Editing Peripheral Component Interconnect (section)

===Address phase===
A PCI bus transaction begins with an ''address phase''. The initiator (usually a [[chipset]]), seeing that it has GNT# and the bus is idle, drives the target address onto the AD[31:0] lines, the associated command (e.g. memory read, or I/O write) on the C/BE[3:0]# lines, and pulls FRAME# low.

Each other device examines the address and command and decides whether to respond as the target by asserting DEVSEL#. A device must respond by asserting DEVSEL# within 3 cycles. Devices that promise to respond within 1 or 2 cycles are said to have "fast DEVSEL" or "medium DEVSEL", respectively. (Actually, the time to respond is 2.5 cycles, since PCI devices must transmit all signals half a cycle early so that they can be received three cycles later.)

A device must [[latch (electronics)|latch]] the address on the first cycle; the initiator is required to remove the address and command from the bus on the following cycle, even before receiving a DEVSEL# response. The additional time is available only for interpreting the address and command after it is captured.

On the fifth cycle of the address phase (or earlier if all other devices have medium DEVSEL or faster), a catch-all "subtractive decoding" is allowed for some address ranges. This is commonly used by an [[ISA bus]] bridge for addresses within its range (24 bits for memory and 16 bits for I/O).

On the sixth cycle, if there has been no response, the initiator may abort the transaction by deasserting FRAME#. This is known as ''master abort termination'' and it is customary for PCI bus bridges to return all-ones data (0xFFFFFFFF) in this case. PCI devices, therefore, are generally designed to avoid using the all-ones value in important status registers, so that such an error can be easily detected by software.

====Address phase timing====

[[File:AddressPhaseTiming wavedrom.svg]]

Notes:
* GNT# Irrelevant after cycle has started
* Address is only valid for one cycle.
* C/BE will provide the command following by first data phase byte enables

On the rising edge of clock 0, the initiator observes FRAME# and IRDY# both high, and GNT# low, so it drives the address, command, and asserts FRAME# in time for the rising edge of clock 1. Targets latch the address and begin decoding it. They may respond with DEVSEL# in time for clock 2 (fast DEVSEL), 3 (medium) or 4 (slow). Subtractive decode devices, seeing no other response by clock 4, may respond on clock 5. If the master does not see a response by clock 5, it will terminate the transaction and remove FRAME# on clock 6.

TRDY# and STOP# are deasserted (high) during the address phase.  The initiator may assert IRDY# as soon as it is ready to transfer data, which could theoretically be as soon as clock 2.

====Dual-cycle address====
To allow 64-bit addressing, a master will present the address over two consecutive cycles.  First, it sends the low-order address bits with a special "dual-cycle address" command on the C/BE[3:0]#.  On the following cycle, it sends the high-order address bits and the actual command.  Dual-address cycles are forbidden if the high-order address bits are zero, so devices that do not support 64-bit addressing can simply not respond to dual-cycle commands.

<pre style="line-height: 1">
              _  0_  1_  2_  3_  4_  5_  6_
        CLK _/ \_/ \_/ \_/ \_/ \_/ \_/ \_/
            ___
       GNT#    \___/XXXXXXXXXXXXXXXXXXXXXXX
            _______
     FRAME#        \_______________________
                    ___ ___
   AD[31:0] -------<___X___>--------------- (Low, then high bits)
                    ___ ___ _______________
 C/BE[3:0]# -------<___X___X_______________ (DAC, then actual command)
            ___________________________
    DEVSEL#                \___\___\___\___
                         Fast Med Slow
              _   _   _   _   _   _   _   _
        CLK _/ \_/ \_/ \_/ \_/ \_/ \_/ \_/
                 0   1   2   3   4   5   6
</pre>

====Configuration access====
Addresses for PCI configuration space access use special decoding. For these, the low-order address lines specify the offset of the desired PCI configuration register, and the high-order address lines are ignored. Instead, an additional address signal, the IDSEL input, must be high before a device may assert DEVSEL#. Each slot connects a different high-order address line to the IDSEL pin and is selected using [[one-hot]] encoding on the upper address lines.