12.7. External Buses
One of the most recent entries in the field of interface
buses is the whole class of external buses. This includes USB,
FireWire, and IEEE1284 (parallel-port-based external bus). These
interfaces are somewhat similar to older and not-so-external
technology, such as PCMCIA/CardBus and even SCSI.
Conceptually, these buses are neither full-featured interface buses
(like PCI is) nor dumb communication channels (like the serial ports
are). It's hard to classify the software that is
needed to exploit their features, as it's usually
split into two levels: the driver for the hardware controller (like
drivers for PCI SCSI adaptors or PCI controllers introduced in the
Section 12.1) and the driver for the specific
"client" device (like
sd.c handles generic SCSI disks and so-called
PCI drivers deal with cards plugged in the bus).