Inside a PC
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.
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
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 tries to boot the machine from the hard disk. Failing that, it will try the CD-ROM drive, then the floppy drive,
finally displaying a message that it needs 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.
Most PCs ship with the BIOS set to check for the presence of an operating system in the floppy disk
drive first, 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.
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 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.
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.
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.
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.
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 LPX format is a specialised variant of the Baby-AT
used in low profile desktop systems and is a loose specification with a variety of proprietary implementations.
Expansion slots are located on a central riser card,
allowing cards to be mounted horizontally. However, this arrangement can make it difficult to remove the motherboard, and
the more complex engineering required adds to system costs. As the riser card prevents good airflow within the system case,
additional chassis fans are almost always needed.
The Intel Advanced/ML motherboard, launched in 1996, was designed to solve these issues and marked
the beginning of a new era in motherboard design. Its size and layout are completely different to the BAT format, following
a new scheme known as ATX. The dimensions of
a standard ATX board are 12in wide by 9.6in long; the mini ATX variant is typically of the order 11.2in by 8.2in.
The ATX design gets round the problem by moving the CPU socket and the voltage regulator to the right-hand
side of the expansion bus. Room is made for the CPU by making the card slightly wider, and shrinking or integrating components
such as the Flash BIOS, I/O logic and keyboard controller.
This means the board need only be half as deep as a full size Baby AT, and there's no obstruction whatsoever to the six expansion
slots (two ISA, one ISA/PCI, three PCI).
ATX uses a new specification of power supply that can be powered on or off by a signal from the motherboard. This allows notebook-style
power management and software-controlled shutdown and power-up. A 3.3V output is also provided directly from the power supply.
Accessibility of the processor and memory modules is improved dramatically, and relocation of the peripheral connectors allows
shorter cables to be used. This also helps reduce electromagnetic interference. The ATX power supply has a side vent that
blows air from the outside directly across the processor and memory modules, allowing passive heatsinks to be used in most
cases, thereby reducing system noise.
is simply a smaller version of a full-sized ATX board. On both designs, parallel, serial, PS/2 keyboard and mouse ports are located on a double-height I/O shield at the
rear. Being soldered directly onto the board generally means no need for cable interconnects to the on-board I/O ports. A
consequence of this, however, is that the ATX needs a newly designed case, with correctly positioned cut-outs for the ports,
and neither ATX no Mini-ATX boards can be used in AT-style cases.
Intel's NLX design, introduced in 1997, is an improvement on the LPX design for low-profile
systems, with an emphasis on ease of maintenance. The NLX format is smaller, typically 8.8in wide by 13in long, so well suited
for low-profile desktop cases.
expansion slots, power cables and peripheral connectors are located on an edge-mounted riser card, allowing simple removal
of the main motherboard, which is mounted on rails in the chassis. It uses a full-width I/O shield to allow for different
combinations of rear-panel I/O. The design allows for use of an AGP card, but the slot must be on the motherboard, which reduces
the ease of maintenance when such a card is implemented.
in the late 1990s, the MicroATX is basically a smaller version of Intel's ATX specification, intended for compact, low-cost
consumer systems with limited expansion potential.
The maximum size of the board is 9.6in square, and its designed to fit into either a standard ATX case or
one of the new micro-tower desktop designs. The double-decker I/O shield is the same as that on the ATX design, but there's
only provision for up to four expansion slots as opposed to the seven that ATX allows. The microATX also allows use of a smaller
power supply, such as the SFX design, which is reduced in both size and power output.
FlexATX is a natural evolution of the Intel's microATX form factor which was first unveiled in late 1999. The FlexATX addendum
to the microATX specification addresses the requirements of only the motherboard and not the overall system solution. As such, it does not detail the interfaces, memory or graphics technologies required to
develop a successful product design. These are left to the implementer and system designer. The choice of processor is, however,
limited to socket-only designs.
principal difference between FlexATX and microATX is that the new form factor reduces the size of the motherboard - to 9in
x 7.5in. Not only does this result in lower overall system costs, it also facilitates smaller system designs. The FlexATX
form factor is backwards compatible with both the ATX and micro-ATX specifications - use of the same motherboard mounting
holes as both of its predecessors avoids the need to retool existing chassis.
In the spring of 2000 VIA Technologies announced an even smaller motherboard than the
FlexATX. At 8.5in x 7.5in, the company's ITX form factor is half and inch less wide than it's Intel competitor. The key innovation
that allows the ITX to achieve such a compact form is the specially designed slimline power unit with built in fan. It's dimensions
of 174mm long x 73mm wide x 55mm high compare with a standard ATX power supply unit measuring 140mm x 150mm x 86mm.
The table below compares the dimensions of the microATX,
FlexATX and ITX form factors:
|Form Factor||Max. Width (mm)||Max. Depth (mm)|
Unsurprisingly Intel's FlexATX form factor uses
it's CNR riser architecture, while the ITX uses the
rival ACR architecture.
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
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.
Motherboard integration of the audio, modem, and LAN subsystems is also problematic, due to the potential
for increased noise, which in-turn degrades the performance of each system. The CNR specifically addresses these problems
by physically separating these noise-sensitive systems from the noisy environment of the motherboard.
With a standard riser solution, as defined in this specification, the
system manufacturer is free to implement the audio, modem, and/or LAN subsystems at a lower bill of materials (BOM) cost than
would be possible by deploying the same functions in industry-standard expansion slots or in a proprietary method. With the
added flexibility that hardware scalability brings, a system manufacturer has several motherboard acceleration options available,
all stemming from the baseline CNR interface.
CNR Specification supports the five interfaces:
- AC97 Interface - Supports
audio and modem functions on the CNR card
- LAN Connect Interface
(LCI) - Provides 10/100 LAN or Home Phoneline Networking capabilities for Intel chipset based solutions
- Media Independent Interface (MII) - Provides 10/100 LAN or Home Phoneline Networking
capabilities for CNR platforms using the MII Interface
Serial Bus (USB) - Supports new or emerging technologies such as xDSL or wireless
- System Management Bus (SMBus) - Provides Plug and Play (PnP) functionality on the CNR card.
Each CNR card can utilise a maximum
of four interfaces by choosing the specific LAN interface to support.
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.
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
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.
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 (µPGA) 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:
|Socket 1||169-pin||Found on 486 motherboards, operated at 5 volts and supported 486 chips, plus the DX2, DX4 OverDrive.|
|Socket 2||238-pin||A minor upgrade from
Socket 1 that supported all the same chips. Additionally supported a Pentium OverDrive.|
|Socket 3||237-pin||Operated at 5 volts,
but had the added capability of operating at 3.3 volts, switchable with a jumper setting on the motherboard. Supported all
of the Socket 2 chips with the addition of the 5x86. Considered the last of the 486 sockets. |
|Socket 4||273-pin||The first socket designed
for use with Pentium class processors. Operated at 5 volts and consequently supported only the low-end Pentium-60/66 and the
OverDrive chip. Beginning with the Pentium-75, Intel moved to the 3.3 volt operation. |
|Socket 5||320-pin||Operated at 3.3 volts
and supported Pentium class chips from 75MHz to 133MHz. Not compatible with later chips because of their requirement for an
|Socket 6||235-pin||Designed for use with 486 CPU's, this was an enhanced version of Socket 3 supporting operation at 3.3 volts. Barely
used since it appeared at a time when the 486 was about to be superseded by the Pentium.|
|Socket 7||32-pin||Introduced for the
Pentium MMX, the socket had provision for supplying the split core/IO voltage required by this and later chips. The interface
used for all Pentium clones with a 66MHz bus.|
|Socket 8||387-pin||Used exclusively by the Intel Pentium Pro, the socket proved extremely expensive to manufacture and was quickly dropped
in favour of a cartridge-based design. |
|Slot 1||242-way connector||The circuit board inside the package had up to 512KB of L1 cache on it - consisting of two 256KB chips - which ran at
half the CPU speed. Used by Intel Pentium II, Pentium III and Celeron CPUs.|
|Slot 2||330-way connector||Similar to
Slot 1, but with the capacity to hold up to 2MB of L2 cache running at the full CPU speed. Used on Pentium II/III Xeon CPUs.|
|Slot A||242-way connector||AMD interface
mechanically compatible with Slot 1 but which using a completely different electrical interface. Introduced with the original
|Socket 370||370-pin||Began to replace Slot 1 on the Celeron range from early 1999. Also used by Pentium III Coppermine and Tualatin CPUs
in variants known as FC-PGA and FC-PGA2 respectively.|
|Socket A||462-pin||AMD interface introduced with the first Athlon processors (Thunderbird) with on-die L2 cache. Subsequently adopted throughout
AMD's CPU range.|
|Socket 423||423-pin||Introduced to accommodate the additional pins required for the Pentium 4's completely new FSB. Includes an Integral
Heat Spreader, which both protects the die and provides a surface to which large heat sinks can be attached.|
|Socket 603||603-pin||The connector for
Pentium 4 Xeon CPUs. The additional pins are for providing more power to future CPUs with large on-die (or even off-die) L3
caches, and possibly for accommodating inter-processor-communication signals for systems with multiple CPUs. |
|Socket 478||478-pin||Introduced in anticipation
of the introduction of the 0.13-micron Pentium 4 Northwood CPU at the beginning of 2002. It's micro Pin Grid Array (µPGA)
interface allows both the size of the CPU itself and the space occupied by the socket on the motherboard to be significantly