Explainer: chipsets and Fabric

Early on in the development of personal computers, a variety of different chips were developed and used to handle peripherals such as the keyboard and storage – in those days, generally some form of tape drive or a floppy disk. With the advent of faster storage devices such as the 10 MB hard disk in the IBM PC XT, some of those had to handle higher speed transfers. In time, and with increasing integration, these became what are generally known as northbridge and southbridge, which is the core of what most now refer to as that computer’s chipset.

Northbridge chips interface the main processor to high-speed devices, including memory and graphics controllers. Southbridge chips handle connections to lower-speed devices, which normally include networking via Ethernet, audio, and standard buses such as serial and parallel ports, PCI, ISA and USB.

Chipsets can be developed by manufacturers who don’t make processors themselves. For example, VLSI Technology, which had attempted unsuccessfully to provide the chipset for Apple’s first Mac, went on to supply them for later models using PowerPC processors. It was also one of the three founding partners, alongside Apple and Acorn, of ARM, and went on to provide chipsets for ARM processors.

One of the main goals of the increasing integration which led to the System on a Chip (SoC) has been to incorporate the functions of the chipset. This was simplest with the limited peripherals which connect to iPhones, but has been one of the major challenges in the development of the M1 series. Apple terms the sections of an M1 chip which replace the traditional chipset its Fabric. Unfortunately, this can be a confusing term, as fabric computing has been used since the mid-1990s to describe high-performance cloud-like computing and what is also known as grid computing.

The Fabric in M1 series chips does a great deal more than any equivalent in an iPhone, including most importantly Thunderbolt 3 and (in the M1 Pro and Max) Thunderbolt 4. Thunderbolt was pioneered by Apple and Intel, appearing first in a MacBook Pro ten years ago, but until last year (2020) Intel was the sole supplier of chipsets supporting Thunderbolt, which wasn’t surprising as it only certified its own chipsets as being compliant. Apple appears to be the first SoC designer which has incorporated its own Thunderbolt 3 and 4 in its chips, a vital step in developing any SoC intended for use in ‘proper’ computers.

Although details of the Fabric are vague, it’s thought that each of its main functional areas is run by its own ARM core, which resembles a cut-down version of the Efficiency (Icestorm) cores of the original M1. These load their own substantial firmware during booting of the M1 series hardware, which has allowed Apple to address early shortcomings in their behaviour without having to change the Fabric itself. It’s believed that the Fabric of the original M1 probably contains as many as a dozen of these ARM cores.

M1 Pro and Max chips have greatly enhanced Fabric when compared to the original M1. This enables them to support more memory, faster GPUs, more displays, and to deliver higher transfer rates with the internal SSD, which have almost doubled from around 4 GB/s to over 7 GB/s. Without these improvements in the performance of the Fabric, much of the potential of the new CPU and GPUs wouldn’t have been realised. Fabric is the unknown and unsung hero of Apple’s M1 series.

Meanwhile, conventional PC logic boards still largely rely on chipsets. Some of their northbridge functions such as graphics control have been integrated into recent CPUs, and most remaining functions have been concentrated in what Intel terms the Platform Controller Hub (PCH). At the moment, it’s AMD which is moving faster towards a fully integrated SoC with its own Fabric, but I doubt whether it will use that term. Meanwhile, pull up your M1 Mac and enjoy the wonders woven into its Fabric.