Tài liệu the PCI Bus demystified phần 4

The Special Cycle provides a mechanism to broadcast information
simultaneously to multiple targets. 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.
One use for this facility is to broadcast processor status such as Halt
and Shutdown.
By definition, a Special Cycle is not directed at a specific target
but rather to any and all targets that have an interest in the message
being broadcast. This has several consequences