Most modern computers use three bus types: the ISA bus for slower, older peripherals, the PCI bus, and the AGP Bus. The chipset controls these buses. It transfers information to and from them and the processor and memory. The chipset’s capabilities in this area determine what kinds of buses the system supports and how fast they can get. For this reason, Intel calls its chipsets “PCIsets” or “AGPSets”. Most modern chipsets support the ISA bus more for legacy reasons, while offering full support for PCI as well as AGP. The speed of the AGP bus (2X,4X, etc) depends on the chipset, of course.
Bus Bridges
A “bridge” is a networking term that refers to a piece of hardware that connects two dissimilar networks and passes information from the computers on one network to those on the other, and vice-versa. In the same fashion, the chipset must use bus bridges to connect together the different bus types it controls. To see the basics of how this is used, let me briefly address the bus setup in a typical PC. Coming directly from the processor, you have the primary frontside bus. This is the main line of communication coming to and from the processor. The AGP bus is a direct line off of the frontside bus. This is the primary reason behind AGP: by giving your graphics card full access to the full speed of the frontside bus, you give it direct access to the processor and system memory without being hampered by the slower frequencies of the PCI bus. There is a bus bridge between the main system bus and the PCI bus in order to do the translation. And, again, we have a PCI-ISA bridge for use for the ISA bus.
You can see what I am talking about at the graphic to the right, from HowStuffWorks.com.
IDE/ATA Hard Disk Controller
Almost all motherboards now have support for four IDE (ATA) hard disks integrated into them, two on each of two channels. Integrating this support makes sense for a number of reasons, among them the fact that these drives are on the PCI bus, so this saves an expansion slot and reduces cost. The data transfer rate of IDE drives is based on their using programmed I/O (PIO) modes, and use of the fastest of these modes depends on support from the PCI bus and chipset. The ability to set a different PIO mode for each of the two devices on a single IDE channel, called independent device timing, is also a function of the chipset. Without this feature, both devices must run at the speed of the slowest drive.
The IDE controller should actually more accurately be called the host controller. After all, we are connecting complete, self-contained devices to a host, the computer. The actual IDE controllers are not on the motherboard, but on the circuit boards underneath each of your IDE devices.
DMA Mode Support and Bus Mastering
Direct memory access (DMA) provides a way for devices to transfer information directly to and from memory, without the processor’s intervention. It is still used by many devices, although newer transfer modes are now used for high-performance devices like hard disks. DMA is controlled by the chipset’s DMA controller, and the newer the controller, the more DMA modes its supports.
Bus mastering is an enhancement of DMA whereby the remote device not only can send data to the memory directly, it actually takes control of the bus, and performs the transfer itself instead of using the DMA controller. This cuts down on the overhead of having the slow DMA controller talk to the device doing the transfer, further improving performance. Bus mastering support is provided by the chipset.
USB Support
USB (Universal Serial Bus) is a technology intended to replace the current ports used for keyboards and mice. It is designed to be an external peripheral connection bus that has high bandwidth while also being easy enough to simply plug a device in and go. USB has been around for a while now and is in use for many types of external devices, including printers, scanners, cameras, PDA connections, etc. Since the USB bus is, like the name suggests, a bus, it is controlled via the chipset.
More recently, we are starting to see USB 2.0 support on chipsets. USB 2.0 is fully back-ward compatible with the previous version of USB, but benefits from a significant increase in bandwidth capability, up to 480 MB/s to be more accurate.
Plug and Play
Plug and Play (PnP) is a specification that uses technology enhancements in hardware, BIOSes and operating systems, to enable supported devices to have their system resource usage set automatically. Intended to help make installation easier by eliminating some of the problems with getting peripheral devices to work together, PnP requires support from the chipset as well.
You can get more information on plug-and-play here.
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