The PC evolves constantly since it’s debuted. 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 standardization, which has promoted competition and, in turn, technical innovation.

The heart of a PC system – the processor (CPU), is no different 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 standardized. 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. This has changed once other manufacturers particularly AMD started to catch on.

The CPU is connected to the 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.

Over the years as CPUs improved and grew, various slots and sockets where developed to allow new features, higher clockrates, greater bandwidth and more. There are a bunch of different connectors used by CPUs. Here’s a short list of their names, codenames:

FC-PGA – With Intel’s high-density 0.18 micron process they can now fit the level-2 cache on the main CPU chip, so the Slot-1 daughter board is no longer necessary. The form factor for these new chips is physically the same as the old Socket-370, but the pinouts are somewhat different. Still, you can get an adaptor to run an FC-PGA CPU in a Socket-370 motherboard.

FC-PGA2 – An update to the FC-PGA socket. Backwards compatible – FC-PGA CPUs can be used in FC-PGA2 sockets. Current info is that FC-PGA2 CPUs have a heat-spreader added, but it is unclear what difference this makes for the socket.

LGA775 – Also known as Socket-T. Third-generation Pentium IV socket – started showing up in mid-2004.

mPGA603 / mPGA604 – These are Intel’s sockets specifically for the Pentium-IV Xeon. The socket equivalent of Slot-2. The difference between the 603-pin version and the 604-pin version is that Xeons with a 533MHz or 800MHz FSB have the extra pin, therefore they don’t fit into boards with the 603-pin socket. The extra pin isn’t connected to anything, it’s just there to enforce the bus speed difference. The older 603-pin Xeons will fit into the 604-pin sockets and function correctly, at their usual 400MHz bus speed.

Slot-1 – (242-leads) With the later Pentium IIs and early Celerons, Intel started packaging the main CPU chip plus some separate level-2 cache memory chips on a little daughter board. This is just because it wouldn’t all fit onto a single chip. Slot-1 was also used by the Pentium III line.

Slot-2 – (330-leads) Intel’s server version of the Slot 1. Used by Intel’s Xeon processors. Similar to Slot-1, but with more memory lines. You can get a very simple adaptor that lets you use a Slot-1 CPU in a Slot-2 motherboard.

Slot-A – (242-leads) With the Athlon Classic, AMD did the same thing as Intel, putting CPU and level-2 cache on a little daughter board. Their board was not compatible with Intel’s Slot-1 though for patent reasons.

Socket-1 – (169-pins) Used for Intel486 SX and Intel486 DX. Operated at 5 volts. This socket takes the Intel OverDrive processors (ODP).

Socket-2 – (238-pins) A minor upgrade from Socket-1 that supported all the same chips. Used for all 5 volt Intel486 SX, Intel486 DX and IntelDX2.

Socket-3 – (237-pins) 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 classic of the 486 sockets.

Socket-4 – (237-pins) Holds the original Pentium processor (the 60 and 66MHz versions). These processor run on 5 volts, and are well known for the massive amount of heat they produce.

Socket-5 – Were originally designed to handle what is now called the ‘Pentium Classic’ processor. These include Pentium 75 through Pentium 133. Operated at 3.3 volts. Not compatible with later chips because of their requirement for an additional pin.

Socket-6 – (235-pins) Those sockets are extremely rare and are the final upgrade option for 486-class processors. They have a 235-pin ZIF and support low voltage (3.3v and 3.45v) processors such as the Intel 486DX4-75 and 100

Socket-7 – (321-pins) There are actually three versions of the Socket 7 motherboard. The original socket was unable to support split voltage processors such as the 6x86L and various MMX-compatible processors. The second type of board supports various split-voltages (2.8v and 2.9v in particular) and sometimes has all three multiplier pins. The newest, called Super-7, has a full range of voltages from 2.0v up through 3.5v in 0.1v increments, all three multiplier pins, supports 100MHz bus speeds, and adds an AGP graphics slot. Not all Socket 7 motherboards support the unusual 40, 55, 75, 83, and 95MHz bus speeds found in various processors.

Socket-8 – (387-pins) Socket 8 are Intel’s proprietary socket for Intel Pentium Pro processor. The socket is a unique modified staggered pin grid array. The Pentium Pro was Intel’s high-end server chip, so you will often see dual and even quad configurations on Socket 8 boards. The Pentium Pro is not only physically and electrically dissimilar to its Socket 7 cousins, but it also uses entirely different chipsets than the other CPUs.

Socket-370 – (370-pins) 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 (look above). Used by alsoby VIA’s M-III CPU.

Socket-423 – (423-pins) In late 2000 Intel introduced the Pentium IV processor, and it uses a new socket design with 423 pins. This is not compatible with the 370-pin socket used by Pentium IIIs, nor (of course) with AMD’s 453-pin Socket-A.

Socket-478 – (478-pins) This is a micro-PGA socket that holds Intel’s Pentium 4 processor. 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 reduced.

Socket-603 – (603-pins) 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-745 – AMD’s socket for the Athlon 64.

Socket-748 – Second-generation Pentium IV socket.

Socket-754 – (754-pins) AMD’s 754-pin CPU interface form factor introduced with its 64-bit Athlon 64 processor in the autumn of 2003.

Socket-939 – (939-pins) 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. Unbuffered memory can be used with this socket.

Socket-940 – (940-pins) AMD’s socket for the Opteron and the Athlon 64-FX. Memory used on this socket must be buffered.

Socket-A – (462-pins) In mid-2000 AMD introduced the new Duron processor and the redesigned Athlon, with on-chip full speed level-2 cache just like Intel’s latest CPUs. At that point they switched from a slot form back to a socket.

Socket-T – See LGA775

Super-7 – Not really a different connector, this refers to motherboards which use Socket-7 but run it at 100MHz instead of 66MHz. Still used by AMD’s low-end CPUs, the K6-2 and K6-III.