Here, PCI stands for Peripheral Component Interconnect.
The PCI bus was developed by Intel and became widespread around 1994. It was used to add expansion cards such as extra serial or USB ports, network interfaces, sound cards, modems, disk controllers, or video cards.
Many devices that formerly required an expansion card can now be connected via USB 2.0 or 3.0.
Instead, more standard ports such as Ethernet and SATA are now integrated into the motherboard.
Some PCI command codes:
0000: Interrupt Acknowledge
0001: Special Cycle
0010: I/O Read
0011: I/O Write
0110: Memory Read
0111: Memory Write
1010: Configuration Read
1011: Configuration WritePCI bus bridge
Bus bridges are permitted by the PCI specification to connect multiple independent PCI buses.
Bus bridges will forward operations on one bus to another when required.