Along Comes PCI

Along Comes PCI:
During the early 1990s, Intel introduced a new bus standard for consideration, the Peripheral Component Interconnect (PCI). PCI presents a hybrid of sorts between ISA and VL-Bus. It provides direct access to system memory for connected devices, but uses a bridge to connect to the frontside bus and therefore to the CPU. Basically, this means that it is capable of even higher performance than VL-Bus while eliminating the potential for interference with the CPU. PCI can connect more devices than VL-Bus, up to five external components. Each of the five connectors for an external component can be replaced with two fixed devices on the motherboard. Also, you can have more than one PCI bus on the same computer, although this is rarely done. The PCI bridge chip regulates the speed of the PCI bus independently of the CPU's speed. This provides a higher degree of reliability and ensures that PCI-hardware manufacturers know exactly what to design for.PCI originally operated at 33 MHz using a 32-bit-wide path. Revisions to the standard include increasing the speed from 33 MHz to 66 MHz and doubling the bit count to 64. Currently, PCI-X provides for 64-bit transfers at a speed of 133 MHz for an amazing 1-GBps (gigabyte per second) transfer rate! PCI cards use 47 pins to connect (49 pins for a mastering card, which can control the PCI bus without CPU intervention). The PCI bus is able to work with so few pins because of hardware multiplexing, which means that the device sends more than one signal over a single pin. Also, PCI supports devices that use either 5 volts or 3.3 volts. Bus Type Bus Width Bus Speed MB/secISA 16 bits 8 MHz 16 MBpsEISA 32 bits 8 MHz 32 MBpsVL-bus 32 bits 25 MHz 100 MBpsVL-bus 32 bits 33 MHz 132 MBpsPCI 32 bits 33 MHz 132 MBpsPCI 64 bits 33 MHz 264 MBpsPCI 64 bits 66 MHz 512 MBpsPCI 64 bits 133 MHz 1 GBpsAlthough Intel proposed the PCI standard in 1991, it did not achieve popularity until the arrival of Windows 95 (in 1995). This sudden interest in PCI was due to the fact that Windows 95 supported a feature called Plug and Play (PnP), which we'll talk about in the next section.