Why Some Systems Work Better Than Others

If the CPU is the brain of a personal computer, the motherboard is its nervous system — the foundation or platform that supports and provides the data-transfer connections between the processor, memory, AGP and PCI expansion cards, disk drives, and external peripherals. And next to the CPU, the most important part of a motherboard is its core logic chipset.

One manufacturer, Nvidia Corp., prefers the term “platform processor” to “chipset.” It’s marketing jargon that makes sense, in that a chipset determines a motherboard’s and hence computer’s capabilities — everything from what kinds of peripherals you can plug into it to what kinds of CPU and memory it can use. Nowadays, thanks to TV commercials, even casual home-PC buyers are likely to ask which processor a system has. This article will explain why truly smart shoppers learn which chipset it has, too.

North and South

A chipset’s functions are divided into two groups, which are usually handled by two chips — the Northbridge and Southbridge, which you can think of as “inner” and “outer” or adjacent to the CPU and peripherals, respectively. Names vary — Intel calls these components hubs; SiS calls them controllers — but the purpose is the same: the Northbridge and Southbridge provide data bridges between specific sets of bus peripherals.

Intel’s block diagram of its recently introduced 845PE Pentium 4 chipset, reproduced below, illustrates the architecture of most standard chipset designs — the Northbridge handling the more data-intensive pathways such as the memory and AGP (Advanced Graphics Port or screen display) buses, while the Southbridge takes care of secondary connections such as those to ATA/IDE disk drives and USB peripherals. There is a strict division of duties between the Northbridge and Southbridge, as well as a high-speed interface between the two.

As an alternative, some vendors such as SiS have created single-chip designs (see the SiS 745 chipset diagram below), putting both Northbridge and Southbridge functions on a single die. At least in theory, this has obvious benefits, such as lower production costs (for potentially cheaper motherboards) and the fastest possible communication between Northbridge and Southbridge components.

It also has obvious disadvantages, such as the need to create a totally new chipset design just to add one new Southbridge feature (upgrading from USB 1.1 to USB 2.0 support, for instance). Whether for these reasons or simply for the sake of playing by the standard rulebook, SiS’ more recent 746 Athlon XP chipset went back to the dual-chip design — using, in fact, the same 963 Southbridge as the company’s SiS 648 Pentium 4 chipset.

The Northbridge

The main function of the Northbridge is to handle the memory, CPU, and AGP data-transfer duties and to make the most efficient use of available resources. These are mission-critical jobs, as control of the CPU and memory makes up a high percentage of overall system activity. The Northbridge component is the key performance area of any motherboard, and it’s usually where benchmark-test races are won or lost.

Out of the three main Northbridge duties, controlling access to system memory is arguably the most important, as it involves playing traffic cop to myriad requests from the CPU and AGP buses as well as the data link to the Southbridge functions. Keeping all these components supplied with data is the core duty of the Northbridge, and its performance is a a factor of its timings, buffers, and memory clock speeds.

The last is the easiest distinction to make — due to extra memory bandwidth, a 2.0GHz Pentium 4 running with DDR400 system memory will outperform one using DDR266. The actual memory timings of the Northbridge itself are a bit harder to grasp, but even slight enhancements can have a significant impact.

Intel’s 845PE is a good example: This chipset upgrades the i845E to DDR333 support, so it’s no surprise that it surpasses its predecessor in overall performance, especially with memory-intensive applications. What’s unexpected, however, is that the i845PE is also noticeably faster than the older, DDR333-equipped i845G platform. We know Intel did some revision work with the i845PE and GE chipsets, and it seems likely that faster memory timings were among the results.

Controlling the CPU bus — moving data to and from the processor and interacting with various levels of cache — is another important function of the Northbridge. The faster the chipset can do this, the more efficient the CPU can be in terms of making the most of its clock speed or internal performance.

The Northbridge also determines the motherboard’s level of CPU support. Early Intel 845-series chipsets, for example, supported only then-current Pentium 4 processors’ 400MHz front-side bus, not the newer 533MHz models. In the vast majority of motherboard architectures, the front-side or system bus drives the other bus speeds, so running a 533MHz Pentium 4 on a nonsupported platform will raise other bus speeds by a corresponding amount — probably well out of specifications. Even the Athlon XP, a relatively minor revision to the Athlon core, ran into problems with some older chipsets, due to the strict chipset compatibility guidelines that both AMD and Intel require.

Between the Bridges

As you can imagine, the Northbridge-to-Southbridge interface is also an extremely important facet of chipset design. Older chipsets used the PCI bus for this data link, but while the overall bandwidth (133MB/sec at 33MHz) was adequate in those days, the number of other components riding the PCI bus served to lower available bandwidth quite significantly. With hard drives, sound cards, network interface cards, and other peripherals sharing the PCI bus, the latter became a real system bottleneck as CPU, AGP, and memory speeds continued to ramp up.

The solution was to create a dedicated Northbridge-to-Southbridge link apart from any other system bus. This really started to become prevalent once high-bandwidth DDR memory came into vogue; VIA, for instance, instituted a dedicated 266MB/sec V-Link for its KT266 and P4X266 chipsets and upgraded it to 533MB/sec for the newer KT400 and P4X400.

As mentioned above, this is one area where single-chip design can yield significant benefits — witness the 1.2GB/sec bandwidth boast of the one-piece SiS 745 and 735. By contrast, the HyperTransport link joining the two halves of Nvidia’s nForce chipset is one of the fastest such connections at 800MB/sec.

Connecting to Everything Else

Compared to the Northbridge, the chipset’s Southbridge component provides support for a wide variety of devices with many differing bus speeds and designs. Control over secondary buses — USB, IDE, PS/2, Ethernet — is the Southbridge’s domain; if a new interface technology like IEEE 1394 FireWire or Gigabit Ethernet is announced, chances are that it’ll show up in a Southbridge.

It’s certainly nice to have access to the newest technologies, but the performance of the Southbridge is not to be overlooked. Hard-disk transfer rates are one of the most common system bottlenecks, and if an enhanced Southbridge offers faster burst-mode transfers or better use of on-drive cache, then overall application response will improve. Intel chipsets use a relatively slow 266MB/sec interface between the Northbridge and Southbridge components, but at the same time are well-known for first-class IDE performance levels. This would seem to be a paradox, but really illustrates once again that efficient use of available resources is key to winning at the chipset game.

The process of integrating additional features into the chipset (usually meaning the Southbridge), as opposed to other motherboard chips or expansion cards, seems to be moving ever faster as system integrators try to hit ever-decreasing price points. By integrating peripheral functionality into the chipset, many times all that is required for a basic PC platform is a motherboard with memory slots.

Examples include standard Southbridge features such as LAN and USB adapters and audio, which replace dedicated hardware and allow motherboards to reach incredibly low per-unit prices. Some motherboard vendors such as ECS have made integration their bread and butter, making many of their designs a perfect fit for low-cost home or business desktops.

Alternatives to AGP Cards

Perhaps the most radical departure — and one of the only integrated features affecting the Northbridge, since it requires direct access to system memory — involves on-chip graphics. The biggest popularizer of this cost-cutter, Intel, recently updated its “Extreme Graphics” with the 845GE and 845GV, while both VIA and SiS are reportedly hard at work building chipsets that incorporate their latest Savage3D and Xabre graphics cores, respectively.

The mover and shaker in this category continues to be Nvidia, whose nForce and nForce2 are arguably the only graphics-built-in chipsets that deliver performance on par with at least entry-level dedicated graphics cards. The speed of integrated video depends on a great many factors, from graphics core specifications to memory bus design and (since some system memory is borrowed for the display) DDR speeds, as well as support for Microsoft’s DirectX 8.1/9.0 and other 3D and multimedia APIs.

But again, with the nForce products as an exception, most integrated graphics to date have been more suitable for economy models than high-performance PCs, for office productivity applications rather than image or video editing or 3D gaming. The reason it’d be nice for home-PC shoppers to know whether their prospective purchase uses an Intel 845G or 845GL, for example, is that while neither will set Quake III Arena or Unreal Tournament on fire, the latter doesn’t support an AGP slot while the former at least makes graphics upgrades possible.

Looking ahead, we see chipsets continuing to advance in two areas — a higher-performance Northbridge and more highly integrated Southbridge components. Expect upcoming designs to offer dual-channel DDR memory controllers, faster timings, and enhanced CPU and memory performance, while including integrated features that will give the likes of Creative Labs and 3Com headaches.

Categories: Technology