Chapter 4 – Optional and Advanced Features

This chapter discusses the advanced and optional features of PCI, including interrupt handling, the “Special” cycle command and 64-bit extensions. The Special cycle provides a mechanism to broadcast information to multiple targets simultaneously. The specification suggests that it is a useful way to convey sideband information to one or more devices without the need for additional wires on the backplane. The specification provides for four interrupt signals from each Peripheral Component Interconnect (PCI) device. A single function device may only use one of the interrupt signals. The routing of the four signals among the devices in a system is at the discretion of the designer. The Special cycle command is a broadcast mechanism that may, in certain cases, substitute for sideband signals. Special cycles are not directed at a specific target and so, no target responds. It is always terminated by a Master Abort. The PCI specification defines an optional extension to 64 bits for memory targets, which allows 64-bit agents to seamlessly interoperate with 32-bit agents. 64-bit transfers occur only if both the initiator and target support 64 bits. Otherwise, transfers default to 32 bits. The “negotiation’ to transfer 64 bits occurs on a per transaction basis and is facilitated by two optional signals: REQ64# and ACK64#.