The southbridge is one of the two chips in the core logic chipset on a personal computer (PC) motherboard, the other being the northbridge. The southbridge typically implements the slower capabilities of the motherboard in a northbridge/southbridge chipset computer architecture. In systems with Intel chipsets, the southbridge is named I/O Controller Hub (ICH), while AMD has named its southbridge Fusion Controller Hub (FCH) since the introduction of its Fusion APUs.
The southbridge can usually be distinguished from the northbridge by not being directly connected to the CPU. Rather, the northbridge ties the southbridge to the CPU. Through the use of controller integrated channel circuitry, the northbridge can directly link signals from the I/O units to the CPU for data control and access.
Due to the push for system-on-a-chip (SoC) processors, modern devices increasingly have the northbridge integrated into the CPU die itself; examples are Intel's Sandy Bridge and AMD's Fusion processors, both released in 2011. The southbridge became redundant and it was replaced by the Platform Controller Hub (PCH) architecture introduced with the Intel 5 Series chipset in 2008. All southbridge features and remaining I/O functions are managed by the PCH which is directly connected to the CPU via the Direct Media Interface (DMI).
A southbridge chipset handles all of a computer's I/O functions, such as USB, audio, serial, the system BIOS, the ISA bus, the interrupt controller and the IDE channels. Different combinations of Southbridge and Northbridge chips are possible, but these two kinds of chips must be designed to work together; there is no industry-wide standard for interoperability between different core logic chipset designs. Traditionally, the interface between a northbridge and southbridge was the PCI bus. The main bridging interfaces used now are DMI (Intel) and UMI (AMD).
The name is derived from representing the architecture in the fashion of a map and was first described as such with the introduction of the PCI Local Bus Architecture in 1991. At Intel, the authors of the PCI specification viewed the PCI local bus as being at the very centre of the PC platform architecture (i.e., at the Equator).
The northbridge extends to the north of the PCI bus backbone in support of CPU, memory/cache, and other performance-critical capabilities. Likewise the southbridge extends to the south of the PCI bus backbone and bridges to less performance-critical I/O capabilities such as the disk interface, audio, etc.
The CPU is located at the top of the map at due north. The CPU is connected to the chipset via a fast bridge (the northbridge) located north of other system devices as drawn. The northbridge is connected to the rest of the chipset via a slow bridge (the southbridge) located south of other system devices as drawn.
Although the current PC platform architecture has replaced the PCI bus backbone with faster I/O backbones, the bridge naming convention remains.
The functionality found in a contemporary southbridge includes:
Optionally, a southbridge also includes support for Ethernet, RAID, USB, audio codec, and FireWire. Where support is provided for non-USB keyboard, mouse, and serial ports, a machine normally does so through a device referred to as a Super I/O; still more rarely, a southbridge may directly support the keyboard, mouse, and serial ports.
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