AMD took a gamble with its Athlon family of processors (Athlon, Athlon XP, Athlon MP, and the now-discontinued Duron). With these processors, AMD decided for the first time to create a chip that was Intel compatible with regards to software but not directly hardware or pin compatible.

Whereas the K6 series would plug into the same Socket 7 that Intel designed for the Pentium processor line, the AMD Athlon and Duron would not be pin compatible with the Pentium II/III and Celeron chips.

This also meant that AMD could not take advantage of the previously existing chipsets and motherboards when the Athlon and Duron were introduced; instead, AMD would have to either create its own chipsets and motherboards or find other companies who would.

The gamble seems to have paid off. AMD bootstrapped the market by introducing its own chipset, referred to as the AMD-750 chipset (code named Irongate). The AMD 750 chipset consists of the 751 System Controller (North Bridge) and the 756 Peripheral Bus Controller (South Bridge).

More recently, AMD introduced the AMD-760 chipset for the Athlon/Duron processors, which is the first major chipset on the market supporting DDR SDRAM for memory. It consists of two chips—the AMD-761 System Bus Controller (North Bridge) and the AMD-766 Peripheral Bus Controller (South Bridge).

Although AMD no longer puts much emphasis on chipset sales, its pioneering efforts have inspired other companies, such as VIA Technologies, NVIDIA, and SiS, to develop chipsets specifically designed for the Slot A and current Socket A and Socket 754 processors from AMD.

This has enabled the motherboard companies to make a variety of boards supporting these chips and the Athlon processors to take a fair amount of market share away from Intel in the process.

North/South Bridge Architecture

Most of Intel's earlier chipsets (and virtually all non-Intel chipsets) are broken into a multi-tiered architecture incorporating what are referred to as North and South Bridge components, as well as a Super I/O chip:

• The North Bridge. So named because it is the connection between the high-speed processor bus (400/266/200/133/100/66MHz) and the slower AGP (533/266/133/66MHz) and PCI (33MHz) buses. The North Bridge is what the chipset is named after, meaning that.

For example, what we call the 440BX chipset is derived from the fact that the actual North Bridge chip part number for that set is 82443BX.

• The South Bridge. So named because it is the bridge between the PCI bus (66/33MHz) and the even slower ISA bus (8MHz).

• The Super I/O chip. It's a separate chip attached to the ISA bus that is not really considered part of the chipset and often comes from a third party, such as National Semiconductor or Standard MicroSystems Corp. (SMSC). The Super I/O chip contains commonly used peripheral items all combined into a single chip.

Note that most recent South Bridge chips now include Super I/O functions, so that most recent motherboards no longer include a separate Super I/O chip.

Chipsets have evolved over the years to support various processors, bus speeds, peripheral connections, and features.

The North Bridge is sometimes referred to as the PAC (PCI/AGP Controller). It is essentially the main component of the motherboard and is the only motherboard circuit besides the processor that normally runs at full motherboard (processor bus) speed.

Most modern chipsets use a single-chip North Bridge; however, some of the older ones actually consisted of up to three individual chips to make up the complete North Bridge circuit. The South Bridge is the lower-speed component in the chipset and has always been a single individual chip.

The South Bridge is a somewhat interchangeable component in that different chipsets (North Bridge chips) often are designed to use the same South Bridge component. This modular design of the chipset allows for lower cost and greater flexibility for motherboard manufacturers.

The South Bridge connects to the 33MHz PCI bus and contains the interface or bridge to the 8MHz ISA bus. It also typically contains dual IDE hard disk controller interfaces, one or two USB interfaces, and in later designs even the CMOS RAM and real-time clock functions.

The South Bridge contains all the components that make up the ISA bus, including the interrupt and DMA controllers. The third motherboard component, the Super I/O chip, is connected to the 8MHz ISA bus and contains all the standard peripherals that are built in to a motherboard.

For example, most Super I/O chips contain the serial ports, parallel port, floppy controller, and keyboard/mouse interface. Optionally, they might contain the CMOS RAM/Clock, IDE controllers, and game port interface as well. Systems that integrate IEEE-1394 and SCSI ports use separate chips for these port types.

Most recent motherboards that use North/South Bridge chipset designs incorporate a Super-South Bridge, which incorporates the South Bridge and Super I/O functions into a single chip.