In order to better understand what Hyper Transport technology is and why it is important, I have included a quick explanation about Peripheral Component Interconnect (PCI), the standard in computer component communication today.

Peripheral Component Interconnect was introduced by Intel in 1992 (the first version) and 1993 (Release 2.0). It supports up to 16 physical slots—an addressing limitation, which won't be reached because of the electrical limitation of 10 loads (which will typically amount to three or four plug-in PCI cards) residing on each PCI bus. PCs can have two or more PCI buses, so there can be six or more PCI cards per PC.

Thirty-two and 64-bit-wide bus implementations are defined. Sixty-four bit support uses an additional in-line connector (similar to the AT bus's extra connector). Thirty-two and 64-bit cards can be installed in 64- and 32-bit slots (and the other way around too—the cards and buses detect this and work properly). When a 64-bit card is installed in a 32-bit slot, the extra pins just overhang, without plugging into anything.

Implementations have a separate (from the processor's) clock, running at DC to 33 MHz (though usually at 33 MHz). Slowing down the bus's clock speed is needed to reduce PC power consumption when the PC is not being used. Because the bus is multiplexed (the same pins carry address and data), two bus cycles (one to send the address, the other to send the data) are required per 32- or 64-bit transfer. A burst mode is defined for reads and writes (though the 486 supports only read bursts), which allows any number of data cycles to follow a single address cycle.

A 32-bit, 33-MHz PCI bus implementation would have a nonburst peak transfer rate of 66 Mbps and a burst peak transfer rate of up to 132 Mbps. The 32-bit PCI has a typical sustained burst transfer rate of 80 Mbps—enough to handle 24-bit color at 30 frames per second (full-color, full-motion video).

ISA, EISA, and MCA buses can be driven by a PCI (using a bridge chip set), so non-PCI peripherals can be used in the same PC. Because PCI is not processor-specific (VL-bus is 486-specific), it can be used for other processors such as DEC's Alpha and the PowerPC (so Macintoshes can use PCI peripherals).

PCI cards have between 64 to 256 bytes of configuration memory. Sixteen bits are reserved for a vendor identification code (each vendor gets a unique number). Sixteen bits are reserved for a device identification (vendors assign a unique number to each of their products). The remainder of the first 64 bytes are reserved for future use. The rest of the memory is available for vendor-specific use.

PCI: