requires an additional data phase: Disconnect-A If the initiator observes STOP# before asserting its own IRDY#, then it can end the burst by deasserting FRAME# at the same time as it asserts IRDY#, ending the burst after the current data phase. Disconnect-B If the initiator has already asserted IRDY# (without deasserting FRAME#) by the time it observes the target's STOP#, it is committed to an additional data phase. The target must wait through an additional data phase, holding STOP# asserted without TRDY#, before the transaction can end. If the initiator ends the burst at the same time as the target requests disconnection, there is no additional bus cycle. Burst addressing For memory space accesses, the words in a burst may be accessed in several orders. The unnecessary low-order address bits AD[1:0] are used to convey the initiator's requested order. A target which does not support a particular order must terminate the burst after the first word. Some of these orders depend on the cache line size, which is configurable on all PCI devices. If the starting offset within the cache line is zero, all of these modes reduce to the same order. Cache line toggle and cache line wrap modes are two forms of critical-word-first cache line fetching. Toggle mode XORs the supplied address with an incrementing counter. This is the native order for Intel 486 and Pentium processors. It has the advantage that it is not necessary to know the cache line size to implement it. PCI version 2.1 obsoleted toggle mode and added the cache line wrap mode, where fetching proceeds linearly, wrapping around at the end of each cache line. When one cache line is completely fetched, fetching jumps to the starting offset in the next cache line. Note that most PCI devices only support a limited range of typical cache line sizes; if the cache line size is programmed to an unexpected value, they force single-word access. PCI also supports burst access to I/O and configuration space, but only linear mode is supported. (This is rarely used, and may be buggy in some devices; they may not support it, but not properly force single-word access either.) Transaction examples This is the highest-possible speed four-word write burst, terminated by the master: 0_ 1_ 2_ 3_ 4_ 5_ 6_ 7_ CLK _/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \ ___ ___ ___ ___ ___ AD[31:0] ---<__X__X___>---<___> ___