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  Service 88H (decimal 136) returns the amount of extended (protected-mode)
  memory installed in a PC/AT or PS/2 Model 50, 60, or 80. The value, in
  kilobytes, is returned in register AX.

  The amount of extended memory is established by the ROM BIOS POST
  routines. It includes extended memory installed beyond the first megabyte;
  that is, memory starting at 10000:0000H. Lotus/Intel/Microsoft "expanded"
  memory is not included in the value returned by service 88H.