The motherboard is the main circuit board inside the PC which holds the processor, memory and expansion slots and connects directly or indirectly to every part of the PC. It's made up of a chipset (known as the "glue logic"), some code in ROM and the various interconnections or buses. PC designs today use many different buses to link their various components. Wide, high-speed buses are difficult and expensive to produce: the signals travel at such a rate that even distances of just a few centimetres cause timing problems, while the metal tracks on the circuit board act as miniature radio antennae, transmitting electromagnetic noise that introduces interference with signals elsewhere in the system. For these reasons, PC design engineers try to keep the fastest buses confined to the smallest area of the motherboard and use slower, more robust buses, for other parts.
   This section focuses on basic functionality and layout - the motherboard's various interfaces, buses and chipsets being covered elsewhere.
   The original PC had a minimum of integrated devices, just ports for a keyboard and a cassette deck (for storage). Everything else, including a display adapter and floppy or hard disk controllers, were add-in components, connected via expansion slots.
   Over time, more devices have been integrated into the motherboard. It's a slow trend though, as I/O ports and disk controllers were often mounted on expansion cards as recently as 1995. Other components - typically graphics, networking, SCSI and sound - usually remain separate. Many manufacturers have experimented with different levels of integration, building in some or even all of these components. However, there are drawbacks. It's harder to upgrade the specification if integrated components can't be removed, and highly integrated motherboards often require non-standard cases. Furthermore, replacing a single faulty component may mean buying an entire new motherboard.
   Consequently, those parts of the system whose specification changes fastest - RAM, CPU and graphics - tend to remain in sockets or slots for easy replacement. Similarly, parts that not all users need, such as networking or SCSI, are usually left out of the base specification to keep costs down.
The basic changes in motherboard form factors over the years are covered later in this section - the diagrams below provide a detailed look at the various components on two motherboards. The first a Baby AT design, sporting the ubiquitous Socket 7 processor connector, circa 1995. The second is an ATX design, with a Pentium II Slot 1 type processor connector, typical of motherboards on the market in late 1998.

   Motherboard development consists largely of isolating performance-critical components from slower ones. As higher speed devices become available, they are linked by faster buses - and the lower-speed buses are relegated to supporting roles. In the late 1990s there was also trend towards putting peripherals designed as integrated chips directly onto the motherboard. Initially this was confined to audio and video chips - obviating the need for separate sound or graphics adapter cards - but in time the peripherals integrated in this way became more diverse and included items such as SCSI, LAN and even RAID controllers. While there are cost benefits to this approach the biggest downside is the restriction of future upgrade options.

   BIOS

   All motherboards include a small block of Read Only Memory (ROM) which is separate from the main system memory used for loading and running software. The ROM contains the PC's Basic Input/Output System (BIOS). This offers two advantages: the code and data in the ROM BIOS need not be reloaded each time the computer is started, and they cannot be corrupted by wayward applications that write into the wrong part of memory. A Flash upgradeable BIOS may be updated via a floppy diskette to ensure future compatibility with new chips, add-on cards etc.
   The BIOS comprises several separate routines, serving different functions. The first part runs as soon as the machine is powered on. It inspects the computer to determine what hardware is fitted and then conducts some simple tests to check that everything is functioning normally - a process called the power-on self test (POST). If any of the peripherals are plug and play devices, it's at this point that the BIOS assigns their resources. There's also an option to enter the Setup program. This allows the user to tell the PC what hardware is fitted, but thanks to automatic self-configuring BIOSes this isn't used so much now.
   If all the tests are passed, the ROM then tries to determine which drive to boot the machine from. Most PCs ship with the BIOS set to check for the presence of an operating system in the floppy disk drive first (A:), then on the primary hard disk drive. Any modern BIOS will allow the floppy drive to be moved down the list so as to reduce normal boot time by a few seconds. To accommodate PCs that ship with a bootable CD-ROM, some BIOSes allow the CD-ROM drive to be assigned as the boot drive. Some also allow booting from a hard disk drive other than the primary IDE drive. In this case it would be possible to have different operating systems - or separate instances of the same OS - on different drives. Many BIOSes allow the start-up process to be interrupted to specify the first boot device without actually having to enter the BIOS setup utility itself. If no bootable drive is detected, a message is displayed indicating that the system requires a system disk. Once the machine has booted, the BIOS serves a different purpose by presenting DOS with a standardised API for the PC hardware. In the days before Windows, this was a vital function, but 32-bit "protect mode" software doesn't use the BIOS, so again it's of less benefit today.
   Windows 98 (and later) provides multiple display support. Since most PCs have only a single AGP slot, users wishing to take advantage of this will generally install a second graphics card in a PCI slot. In such cases, most BIOSes will treat the PCI card as the main graphics card by default. Some, however, allow either the AGP card or the PCI card to be designated as the primary graphics card.
Whilst the PCI interface has helped - by allowing IRQs to be shared more easily - the limited number of IRQ settings available to a PC remains a problem for many users. For this reason, most BIOSes allow ports that are not in use to be disabled. With the increasing popularity of cable and ADSL Internet connections and the ever-increasing availability of peripherals that use the USB interface, it will often be possible to get by without needing either a serial or a parallel port.

   CMOS RAM

   Motherboards also include a separate block of memory made from very low power consumption CMOS (complementary metal oxide silicon) RAM chips, which is kept "alive" by a battery even when the PC's power is off. This is used to store basic information about the PC's configuration: number and type of hard and floppy drives, how much memory, what kind and so on. All this used to be entered manually, but modern auto-configuring BIOSes do much of this work, in which case the more important settings are advanced settings such as DRAM timings. The other important data kept in CMOS memory is the time and date, which is updated by a Real Time Clock (RTC). The clock, CMOS RAM and battery are usually all integrated into a single chip. The PC reads the time from the RTC when it boots up, after which the CPU keeps time - which is why system clocks are sometimes out of sync. Rebooting the PC causes the RTC to be reread, increasing their accuracy.

   Form factor

   Early PCs used the AT form factor and 12in wide motherboards. The sheer size of an AT motherboard caused problems for upgrading PCs and did not allow use of the increasingly popular slimline desktop cases. These problems were largely addressed by the smaller version of the full AT form factor, the Baby AT, introduced in 1989. Whilst this remains a common form factor, there have been several improvements since. All designs are open standards and as such don't require certification. A consequence is that there can be some quite wide variation in design detail between different manufacturers' motherboards.

   BAT

   The Baby AT (BAT) format reduced the dimensions of the motherboard to a typical 9in wide by 10in long, and BAT motherboards are generally characterised by their shape, an AT-style keyboard connector soldered to the board and serial and parallel port connectors which are attached using cables between the physical ports mounted on the system case and corresponding connectors located on the motherboard. With the BAT design the processor socket is located at the front of the motherboard, and full-length expansion cards are intended to extend over it. This means that removing the processor requires the removal of some or all expansion cards first. Problems were exacerbated by the increasing speeds of Pentium-class processors. System cooling relied on the AT power supply blowing air out of the chassis enclosure and, due to the distance between the power supply and the CPU, an additional chassis fan or active heatsink became a necessity to maintain good airflow across the CPU. AT power supplies only provide 12V and 5V outputs to the motherboard, requiring additional regulators on the motherboard if 3.3V components (PCI cards or CPUs) are used. Sometimes a second heatsink was also required on these voltage regulators and together the various additional heat dissipation components caused serious obstruction for expansion slots. Some BAT designs allow the use of either AT or ATX power supplies, and some ATX cases might allow the use of a Baby-AT motherboard.

The table below compares the dimensions of the microATX, FlexATX and ITX form factors:

   Riser architectures

   In the late 1990s, the PC industry developed a need for a riser architecture that would contribute towards reduced overall system costs and at the same time increase the flexibility of the system manufacturing process. The Audio/Modem Riser (AMR) specification, introduced in the summer of 1998, was the beginning of a new riser architecture approach. AMR had the capability to support both audio and modem functions. However, it did have some shortcomings, which were identified after the release of the specification. These shortcomings included the lack of Plug and Play (PnP) support, as well as the consumption of a PCI connector location.
   Consequently, new riser architecture specifications were defined which combine more functions onto a single card. These new riser architectures combine audio, modem, broadband technologies, and LAN interfaces onto a single card. They continue to give motherboard OEMs the flexibility to create a generic motherboard for a variety of customers. The riser card allows OEMs and system integrators to provide a customised solution for each customer's needs. Two of the most recent riser architecture specifications include CNR and ACR.
   Intel's CNR Communication and Networking Riser) specification defines a hardware scalable OEM motherboard riser and interface that supports the audio, modem, and LAN interfaces of core logic chipsets. The main objective of this specification is to reduce the baseline implementation cost of features that are widely used in the "Connected PC", while also addressing specific functional limitations of today's audio, modem, and LAN subsystems.
   PC users' demand for feature-rich PCs, combined with the industry's current trend towards lower cost, mandates higher levels of integration at all levels of the PC platform. Motherboard integration of communication technologies has been problematic to date, for a variety of reasons, including FCC and international telecom certification processes, motherboard space, and other manufacturer specific requirements.
   PC users' demand for feature-rich PCs, combined with the industry's current trend towards lower cost, mandates higher levels of integration at all levels of the PC platform. Motherboard integration of communication technologies has been problematic to date, for a variety of reasons, including FCC and international telecom certification processes, motherboard space, and other manufacturer specific requirements.

   The rival ACR specification is supported by an alliance of leading computing and communication companies, whose founders include 3COM, AMD, VIA Technologies and Lucent Technologies. Like CNR, it defines a form factor and interfaces for multiple and varied communications and audio subsystem designs in desktop OEM personal computers. Building on first generation PC motherboard riser architecture, ACR expands the riser card definition beyond the limitation of audio and modem codecs, while maintaining backward compatibility with legacy riser designs through an industry standard connector scheme. The ACR interface combines several existing communications buses, and introduces new and advanced communications buses answering industry demand for low-cost, high-performance communications peripherals.
   ACR supports modem, audio, LAN, and xDSL. Pins are reserved for future wireless bus support. Beyond the limitations of first generation riser specifications, the ACR specification enables riser-based broadband communications, networking peripheral and audio subsystem designs. ACR accomplishes this in an open-standards context.
   Like the original AMR Specification, the ACR Specification was designed to occupy or replace an existing PCI connector slot. This effectively reduces the number of available PCI slots by one, regardless of whether the ACR connector is used. Though this may be acceptable in a larger form factor motherboard, such as ATX, the loss of a PCI connector in a microATX or FlexATX motherboard - which often provide as few as two expansion slots - may well be viewed as an unacceptable trade-off. The CNR specification overcomes this issue by implementing a shared slot strategy, much like the shared ISA /PCI slots of the recent past. In a shared slot strategy, both the CNR and PCI connectors effectively use the same I/O bracket space. Unlike the ACR architecture, when the system integrator chooses not to use a CNR card, the shared PCI slot is still available.
   Although the two specifications both offer similar functionality, the way in which they are implemented are quite dissimilar. In addition to the PCI connector/shared slot issue, the principal differences are as follows:
   — ACR is backwards compatible with AMR, CNR isn't
   — ACR provides support xDSL technologies via its Integrated Packet Bus (IPB) technology; CNR provides such support via the well-established USB interface
   — ACR provides for concurrent support for LCI (LAN Connect Interface) and MII (Media Independent Interface) LAN interfaces; CNR supports either, but not        both at the same time
   — The ACR Specification has already reserved pins for a future wireless interface; the CNR specification has the pins available but will only define them when        the wireless market has become more mature.
   Ultimately, motherboard manufacturers are going to have to decide whether the ACR specification's additional features are worth the extra cost.

   CPU interfaces

   The PC's ability to evolve many different interfaces allowing the connection of many different classes of add-on component and peripheral device has been one of the principal reasons for its success. The key to this has been standardisation, which has promoted competition and, in turn, technical innovation.
The heart of a PC system - the processor - is no different in this respect than any other component or device. Intel's policy in the early 1990s of producing OverDrive CPUs that were actually designed for upgrade purposes required that the interface by which they were connected to the motherboard be standardised. A consequence of this is that it enabled rival manufacturers to design and develop processors that would work in the same system. The rest is history.
In essence, a CPU is a flat square sliver of silicon with circuits etched on its surface. This chip is linked to connector pins and the whole contraption encased some form of packaging - either ceramic or plastic - with pins running along the flat underside or along one edge. The CPU package is connected to a motherboard via some form of CPU interface, either a slot or a socket. For many years the socket style of CPU was dominant. Then both major PC chip manufacturers switched to a slot style of interface. After a relatively short period of time they both changed their minds and the socket was back in favour!
The older 386, 486, classic Pentium and Pentium MMX processors came in a flat square package with an array of pins on the underside - called Pin Grid Array (PGA) - which plugged into a socket-style CPU interface on the motherboard. The earliest such interface for which many motherboards and working systems remain to this day - not least because it supported CPUs from so many different chip manufacturers - is Socket 7. Originally developed by Intel as the successor to Socket 5, it was the same size but had different electrical characteristics including a system bus that ran at 66MHz. Socket 7 was the interface used by most Pentium systems from the 75MHz version and beyond.
   Socket 8 was developed for Intel's Pentium Pro CPU - introduced in late 1995 - and specifically to handle its unusual dual-cavity, rectangular package. To accommodate L2 cache - in the package but not on the core - this contained up to three separate dice mounted on a small circuit board. The complicated arrangement proved extremely expensive to manufacture and was quickly abandoned.
   With the introduction of their Pentium II CPU, Intel switched to a much cheaper solution for packaging chips that consisted of more than a single die. Internally, the SECC package was really a circuit board containing the core processor chip and cache memory chips. The cartridge had pins running along one side which enabled it to be mounted perpendicularly to the motherboard - in much the same way as the graphics or sound card is mounted into an expansion slot - into an interface that was referred to as Slot 1. The up to two 256KB L2 cache chips ran at half the CPU speed. When Intel reverted - from the Pentium III Coppermine core - to locating L2 cache on the processor die, they continued to use cacheless Slot 1 packaging for a while for reasons of compatibility.
   Pentium II Xeon's - unlike their desktop counterparts - ran their L2 cache at full clock speed. This necessitated a bigger heatsink which in turn required a taller cartridge. The solution was Slot 2, which also sported more connectors than Slot 1, to support a more aggressive multi-processor protocol amongst other features.
   When Intel stopped making its MMX processor in mid-1998 it effectively left the Socket 7 field entirely to its competitors, principally AMD and Cyrix. With the co-operation of both motherboard and chipset manufacturers their ambitious plans for extending the life of the "legacy" form factor was largely successful.
   AMD's determination to match Intel's proprietary Slot 1 architecture on Socket 7 boards was amply illustrated by their 0.25-micron K6-2 processor, launched at the end of May 1998, which marked a significant development of the architecture. AMD referred to this as the "Super7" platform initiative, and its aim was to keep the platform viable throughout 1999 and into the year 2000. Developed by AMD and key industry partners, the Super7 platform supercharged Socket 7 by adding support for 100MHz and 95MHz bus interfaces and the Accelerated Graphics Port (AGP) specification and by delivering other leading-edge features, including 100MHz SDRAM, USB, Ultra DMA and ACPI.
   When AMD introduced their Athlon processor in mid-1999 they emulated Intel's move away from a socket-based CPU interface in favour of a slot-based CPU interface, in their case "Slot A". This was physically identical to Slot 1, but it communicated across the connector using a completely different protocol - originally created by Digital and called EV6 - which allowed RAM to CPU transfers via a 200MHz FSB. Featuring an SECC slot with 242 leads, Slot A used a Voltage Regulator Module (VRM), putting the onus on the CPU to set the correct operating voltage - which in the case of Slot A CPUs was a range between 1.3V and 2.05V.
   Slot-based processors are overkill for single-chip dies. Consequently, in early 1999 Intel moved back to a square PGA packaging for its single die, integrated L2 cache, Celeron range of CPUs. Specifically these used a PPGA 370 packaging, which connected to the motherboard via a Socket 370 CPU interface. This move marked the beginning of Intel's strategy for moving its complete range of processors back to a socket-based interface. Socket 370 has proved to be one of the more enduring socket types, not least because of the popularity of the cheap and overclockable Celeron range. Indeed, Intel is not the only processor manufacturer which produces CPUs that require Socket 370 - the Cyrix MIII (VIA C3) range also utilising it.
   The sudden abandonment of Slot 1 in favour of Socket 370 created a need for adapters to allow PPGA-packaged CPUs to be used in Slot 1 motherboards. Fortunately, the industry responded, with Abit being the first off the mark with its original "SlotKET" adapter. Many were soon to follow, ensuring that Slot 1 motherboard owners were not left high and dry. A Slot 1 to Socket 370 converter that enables Socket 370-based CPUs to be plugged into a Slot 1 motherboard was also produced. Where required, these converters don't just provide the appropriate connector, they also make provision for voltage conversion.
   Unfortunately users were more inconvenienced by Intel's introduction of the FC-PGA (Flip Chip-Pin Grid Array) and FC-PGA2 variants of the Socket 370 interface - for use with Pentium III Coppermine and Tualatin CPUs respectively - some time later. The advantage with this packaging design is that the hottest part of the chip is located on the side that is away from the motherboard, thereby improving heat dissipation. The FC-PGA2 package adds an Integral Heat Spreader, improving heat conduction still further. Whilst FC-PGA and FC-PGA2 are both mechanically compatible with Socket 370, electrically they're incompatible and therefore require different motherboards. Specifically, FC-PGA processors require motherboards that support VRM 8.4 specifications while FC-PGA2 processors require support for the later VRM 8.8 specifications.
   Like Intel's Slot 1, AMD's proprietary Slot A interface was also to prove to be relatively short-lived. With the advent of the Athlon Thunderbird and Spitfire cores, the chipmaker followed the lead of the industry leader by also reverting to a PPGA-style packaging for its new family of Athlon and Duron processors. This connects to a motherboard via what AMD calls a "Socket A" interface. This has 462 pin holes - of which 453 are used by the CPU - and supports both the 200MHz EV6 bus and newer 266MHz EV6 bus. AMD's subsequent Palomino and Morgan cores are also Socket A compliant.
   With the release of the Pentium 4 in late 2000, Intel introduced yet another socket to its line-up, namely Socket 423. Indicative of the trend for processors to consume ever decreasing amounts of power, the PGA-style Socket 423 has a VRM operational range of between 1.0V and 1.85V.
   Socket 423 had been in use for only a matter of months when Intel muddied the waters still further with the announcement of the new Socket 478 form factor. The principal difference between this and its predecessor is that the newer format socket features a much more densely packed arrangement of pins known as a micro Pin Grid Array (¦ÌPGA) interface, which allows both the size of the CPU itself and the space occupied by the interface socket on the motherboard to be significantly reduced. Socket 423 was introduced to accommodate the 0.13-micron Pentium 4 Northwood core, launched at the beginning of 2002.
   The table below identifies all the major CPU interfaces from the time of Intel's Socket 1, the first "OverDrive" socket used by Intel's 486 processor in the early 1990s: