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 (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 478 was introduced to accommodate the 0.13-micron Pentium 4 Northwood core, launched at the beginning of 2002.

In the autumn of 2003 AMD brought two versions of their K8 architecture to the desktop market, branded the Athlon 64 and the Athlon 64 FX. The new chips signalled the end of the line for the venerable Socket A CPU interface. The Athlon 64 saw the introduction of Socket 754 and has a single-channel integrated memory controller. The Athlon 64 FX used the Opteron’s dual-channel memory controller design and the same Socket 940 interface.

The summer of 2004 saw the Athlon 64 line converged to a new platform, Socket 939. Importantly, this allowed both the consumer and high-end models to use unbuffered memory in a dual channel configurations.

At about the same time, Intel unveiled major changes in its platform technology, including new PCI Express compatible chipsets and the innovative LGA775 CPU interface (also referred to as Socket T). The first Pentium 4 processors to use the company’s processor numbers model naming scheme – the Prescott-based 520 to 560 CPUs – all used the new platform technology.

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:

Name Interface Description
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 5×86. 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 additional pin.
Socket 6 235-pin Designed for use with 486 CPUs, 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 7 321-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 Athlon CPU.
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. Its 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 reduced.
Socket 754 754-pin AMD’s 754-pin CPU interface form factor introduced with its 64-bit Athlon 64 processor in the autumn of 2003, finally replacing the longstanding and highly succesful Socket A. Targeted at budget desktop and mobile 64 bit computing.
Socket 940 940-pin AMD’s 940-pin CPU interface form factor originally used by Opteron and FX versions of the Athlon 64 CPU. Subsequently replaced for use by the latter by Socket 939, which allowed for a less-expensive motherboard option, one with only four layers rather than from six to nine.
Socket 939 939-pin AMD’s 939-pin CPU interface form factor introduced in the summer of 2004. The Socket 939 marked the convergence of the mainstream and FX versions of the Athlon 64 CPU, which had previously used different interfaces, the Socket 754 and Socket 940 respectively.

Socket T

775-pin Land Grid Array 775: Intel’s proprietary CPU interface form factor introduced in the summer of 2004. Similar to a pin grid array (PGA), the connection between LGA775 chip packaging and the processor chip is via an array of contacts rather than pins to sockets providing better power distribution to the processor. Used for some Pentium 4, Pentium D, Core 2 Duo and Core 2 Quad CPUs.
Socket AM2 940-pin Released just before the Intel LGA771, the AM2 came as AMD’s replacement for the Socket 939 and and Socket 754. Offering support for an array of processors and DDR2 RAM,

Socket J

771-pin Land Grid Array 771: Released in June 2006, the Socket J processor was released to support Intel’s Server and Workstation based Xeon processor range. The J stands for “Jayhawk,” a processor to have been released simultaneously but cancelled. The LGA771 supports the Dual Core Xeon Dempsey and Woodcrest, Quad Core Clovertown, and Core 2 Extreme processors.
Socket 479 479-pin Also referred to as the mPGA479M socket, Socket 479 is best known as the CPU socket for the Intel Pentium M mobile processor. The format was also used for desktop PCs, Asus making a drop-in board which allowed Socket 479 CPUs to be used in selected desktop motherboards. Intel subsequently announced a new Socket 479 with a revised pinout for its new generation of Core CPUs.
Socket F 1207-pin Released August 2006, AMD created the Socket F as a socket 940 replacement for its server line processors, particularly the Opteron range, though it also the high range Athlon 64 FX series of processors. Supporting AMD’s native quad core Opteron processors and DDR2 RAM, this socket gave AMD a considerable boost in server market share over Intel.
Socket AM2+ 940-pin Released November 2007, this step up from the socket AM2 improved HyperTransport support and energy efficiency, though only on AM2+ compatible chips with fully AM2+ motherboards. Seen as a halfway house to socket AM3, takeup on the socket was arguably disappointing.
LGA1336/ Socket B 1336-pin Land Grid Array 1336: Introduced in November 2008, superseding the LGA775/Socket T, to support Intel’s Core i7 Nehalem CPUs: Bloomfield, Gainestown and Westmere. DDR3 RAM is required for this socket.

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