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─────Run-Time Library───────────────────────────────────────────────────────
 
     For hardware and mode configurations with enough memory to
     support multiple-screen pages, _getactivepage, _getvisualpage,
     _setactivepage, and _setvisualpage can be used to manipulate
     screen pages. The active page is the video area where text is
     written. The visual page is the video area displayed to the user.
 
     When active and visual pages are the same (the default), text
     becomes visible when it is written. Screen animation can be done
     by writing different text to each active page, then alternating
     the visual page.
 
     Use _setactivepage to set the page for output. Use _setvisualpage
     to set the page for display. For both functions, the <page>
     argument selects the current page. The default page number is 0.
     The _getactivepage and _getvisualpage functions can be used to
     save the initial pages for later restoration.
 
     These functions only work when text is managed and written with
     text output functions: _outtext, _outmem, _settextposition,
     _gettextposition, _settextcolor, _gettextcolor, and _wrapon. They
     do not work with printf and other standard I/O functions.
 
     The CGA hardware configuration has only 16K of RAM available to
     support multiple video pages, and only in text mode. The EGA and
     VGA configurations can be equipped with up to 256K of RAM for
     multiple video pages in graphics mode. All hardware combinations
     support at least one page (page number 0).
 
     Return Value
 
     If successful, _setactivepage and _setvisualpage return the page
     number of the previous video page. If one of these functions
     fails, it returns -1.
 
     The _getactivepage and _getvisualpage functions return the
     current page number.
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