"note that it is possible to have internal devices on the pci bus even though all the PCI slots are free. For example, most systems today have two ide controllers and a usb controller as devices on the pci bus. Normally the pci ide controllers are mapped to ISA interrupts 14 (primary ide) and 15 (secondary ide), while the usb controller can be mapped to the normally available ISA interrupts 9,10,11, or 5.
The pci bus allows two types of devices to exist, called bus masters (initiators) orslaves (targets). A bus master is a device that can take control of the bus and initiate a transfer. The target device is the intended destination of the transfer. Most current PCI devices can act as both masters and targets, and to be compliant to the PC97 and newer design guides, all PCI slots now must support bus master cards.
The PCI bus is an arbitrated bus. This means that a central arbiter (part of the PCI bus controller in the motherboard chipset) governs all bus transfers, giving fair and controlled access to all the devices on the bus. Before a master can use the bus, it must first ask control from the central arbiter, and then it is granted control for a specified maximum number of clock cycles. This type arbitration allows equal and fair access to all the bus mastering devices, prevents a single device from hogging the bus, and also prevents deadlocks because of simultaneous multiple device access. In this manner, the PCI bus acts much like a LAN (local area network, albeit one that is contained entirely within the system and runs at a much higher speed than conventional external networks between PC's."
uh......, I suggest you purchase the two A+ books most classes use:
"PC Upgrade & Maintenance Guide" Mark Minasi
"Upgrading and Repairing PC's" Scott Mueller
"Troubleshooting, Maintaining and Repairing PC's" Stephen Bigelow is also good
(Latest versions of above, of course)