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  Interrupt 12H (decimal 18) invokes the service that reports the available
  memory size in kilobytes--the same information stored at low-memory
  location 0040:0013H. (See page 55.) The value is reported in AX. The
  memory-size value reflects only the amount of base memory available. In a
  PC/AT or PS/2 with extended (protected-mode) memory, you must use
  interrupt 15H, service 88H (Get Extended Memory Size), to determine the
  amount of extended memory installed.

  In the standard models of the PC, this value is taken from the setting of
  the physical switches inside the system unit. These switches are supposed
  to reflect the actual memory installed, although under some circumstances
  they are set to less memory than is actually present. In the PC/AT and
  PS/2s, the ROM BIOS POST determines the amount of memory in the system by
  exploring available RAM to see what is installed. If the BIOS is using an
  extended data area, this data area is allocated at the highest memory
  address available, so the value returned by this service excludes the
  amount of RAM reserved for the extended data area.