an arbitrary cycle using the FRAME# and STOP# signals. Subtractive decode DEVSEL# takes place two cycles after the "slow DEVSEL#" cycle rather than on the next cycle. After the address phase (and before any device has responded with DEVSEL#), there is an additional 1-cycle "attribute phase", during which 36 additional bits (both AD and C/BE# lines are used) of information about the operation are transmitted. These include 16 bits of requester identification (PCI bus, device and function number), 12 bits of burst length, 5 bits of tag (for associating split transactions), and 3 bits of additional status. Versions Essentially all PCI-X cards or slots have a 64-bit implementation and vary as follows: Cards 66 MHz (added in Rev. 1.0) 100 MHz (works in 133 MHz slots by forcing a downclock of the bus to 100 MHz) 133 MHz (added in Rev. 1.0) 266 MHz (added in Rev. 2.0) 533 MHz (added in Rev. 2.0) Slots 66 MHz (speed as 66 MHz 64-bit PCI, can be found on older servers) 133 MHz (most common) 266 MHz (rare on x86, main bus on IBM pSeries from the era) 533 MHz (rare) Mixing of 32-bit and 64-bit PCI cards in different width slots Most 32-bit PCI cards will function properly in 64-bit PCI-X slots, but the bus speed will be limited to the clock frequency of the slowest card, an inherent limitation of PCI's shared bus topology. For example, when a PCI 2.3 66-MHz card is installed into a PCI-X bus capable of 133 MHz, the entire bus backplane will be limited to 66 MHz. To get around this limitation, many motherboards have multiple PCI/PCI-X buses, with one bus intended for use with high-speed PCI-X peripherals, and the other bus intended for general-purpose peripherals. Many 64-bit PCI-X cards are designed to work in 32-bit mode if inserted in shorter 32-bit connectors, with some loss of speed. An example of this is the Adaptec 29160 64-bit SCSI interface card. However some 64-bit PCI-X cards do not work in standard 32-bit PCI slots. Even if it would work, installing a 64-bit PCI-X card in a 32-bit slot will leave the 64-bit portion of the card edge connector not connected and overhanging, which requires that there be no motherboard components positioned so as to mechanically obstruct the overhanging portion of the card edge connector. Comparison with PCI-Express PCI-X is often confused by name with similar-sounding PCI