The
ast driver provides support for boards that multiplex together up to four EIA RS-232C (CCITT V.28) communications interfaces. Apparently the original maker of hardware using this multiplexing protocol was AST.
Each
ast device is the master device for up to four
com devices. The kernel configuration specifies these
com devices as slave devices of the
ast device, as shown in the synopsis. The slave ID given for each
com device determines which bit in the interrupt multiplexing register is tested to find interrupts for that device. The port specification for the
ast device is used to compute the base addresses for the
com subdevices and the port for the interrupt multiplexing register.