 // SetScreenMode
 TSetScreenMode struct
  Result                sbyte         // The function call result, always NO_ERR.
  ScreenMode    dword         // The requested screen mode.
 TSetScreenMode ends

The standard NXT firmware only supports setting the ScreenMode to SCREEN_MODE_RESTORE. If you install the NBC/NXC enhanced standard NXT firmware this system function also supports setting the ScreenMode to SCREEN_MODE_CLEAR.