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 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 which do not support 64-bit addressing can simply not respond to dual cycle commands. _ 0_ 1_ 2_ 3_ 4_ 5_ 6_ CLK _/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ ___ GNT# \___/XXXXXXXXXXXXXXXXXXXXXXX ___ FRAME# \___ ___ ___ AD[31:0] -------<__>--------------- (Low, then high bits) ___ ___ ___ C/BE[3:0]# -------<__X___ (DAC, then actual command) ___ DEVSEL# \___\___\___\___ Fast Med Slow _ _ _ _ _ _ _ _ CLK _/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ 0 1 2 3 4 5 6 Configuration access Addresses for PCI configuration space access are decoded specially. 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