Editing Peripheral Component Interconnect (section)

===Parity===
The PCI bus detects parity errors, but does not attempt to correct them by retrying operations; it is purely a failure indication.  Due to this, there is no need to detect the parity error before it has happened, and the PCI bus actually detects it a few cycles later.  During a data phase, whichever device is driving the AD[31:0] lines computes even parity over them and the C/BE[3:0]# lines, and sends that out the PAR line one cycle later.  All access rules and turnaround cycles for the AD bus apply to the PAR line, just one cycle later.  The device listening on the AD bus checks the received parity and asserts the PERR# (parity error) line one cycle after that.  This generally generates a processor interrupt, and the processor can search the PCI bus for the device which detected the error.

The PERR# line is only used during data phases, once a target has been selected.  If a parity error is detected during an address phase (or the data phase of a Special Cycle), the devices which observe it assert the SERR# (System error) line.

Even when some bytes are masked by the C/BE# lines and not in use, they must still have ''some'' defined value, and this value must be used to compute the parity.