XPpoke()
  Overwrite any part of the Computer's memory
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   Function:   XPpoke()

               This function makes it possible to write any data
               you want at any possible computer location. Note that it
               is not possible to actually write to the ROM-BIOS, whilst
               you can still read from it using XPpeek(). XPpoke() can
               be used in any place without exception, although great care
               and knowledge of the Operation System is required. While the
               Memory is written to, Interrupts are disabled.

   Syntax:     XPpoke(nSeg,nOfs,cBuffer) --> NIL

   Arguments:  nSeg and nOfs define the space of memory
               where you want to "poke". nBuffer is the buffer
               to place at that Memory spot. Note that unpredictable
               errors can occur when using this function without
               proper knowledge of the items involved.

   Returns:    Nothing

   Usage:      * - let's clear the EGA/VGA Memory bank
               * - (Clear's the screen in the 320x200x256 mode)

               * Put at A000:0000 320x200 bytes of ASCII NULL.
               XPpoke( 655360,0,Replicate( chr(0), 320 * 200))