29 Dec, 2006

The Mainboard

After the CPU chip, the mainboard or "motherboard" is the most important component of any personal computer. Intel makes a small number of its own boards, but most systems use motherboards from companies such as Abit, Asus, MSI, SuperMicro, Tyan, …

Mainboards come in sizes. A typical full sized computer uses an ATX mainboard which has room for 4 memory sticks and 7 PCI or PCI-e adapter cards. A smaller size board and case supports the MATX standard, which typically has only two memory slots and room for 4 adapter cards. Although the mainboard is smaller, MATX vendors typically find room for integrated video on the mainboard so you don’t necessarily need to use up an adapter card slot. There are oversized boards, but they are only used in servers. There are tiny boards used in specialty devices.

The mainboard is attached to a tray in the bottom or side of the case by nine screws that screw into metal “standoffs” that keep the bottom of the mainboard a safe distance from the metal of the case. Everything else plugs into the mainboard:

The CPU drops into the mainboard socket.

A mainboard for Intel CPUs has an LGA 775 socket, but the speed of the clock generated by the mainboard to run the CPU has to be fast enough to support newer CPU models.
AMD Athlon 64 CPUs come in versions for Socket 939 and AM2. The difference is that 939 CPUs support older DDR memory and AM2 CPUs support DDR2 memory.
Server boards support a different Intel Athlon or AMD Opteron socket, but ordinary users will typically not encounter them. However, some Opteron chips are made for the 939 and AM2 sockets. They are equivalent to Althlon 64 processors, but with a larger 1 megabyte of cache.

DDR and DDR2 memory is rated by speed. A mainboard will be rated for a certain maximum speed, but it will slow down to support slower memory. Memory will be rated for a maximum speed, but it will also slow down if plugged into a slower board. So the only compatibility issue is to put DDR memory into a DDR mainboard, and DDR2 memory into a DDR2 mainboard.
Memory is rated for a certain speed, and the mainboard is rated for a certain maximum speed. However, in some cases the combination of a particular brand of low cost memory plus a particular brand of mainboard will cause trouble. The mainboard will try to run at the rated top speed, but occasionally the data will be corrupted. The problem can be solved by entering the BIOS configuration panels at power up and manually setting the memory speed to the next slower setting. There is no message telling you that you have the problem, and until you realize it the computer will crash in many different ways at different times. In order for the computer to detect a memory problem, it needs an extra memory feature called ECC. All servers use ECC. Microsoft would love computer makers to put ECC memory in every computer, because they get blamed for all the failures that are really caused by memory problems. However, ECC costs a bit more and computer makers unwisely shave a few bucks off the cost of a system by using cheaper unchecked memory. If you can find a combination of mainboard, CPU, and memory that supports ECC, it is the single most valuable upgrade you can consider.
A modern mainboard has some combination of old flat “parallel” ATA connectors and new, small Serial ATA (SATA) connectors. SATA is better, but until recently you needed the old flat cable for DVD drives. During 2007 we may see a transition to all SATA.
The two leading makers of video chips are Nvidia and ATI. AMD bought ATI last year, and Nvidia makes a very popular and powerful set of chips for controlling mainboards. As a result, the quality and capability of mainboards that come with integrated graphics is improving. Anyone playing video games still wants a separate video card. Integrated video typically uses some of the computer’s main memory, and contention for memory between ordinary programs running on the CPU and video requirements has an impact on overall system performance. However, business users and adults who just want to run Windows, Office, and view TV or DVDs may be quite happy with the video that comes on a mainboard. If you need more, you can always buy a video card later on.
Mainboards also have integrated audio. If your tastes are simple, this will be perfectly adequate. However, games may require computer generated sounds, and when you have the power, computer processing of even recorded sound can add impressive effects that make ordinary headphones sound richer. An add-on audio card from companies like Creative will be able to do far more sound processing, if you need it.
All mainboards have USB capability. However, to run external disks at their full speed, a external form of SATA called “eSATA” is becoming popular. New mainboards may have one or two eSATA slots on the back panel next to the USB and Ethernet.
Modern mainboards have some combination of modern PCI Express (PCI-e) slots and older PCI slots. PCI-e card slots come in sizes. Video always uses a long “x16” slot. Other types of cards can fit into smaller x1 and x4 slots. PCI-e cards have a size, but they can always be plugged into a larger mainboard slot. An x1 card will also plug into an x4 or x16 slot. However, a larger card will not fit into a smaller size slot. Other than video, PCI-e cards are still rather exotic devices. There are a few disk controllers and TV tuners. There is currently no PCI-e add-in sound card. The high end boards sold to people who play video games have room for two oversize video cards, but then they can support only two old PCI cards. Less expensive boards with one PCI-e video slot and two PCI-e x1 or x4 slots leave room for up to four old PCI slots and may be more useful to mainstream users.

Chip Set

The core of each mainboard is a pair of chips collectively referred to as "the Chip Set". They sit in the middle of the mainboard and are connected to everything else.

Intel makes its own Chip Sets for people who want a high quality, conservative, middle of the road system. A bit more function at lower cost is provided by alternate chip sets from companies named VIA, SIS, and Nvidia. The support for CPU, memory, and PCI is pretty much the same from all vendors, so the choice of mainboard and chipset may be driven by video, USB 2, FireWire, audio, and integrated LAN.

Each vendor has different Chip Sets for Intel and AMD systems. Even for an Intel Pentium IV, however, there are different FSB CPU speeds (400, 533, 800), different DDR memory speeds (266, 333, 400, and dual bus 400).

The first chip in the set is called the "Northbridge". It connects to the three high speed devices: the CPU, memory, and video card. Most of the time the Northbridge moves data between the CPU and memory.

The second chip, called the "Southbridge", provides the control function for all the other devices.

The Southbridge generates the PCI bus and typically generates a few extra PCI Express lines.
The Southbridge contains the controller function for the parallel and Serial ATA hard disks and DVD drive. Some mainboards have a secondary SATA controller chip connected to the Southbridge using one PCI-e lane.
The Southbridge generates ports for USB and for all the standard low speed devices (keyboard, mouse, serial port, printer port).
One Ethernet port is supported by the Southbridge, but some mainboards add a second Ethernet port supported by an external chip connected to the Southbridge.

Each control function of the Southbridge started out as a separate device with its own controller chip. In fact, in the first IBM PC, the Serial Port and Printer Port each came on separate adapter cards. Over the next 25 years each of the old support chips were combined, new control functions were added, and then they two were combined with the old functions. The result is a single Southbridge chip that combines functions that at one time were on dozens of separate chips.

In order to maintain compatibility with all the operating systems and applications previously written, new chips continue to pretend to be each old chip they replace. Thus the Southbridge doesn't behave like a single device, but rather like dozens of individual devices each with their own I/O addresses, interrupt levels, and individual states and status.

It is important to remember that the Southbridge only provides the control functions of all these devices. Control logic operates at the low voltages associated with a chip's internal processing. The external devices being controlled (keyboards, mice, USB, Ethernet, etc.) have long external cables that require higher voltage and often require additional power lines. So the Southbridge chip isn't directly connected to the keyboard it controls. There have to be additional intermediate circuits (traditionally called "drivers" and "receivers") to take Southbridge signals and step them up to 5 or 12 volts on the way out, and then to accept signals at 5 or 12 volts and step that signal back down to the lower voltage that can be accepted by the Southbridge.

AMD and HyperTransport

Putting a Northbridge chip between the CPU and the memory adds a slight delay. AMD decided a few years ago to avoid the delay and simplify the mainboard by connecting the memory directly to the CPU chip. This makes things slightly more complicated, because the CPU socket changes for each type of memory (DDR or DDR2, with or without ECC). However, it has turned out to be such a good design that Intel intends to adopt it a few years from now.

When the memory is connected to the CPU, then there is no need for a Northbridge chip per se. There are two solutions. For low cost systems, all the PCI-e support (including the 16 lines needed to run a video card) move to a slightly enhanced Southbridge. Such systems have only the one chip and support only one video card. More expensive systems add a second chip that creates additional PCI-e lines to support a second video card, and maybe a few extra SATA connections.

AMD needed a standard connection between the CPU and the one or two external mainboard chips. It adopted an industry standard called HyperTransport. HT is a very high speed connection between chips on a mainboard. When three or more chips are connected together, middle chips act as a “tunnel” to receive and forward on data transmitted between external chips.

The AMD design is particularly good when you want to add additional CPU chips. In the Intel design, more than one CPU (even more than one core inside the same chip) is supported through the Northbridge. In the AMD design, cores that are part of the same chip can talk directly to each other, and one CPU chip talks to another CPU chip directly over a HT link.