Southbridge (computing)

In computing, a southbridge is a component of a traditional two-part chipset architecture on motherboards, historically used in personal computers. It works alongside the northbridge to manage communications between the central processing unit (CPU) and lower-speed peripheral interfaces. The northbridge typically handled high-speed connections such as RAM and GPU interfaces, while the southbridge managed lower-speed functions.

A typical north/southbridge layout

IBM T42 laptop motherboard with the following labels: CPU (central processing unit), NB (northbridge), GPU (graphics processing unit), and SB (southbridge)

The southbridge controls a range of input/output (I/O) functions, including USB, audio, firmware (e.g., BIOS or UEFI), storage interfaces such as SATA, NVMe, and legacy PATA, as well as buses like PCI, LPC, and SPI.^[1][2]

Southbridge and northbridge components were often designed to work in pairs, though there was no universal standard for interoperability.^[3] In the 1990s and early 2000s, they commonly communicated via the PCI bus; more recent chipsets use Direct Media Interface (Intel) or PCI Express (AMD).

Intel referred to its southbridge as the I/O Controller Hub (ICH), later replaced by the Platform Controller Hub (PCH), which connected directly to the CPU in later architectures. Since the mid-2010s, the traditional two-chip design has largely been replaced by single-chip platforms or system-on-chip (SoC) solutions that integrate southbridge functions into a single chipset or the CPU itself.

Current status

Due to the push for system-on-chip (SoC) processors, modern devices increasingly have the northbridge integrated into the CPU die itself;^{[further explanation needed]} examples are Intel's Sandy Bridge^[4] and AMD's Fusion processors,^[5] both released in 2011.

With the Intel 5 Series chipset in 2008, the southbridge became redundant and was replaced by the Platform Controller Hub (PCH) architecture introduced. AMD did the same with the release of their first APUs in 2011, naming the PCH the fusion controller hub (FCH), which was only used on AMD's APUs until 2017 when it began to be used on AMD's Zen architecture while dropping the FCH name.

On Intel platforms, 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).^[6] Intel low-power processors (Haswell-U and onward) and ultra low-power processors (Haswell-Y and onward) also integrate an on-package PCH. Based on its Chiplet design, AMD Ryzen processors also integrated some southbridge functions, such as some USB and SATA/NVMe interfaces.^[7]

Etymology

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.

Functionality

Motherboard diagram, created in 2007, which supports many on-board peripheral functions as well as several expansion slots

The functionality found in a contemporary southbridge includes:^[8][2]

• PCI bus. A south bridge may also include support for PCI-X.
• Low speed PCI Express (PCIe) interfaces usually for Ethernet and NVMe.
• ISA bus or LPC bridge. ISA slots are no longer provided on more recent motherboards. The LPC bridge provides a data and control path to the super I/O (the normal attachment for the PS/2 keyboard and mouse, parallel port, serial port, IR port, and floppy controller).
• I2C and SMBus controller.
• DMA controller. The 8237 DMA controller allows ISA or LPC devices direct access to main memory without needing help from the CPU.
• PIC and I/O APIC.
• Mass storage interfaces such as SATA, M.2, and historical PATA. This typically allows attachment of hard drives or SSDs.
• Real-time clock.
• Programmable interval timer.
• High Precision Event Timer.
• ACPI controller or APM controller.
• SPI serial bus mostly used for firmware (e.g., BIOS/UEFI) flash storage access.
• Nonvolatile BIOS memory. The system CMOS (BIOS configuration memory), assisted by battery supplemental power, creates a limited non-volatile storage area for BIOS configuration data.
• Intel HD Audio or AC'97 sound interface.
• USB interfaces.

Optionally, a southbridge also includes support (onboard discrete chip or southbridge-integrated) for Ethernet, Wi-Fi, RAID, Thunderbolt, and Out-of-band management.

See also

• Memory controller
• Northbridge (computing)