Host adapters have a number of connectors allowing the connection of PATA and/or SATA devices. The host adapter can have multiple forms, in the past host adapters were always extension boards, nowadays the chip-set is usually integrated with the main board (‘on-board’). Note that this is done assigning a PCI bus of the main-board to the on-board chip-set. From the systems point of view the ‘on-board’ or PCI extension board are equal.
See also: Hardware overview image.