Building your PC:
The Motherboard
Introduction
Though used almost synonymously with the mainboard in computer systems, the motherboard is actually a generic term referring to the mainboard of any device where substantial or complex calculations occur. Thus a computer, games console, PDA or handheld game will usually have a motherboard whilst other complex devices such as a TV, stereo or microwave will have a mainboard. A motherboard can also be called a mainboard, logic board, system board or it may even be named using the abbreviation mobo. A typical modern motherboard provides direct connections for the CPU, memory, graphics card, sound card and network cards. The motherboard also provides controllers and connectors for hard disk drives, CD and DVD drives as well as a floppy drive by means of ribbon cables.
The Motherboard:
History:
Almost by definition the motherboard has been a feature of computers since the very earliest days. However, it was only with the advent of semiconductor devices and automated assembly during the early 1970s that the motherboard as we know it today was born. For the early years of the computer, however, most computers were produced as single units by the machine's manufacturer. Even here motherboard design was evolving from the PC to the XT form factors. IBM made this an 'open standard' allowing a number of other manufacturers to copy their deigns. However, it was not until IBM came up with the AT (advanced technology) design in 1984 that the component-based PC which we know today was born. The AT form factor (see image, left) proved very popular and the various IBM PC clone manufacturers of the time began to use AT-compatible designs. Though not revolutionary in design the AT form factor provided for a remote power switch (now standard) as well as the ability to build a motherboard that would fit into desktop, mini-tower and full tower cases. The power supply for the AT motherboard was also increased from the 84W of previous models to 194W allowing for the addition of far more components and peripherals.
In 1995 Intel developed the ATX (Advanced Technology Extended) motherboard form factor. This represented the first major design evolution in motherboard and case designs for almost a decade and ATX motherboards were not compatible with previous AT designs. Unlike the AT design an ATX power supply does not directly connect to the system power button, allowing the computer to be powered off via software. However, many ATX power supplies have a switch on the back to ensure no power is flowing to the motherboard (a trickle of energy is normally sent to an ATX-style motherboard even if the computer appears to be "off"). Since the ATX PSU uses the motherboard's power switch, to turn on the power for use in situations that do not utilize an ATX motherboard it is possible to activate the power supply by shorting the green wire from the ATX connector to any black wire on the connector (or ground). In addition the power supply's connection to the motherboard was changed. An ATX power supply uses a single connector that can only be plugged into the motherboard in a single orientation (thus alleviating the problem encountered with the AT motherboard that the power supply was reversible and could short-circuit the motherboard). The other major change was to dedicate a single area at the back of the motherboard where all connectors could be placed. (In the AT design only the keyboard connector could go here). ATX motherboards also saw the ubiquitous introduction of the PS/2 keyboard and mouse rather than the large 5-pin DIN connector of the AT design.
In 2005 Intel introduced the BTX (Balanced Technology Extended) form factor for PC motherboards. This motherboard was designed to alleviate some of the power and thermal problems of the ageing ATX design. However, as yet it has not received widespread support and the ATX form factor remains the current industry standard.
The Motherboard: How it Works
Admittedly, the image above is quite 'busy'. Seemingly there's a lot happening. So let's focus on individual components and explain what they do.
1. CPU Zif Socket
This is the socket in which the CPU (the main chip or 'central processing unit') actually sits. In fact the socket (left) is composed of two parts. The white central region where the CPU itself sits and the black outer layer where the CPU's cooling fan sits. The central CPU housing is called a ZIF (which stands for zero insertion force) as it has a plastic upper layer and a metal lower layer. By means of a crank handle the upper and lower parts slide apart leaving a clear path by which the pins on the underside of the CPU housing can be inserted. Once the CPU is firmly in place the handle is levered down and this pinches the pins of the CPU against the metal base plate, making an electrical connection.
Each zif socket is specific for a certain type of CPU. As such the CPU has to be matched exactly to the 'socket type' of the CPU. As of 2006 the current socket types are:
Intel Processors- Socket 6 — 80486DX4
- Socket 7 — Pentium and Pentium MMX, AMD and some Cyrix CPUs)
- Socket 8 — Intel Pentium Pro
- Slot 1 — Intel Pentium II, older Pentium III, and Celeron processors (233 MHz–1.13 GHz)
- Slot 2 — Intel Xeon processors based on Pentium II/III cores
- Socket 370 — Celeron processors and newer Pentium IIIs (800 MHz–1.4 GHz)
- Socket 423 — Intel Pentium 4 and Celeron processors (based on the Willamette core)
- Socket 478 — Intel Pentium 4 and Celeron processors (based on Northwood, Prescott, and Willamette cores)
- Socket 479 — Intel Pentium M and Celeron M processors (based on the Banias and Dothan cores)
- Socket 480 — Intel Pentium M processors (based on the Yonah core)
- Socket 603/604 — Intel Xeon processors based on the Northwood and Willamette Pentium 4 cores
- Socket T/LGA 775 (Land Grid Array) — Intel Pentium 4 and Celeron processors (based on Northwood and Prescott cores)
- Slot A — original AMD Athlon processors
- Socket 462 (aka Socket A) — newer AMD Athlon, Athlon XP, Sempron, and Duron processors
- Socket 754 — lower end AMD Athlon 64 and Sempron processors with single-channel memory support
- Socket 939 — AMD Athlon 64 and AMD Athlon FX processors with dual-channel memory support
- Socket 940 — AMD Opteron and early AMD Athlon FX processors
In matching processors to motherboard the easiest way is to decide on which particular processor you want, check its socket type and then look at all the matching motherboards. Make a list of these and then check their features against what you want your computer to achieve. That way you can decide on purchasing your CPU and motherboard in tandem to ensure an exact match.
2. Northbridge and Southbridge Chipsets
The Northbridge Chipset
This rather unprepossessing pair of chips actually play a very important role on the motherboard. Along with the CPU the Northbridge and Southbridge chips form the 'core logic chipset' of the motherboard. Though both chipsets have been incorporated into a single die in the past it is far more common to see them as two separate microprocessors.
The schematic (left) shows the relationship between the CPU and the Northbridge and Southbridge chipsets. This diagram also explains the names Northbridge and Southbridge. If the CPU is considered as lying due North then the next chip in the sequence is the northernmost 'bridge' between the CPU and the remainder of the system whilst the final (most southernmost bridge) is the Southbridge.
It is the Northbridge chipset that is often the primary factor in deciding the number, speed and type of CPU that can be utilized as well as the amount, speed and type of RAM that can be accessed. It is the Northbridge's clock frequency that is used as a baseline for the CPU to determine its own frequency. Because different processors and RAM require different signalling, a northbridge will typically work with only one or two classes of CPUs and generally only one type of RAM. Also, a Northbridge chip will typically only work with one or two different Southbridge integrated circuits. Thus it could be argued that the northbridge is the most important factor in determining which technologies are available on a given motherboard.
However, the Northbridge may be a technology on the verge of obsolescence. In the latest generation of AMD 64 processors the memory controller has already been incorporated into the CPU itself and with the evolution of PCI express (see below) as a video card technology the need for a Northbridge chipset to drive video controllers is also falling by the wayside.
The Southbridge ChipsetIn contrast with the Northbridge chipset which mediates all the 'fast' capabilities of the computer (memory and graphics) the Southbridge chip effectively implements the 'slower' capabilities of the motherboard. On contemporary motherboards the Southbridge chipset will contain interfaces for the PCI bus (where additional cards are added [see below]), the system management bus (which handles things like the interrupt button and a modem's 'wake on LAN' functionality) the DMA controller (which allows certain subsystems on the motherboard to directly access system memory), the drive controller (both IDE and SATA), the LPC (low pin count controller) that allows low bandwidth devices such as audio controllers and the boot ROM to communicate with the CPU, serial and parallel ports, keyboard and mouse connectors), the RTC (real-time clock) which is the motherboard chip that maintains track of the current date and time even when the computer is turned off, the power management subsystem (which allows the BIOS to perform power management tasks such as slowing the speed of the CPU and the BIOS memory. In addition the Southbridge of modern systems may include support for Ethernet, RAID, audio codec and FireWire devices.
3. Memory Slots
Under the control of the Northbridge chipset, these memory slots represent where the memory modules are inserted in a modern PC. For a modern motherboard the memory will undoubtedly be DDR or DDR2 with the type determined by the clock-speed of the CPU (and ultimately on the clock-speed of the Northbridge chip). To learn more about memory modules and which ones to purchase for your system please visit this page.
4. BIOS
The acronym BIOS stands for Basic Input/Output System and represents the software code that's run by a computer when first turned on. The primary function of this is to prepare the machine so other software programs stored on various media (such as hard drives, floppies, and CDs) can load, execute, and assume control of the computer and it is this process which his generally known as 'booting-up'.
In effect the BIOS is a program that's encoded a chip (an example of which is shown above). Originally the BIOS was encoded onto PROM or EPROM programmable chips, but these days is is more commonly written onto flash memory which is included as an integral part of the motherboard. After initialization the BIOS performs an initial check on system integrity and then decompresses itself from the BIOS memory space on the flash RAM and loads itself into main memory where it starts executing. In addition nearly all BIOS implementations can optionally execute a setup program interfacing the nonvolatile BIOS memory (CMOS) which holds user-defined data accessed by BIOS code.
BIOS can sometimes be referred to as firmware as it is an integral part of the motherboard and thus the system hardware. With the advent of flash memory BIOS it is now possible to update the bios of a motherboard by flashing it (replacing the original code with new code) so that the motherboard can keep pace with updates in technology. This obviously can be a dangerous process because the BIOS can become corrupted and if it does the system will not boot. However, modern BIOSes evaluate their own integrity and if there is a problem with the BIOS (if it is 'corrupt') they will boot to a floppy drive so that the user can try flashing it again (this remains the most convincing reason for purchasing an integral floppy drive with a new computer system).
5. CMOS
On almost every modern motherboard you will notice a tiny button battery (like the one on the left). This is a small rechargeable cell that is used to power the Nonvolatile BIOS memory (generally referred to as CMOS) when the main power is off. In effect, the CMOS is an area of memory that contains BIOS settings and sometimes the code used to initialize the computer and load the operating system. Though modern motherboards utilize a flash memory chip or an EPROM for this function the original versions used a low-power CMOS memory chip and this was kept powered by the back-up battery. These days, however, as the memory itself is non-volatile the function of the battery is to power the RTC chip (this provides the real-time clock function that keeps track of time and date). On occasion, such as when a device is added to or removed from your PC the CMOS internal integrity check may report a CMOS mismatch. In this case the POST (power-on self-test) will fail and the computer will refuse to boot. The only way to overcome this is to re-set the CMOS which can be done by means of a jumper located on the motherboard. All settings are set to default and, with any luck, the problem can be rectified.
6. ATA Headers
ATA stands for Advanced Technology Attachment and represents a standard interface for connecting storage devices such as hard disks and CD/DVD drives inside personal computers. With the introduction of serial ATA technology in 2003 (see below) the standard ATA interface is now known as parallel ATA. Although the standard for this interface as always had the official name "ATA", marketing dictates dubbed an early version of the standard Integrated Drive Electronics (IDE), and the one following it Enhanced IDE (EIDE). The parallel ATA interface itself comprises of an 40-pin connector on the motherboard (above) which connects to an 80-wire ribbon cable that links the motherboard to the drive being attached. It should be noted that the specified maximum cable length for the ATA standard is just 46cm which can make connecting multiple drives on large cases difficult and though longer cables are available (and though they will generally work) they do lie outside the specified parameters of the standard. Modern motherboards generally have two ATA risers which are usually of the ATA-133 standard (often known as Ultra DMA 133 [UDMA133]) where the maximum data transfer rate over the PCI bus on the motherboard is 133 Mb/sec. This standard also removed the bottleneck of older specifications, allowing drives of 200Gb or greater to be attached.
Most modern motherboards have two ATA risers allowing up to four drives to be attached. Each riser can handle two drives; one device 0 (master) and one device 1 (slave). In the ATA133 standard the master device is defined as the drive at the end of the cable and the slave is the device attached to the middle connector. The jumper settings on the attached drives should correspond to this (see the hard drive page for more information).
7. Serial ATA Headers
Serial-ATA (usually abbreviated to S-ATA or SATA) stands for Serial Advanced Technology Attachment and is a standard for the attachment of external drives introduced in 2003. Primarily intended for the attachment of hard disks, SATA is seen as the successor to Parallel ATA technology (see above). First-generation Serial ATA interfaces, also known as SATA/150, run at 1.5 gigahertz, resulting in an actual data transfer rate of 1.2 gigabits per second (Gbit/s), or 150 megabytes per second. This transfer rate is only slightly higher than that provided by the fastest Parallel ATA mode, Ultra ATA at 133 MB/second (UDMA/133), but the relative simplicity of a serial link allows for the use of longer cables and also provides a much easier path for transitioning to higher speeds. Almost all modern motherboards now come with at least one SATA connector (left). In 2004 the SATA/300 specification was released, allowing a doubling of the clock rate to 3GHz for a maximum throughput of 300 MB/s or 2.4 gigabits per second (Gbit/s). SATA/300 is backwards-compatible with SATA/150 devices, allowing SATA/150 hardware to interface with SATA/300 ports and SATA/300 hardware with SATA/150 ports (albeit the latter at the slower 150 MB/s data rate). Somewhere in 2007 it is expected that there will be a further increase in the maximum throughput of Serial ATA to 600 MB/s, 4.8 gigabits per second (Gbit/s).
The most notable difference between parallel-ATA and S-ATA lies in the cabling. he Serial ATA standard defines a data cable using seven conductors and 8 mm wide wafer connectors on each end and these cables can be up to a metre in length.
SATA also drops the master/slave concept, giving each device its own dedicated connection, once again allowing for better data throughput. As all modern motherboards can boot from a SATA drive, if you have a motherboard that supports this technology buy a hard drive that supports it and use this as your main drive. Relegate parallel-ATA to DVD/CD drives and secondary hard drives.
8. FDD Header
The floppy drive is considered a 'legacy' item on the motherboard these days. It is no longer viewed as an essential component of a PC build, though it can still be useful in recovering from a BIOS update problem. However, all motherboards currently include a 34-pin FDD header. This can be connected to a floppy disk drive via a 34-wire ribbon cable which implements an ATA interface specifically for the floppy disk drive. Plugging the floppy drive is a simple matter of adding the drive to the case, plugging-in the power cable and attaching the data cable to the drive and the motherboard. It should be noted that some (especially older) BIOSes require the presence of a floppy drive before they pass the initial POST (power-on self-test) process and begin loading the operating system. For more on the floppy disk drive please view this page.
9. Graphics Connector
In common with many other components of the motherboard, graphics connectors have undergone considerable change, from dedicated cards to standard PCI cards to AGP and now PIC-E. Most modern motherboards (even the ones that come with on-board graphics) will contain either and AGP or a PCI-E slot for a dedicated graphics card.
AGP GraphicsOf the two current formats for graphics cards this is the oldest (the slot for these types of cards is shown above, top. AGP is an acronym for Advanced Graphics Port which is a high-speed speed point-to-point channel for attaching a graphics card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. The original specification of AGP (1x) used a 32-bit channel operating at 66 MHz resulting in a maximum data rate of 266 megabytes per second (MB/s), doubled from the 133 MB/s transfer rate of PCI bus 33 MHz / 32-bit; 3.3 V signaling. Every few years the data transfer rate was doubled until 2004 when the AGP 8x standard was released. This delivered an effective 533 MHz resulting in a maximum data rate of 2133 MB/s (2 GB/s); 0.8 V signaling.
AGP grew from the need for more complex image processing that the limited standard PCI bus could deliver. A dedicated graphics bus and slot allowed better data throughput and more realistic rendering of three-dimensional environments.
PCI-E GraphicsPCI-E stands for PCI-Express (sometimes known as PCIe). This is the latest specification for graphics card technologies and the port for these cards is shown in the bottom pane of the above image. It is an implementation of the PCI computer bus that uses existing PCI programming concepts, but bases it on a completely different and much faster serial physical-layer communications protocol which allows for considerable parallelism in the interface and therefore large data throughput. Because of this parallelism the size of the PCIe socket grows between the 1x, 4x, 8x and 16x versions of this interface.
The PCIe link is built around a bidirectional, serial (1-bit), point-to-point connection known as a "lane". This is in sharp contrast to the PCI connection, which is a bus-based system where all the devices share the same unidirectional, 32-bit, parallel bus. A connection between any two PCIe devices is known as a "link", and is built up from a collection of 1 or more lanes. All devices must minimally support single-lane (x1) links. Devices may optionally support wider links composed of 2, 4, 8, 12, 16, or 32 lanes. As a result of this PCIe is useful in applications other than graphics cards and currently PCI Express appears to be well on its way to becoming the new backplane standard in personal computers.
Most of the latest graphics cards from all the major manufacturers now use PCI-Express and if you're buying a new motherboard it is worth buying one employing this technology.
10. Expansion Ports
The expansion ports as sited at the back of the motherboard and align with the back-plate of the case. It is via these prots that expansion cards such as sound cards, network cards, wireless cards and firewire cards. Originally such devices were attached to an ISA (Industry Standard Architecture) bus which was developed by IBM around 1981. This provided the first standard whereby any manufacturer could provide an add-on card to extend the capabilities of a PC. The original bus was 8-bit but was replaced by a 16-bit architecture in the mid 1980s. The ISA bus survived into the early 1990s when it was finally displaced by the PCI bus.
PCI BusLike its predecessor the VESA bus, the PCI bus (short for Peripheral Component Interconnect) standard specifies a computer bus for attaching peripheral devices to a computer motherboard. The standard itself was released in 1992 and the full specification of the connector and motherboard slot was published in 1993 and it was not until the release of the second generation pentium processors in late 1994 that the PCI bus began to achieve some measure of market penetration in consumer PCs. By the early 2000s the PCI bus had become common, though EISA (Extended Industry Standard Architecture, a 32-bit extension to VESA) slots continued to be added alongside. It was not until 2003 that the EISA bus disappeared entirely and newer motherboards came with PCI buses only (or maybe a single low-throughput CNR slot for modems.
The PCIe BusThe PCI-express (PCIe) bus standard was published in 2003 and was initially adopted for video cards (see above). However, the architecture is multi-purpose and obviates the need for a Northbridge chip. Modern motherboards tend to come with a few PCI connectors and a number of PCIe connectors (see above) and, as well as graphics cards, most new Gigabit ethernet cards and even some wireless cards now use PCIe. Undoubtedly the trend will be to replace PCI with PCIe and other peripheral cards (such as sound cards) will soon be based around the PCIe standard. For the moment ensure you have a PCIe card for your graphics card and make sure you have enough PCI slots for your standard add-on cards.
11. Back Pane Connectors
According to the ATX motherboard specification a square region to the far right of the motherboard's rear edge is reserved for all the connectors and devices directly supported by the motherboard itself. As standard there will be PS/2 D-type connectors for a keyboard and a mouse as well as a parallel prot to support a printer and probably at least one COM port (these are considered the most basic external connector ports on a computer and provide serial connectivity for modems and PDAs). However, all the ports mentioned thus far could be considered legacy systems and all their functionality is now encapsulated within the USB universal serial bus protocol. You should have at least two of these prots on a modern motherboard and many manufacturers have done away with the serial ports entirely so that they can include more USB connectors.
Most modern motherboards will also include an RJ-45 connector for ethernet connection to a local network. Many modern motherboards will also come with audio connectors and a game port connector. It may even have a video port if the motherboard supports on-board graphics.
All these port functions are supported by add-on cards, so the lesson is to pick a motherboard that has all the connections built-in for what you want to do (another option available with some motherboards being FireWire).
12. Additional Connectors
There are a few other connectors on the motherboard that you need to know about and I'll run through these quickly:
ATX Power Connector
This is the main power connector on the motherboard and a single connector from the ATX power supply will plug in here. The connector is directional so that it can only fit one way.
CPU supplementary Power Connector
On many modern ATX motherboards there is an additional power connector that provides power solely to the CPU.
Fan Connector
Next to the CPU Zif socket there is a three-pin connector where the CPU fan plugs in. This both powers the fan and controls the fan speed and power-up cycles. On some motherboards there may be a second one of these to power a case fan.
Audio Connector
Almost all motherboards these days have a supplementary connector to which the audio cable from a CD or DVD can be connected. This links the audio output from those devices to the motherboard's audio system so that sound can be output to external speakers.
Summary
Having read through this document you should now have some idea of the history of motherboard development as well as some knowledge of all the components on a motherboard and what they do. Hopefully this will allow you to know what to look out for when planning to purchase your own motherboard.
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