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Peripheral Control

If your computer didn't have a keyboard, it might be less intimidating, but you probably wouldn't find it very useful. You might make do, drawing pictures with your mouse or trying to teach it to recognize your handwriting, but some devices are so integral to using a computer that you naturally expect them to be a part of it. The list of these mandatory components includes the keyboard (naturally), a mouse, a hard disk, a CD drive, ports, and a display system. Each of these devices requires its own connection system or interface. In early computers, each interface used a separate expansion board that plugged into a slot in the computer. Worse than that, you often had to do the plugging.

You probably don't like the sound of such a system, and computer manufacturers liked it even less. Individual boards were expensive and, with most people likely not as capable as you in plugging in boards, not very reliable. Clever engineers found a clever way to avoid both the problems and expenses. They built the required interfaces into the chipset. At one time, all these functions were built into a single chip called the peripheral controller, an apt description of its purpose. Intel's current chipset design scheme puts all the peripheral control functions inside the I/O controller hub.

As with the system-control functions, most of the peripheral interface functions are well-defined standards that have been in use for years. The time-proven functions usually cause few problems in the design, selection, or operation of a chipset. Some interfaces have a more recent ancestry, and if anything will cause a problem with a chipset, they will. For example, the PCI bus and ATA-133 disk interface (described in Chapter 10, "Interfaces") are newcomers when compared to the decade-old keyboard interface or even-older ISA bus. When a chipset shows teething pains, they usually arise in the circuits relating to these newer functions.

Hard Disk Interface

Most modern chipsets include all the circuitry needed to connect a hard disk to a computer, typically following an enhancement of the AT Attachment standard, such as UDMA. Although this interface is a straightforward extension of the classic ISA bus, it requires a host of control ports for its operation, all provided for in the chipset. Most chipsets now connect the ISA bus through their PCI bridges, which may or may not be an intrinsic part of the south bridge.

The chipset thus determines which modes of the AT Attachment interface your computer supports. Intel's current chipsets, for example, support all ATA modes up to and including ATA/100. They do not support ATA/133, so computers based on Intel chipsets cannot take full advantage of the highest speeds of the latest ATA (IDE) hard disk drives.

Floppy Disk Controller

Chipsets often include a variety of other functions to make the computer designer's life easier. These can include everything from controls for indicator lights for the front panel to floppy disk controllers. Nearly all chipsets incorporate circuitry that mimics the NEC 765 floppy disk controller used by nearly all adapter boards.

In that the basic floppy disk interface dates back to the dark days of the first computers, it is a technological dinosaur. Computer-makers desperately want to eliminate it in favor of more modern (and faster) interface alternatives. Some software, however, attempts to control the floppy disk by reaching deep into the hardware and manipulating the floppy controller directly through its powers. To maintain backward compatibility with new floppy interfaces, the chipset must still mimic this original controller and translate commands meant for it to the new interface.

Keyboard Controller

One additional support chip is necessary in every computer: a keyboard decoder. This special-purpose chip (an Intel 8042 in most computers, an equivalent chip, or part of a chipset that emulates an 8042) links the keyboard to the motherboard. The primary function of the keyboard decoder is to translate the serial data that the keyboard sends out into the parallel form that can be used by your computer. As it receives each character from the keyboard, the keyboard decoder generates an interrupt to make your computer aware you have typed a character. The keyboard decoder also verifies that the character was correctly received (by performing a parity check) and translates the scan code of each character. The keyboard decoder automatically requests the keyboard to retransmit characters that arrive with parity errors.

USB-based keyboards promise the elimination of the keyboard controller. These pass packets of predigested keyboard data through the USB port of your computer instead of through a dedicated keyboard controller. In modern system design, the keyboard controller is therefore a legacy device and may be eliminated from new systems over the next few years.

Input/Output Ports

The peripheral control circuitry of the chipset also powers most peripheral ports. Today's computers usually have legacy serial and parallel ports in addition to two or more USB connections. As with much else of the chipset design, the legacy ports mimic old designs made from discrete logic components. For example, most chipsets emulate 16550 universal asynchronous receiver/transmitters (UARTs) to operate their serial ports.

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