だいたい、こんな。 

                           +-------+ +--------+
                           |KBD/CRT| |HardDisk|
                           +-------+ +--------+
                               |        |
  +-----+  +-----+ +-----+  +-----+  +-----+
  | CPU |  | ROM | | RAM |  | I/O |  |DiskC|
  +-----+  +-----+ +-----+  +-----+  +-----+
     |        |        |        |       |
   ======================================= (BUS)