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  The address bus in the PC, PC/XT, and PS/2 models 25 and 30 uses 20
  signal lines to transmit the addresses of the memory cells and devices
  attached to the bus. (Memory addressing is discussed more fully on page
  13 and in Chapter 3.) Because two possible values (either 1 or 0) can
  travel along each of the 20 address lines, these computers can specify
  220 addresses--the limit of the addressing capability of the 8088 and
  8086 microprocessors. This amounts to more than a million possible
  addresses.

  The 80286 used in the PC/AT can address 224 bytes of memory, so the AT
  has a 24-line address bus. The bus in the 80286-based PS/2 models 50 and
  60 also supports 24-bit memory addressing; in the 80386-based PS/2 Model
  80, the bus has 32-bit addressing capability.