9.2 The PCI Bus
Since DOS provided for 4 serial ports on the old ISA bus: COM1-COM4, ttyS0-ttyS3 (tts/0-tts/3) most serial ports on the newer PCI bus use higher numbers such as ttyS4 (tts/4) or ttyS14 (tts/14) for kernel 2.6. This permits one to have both ISA serial ports and PCI serial ports on the same PC with no name conflicts. 0-3 are reserved for the old ISA bus and 4-upward (or 14-upward) are used for PCI. It's not required to be this way but it often is. On-board serial ports on motherboards which have both PCI and ISA slots are likely to still be ISA ports. Even for all-PCI-slot motherboards, the serial ports are often not PCI. They are either ISA, on an internal ISA bus or on a LPC bus which is intended for slow legacy I/O devices: serial/parallel ports and floppy drives.
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