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  A program running in real mode can use service 87H to transfer data to or
  from extended (protected-mode) memory on a PC/AT or PS/2 Model 50, 60, or
  80. This service is designed to be used by a protected-mode operating
  system. The IBM-supplied VDISK utility also uses this function to copy
  data to and from a virtual disk in extended memory. See the IBM BIOS
  Interface Technical Reference Manual for details.