What Is A PCI Bus And What Are The Specificat

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Revision Date: 29-Feb-96 1:42:15 PM

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Symptoms: What is a PCI bus and what are its specifications? Solutions: Peripheral Component Interconnect (PCI) is an open standard bus architecture developed by Intel Corporation. It offers greater speed and configuration benefits over current bus designs such as ISA, MCA, EISA, VESA local bus (VL-Bus) and other proprietary local bus technologies. To access peripheral devices located in the expansion slots, the original PC provided an 8-bit expansion bus that ran at 4.77MHz. The AT improved this by providing a 16-bit wide data path that ran at 8.33MHz, and allowed a maximum throughput of between 4M per second and 8M per second. This bus design is now commonly called an ISA (Industry Standard Architecture) bus. Micro Channel Architecture (MCA) and Extended Industry Standard Architecture (EISA) expansion buses were developed to further increase data transfer rates between the CPU and I/O devices. But maximum peripheral data throughput wasn't obtained until computer designers bypassed the expansion bus entirely, and provided a separate data bus/path that connected directly onto the local bus of a 486 CPU. The most common local bus standard is the VL-Bus, which was designed by the Video Electronics Standards Association (VESA). Current 32-bit implementations of both VL-Bus and PCI provide similar data throughput at approximately 132M per second (far faster than ISA's maximum of approximately 8Mps). However, Intel is reportedly developing a true 64-bit PCI chip set that should double the bandwidth of the bus to 264Mps, enough to handle HDTV and real-time 3-D rendering. And IBM Microelectronics plans to provide the PowerPC with PCI through a bridge chip set that supports 64-bit data transfers as well. PCI also boasts other advantages over other true local bus architectures. First, it is actually a 'mezzanine' bus; one step removed from the processor bus, it uses this removal from the local bus to support more devices (up to 10) than VL-Bus (usually only two or three). This 'bridging' to the CPU's local bus versus direct attachment to it is also referred to as being 'processor independent'. And the main advantage of processor independence is that the same PCI standard can be implemented on numerous processor designs, from present CPU chips such as the Pentium, PowerPC, PowerMAC, RS/6000, the DEC Alpha chip and all varieties of 486's, etc., to ones which haven't yet been created. Therefore, in theory, one could take the same PCI card (like a SCSI host adapter or Hi-Res video card) and plug it into any PCI machine and maintain superior compatibility. Another advantage is the way in which it lets peripherals connected to it to operate concurrently with the CPU. This allows the CPU to perform device-writes to PCI peripherals, which the PCI bus intercepts and buffers/caches. The PCI bus then sends a ready signal back to the CPU that releases it to perform other functions (instead of the CPU having to wait for peripherals to finish their work before returning full control back to it). The PCI bus then proceeds independently of the CPU in performing the actual device-write request(s) itself. Without extolling its many other virtues, one last highlight is that it provides automatic configuration of peripheral cards (finally -- true plug-and-play). Users no longer have to worry about setting up unique IRQs, I/O addresses and the like for each component to avoid hardware conflicts. PCI does it all.

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