Socket 370
The Socket 370 processor socket, a ZIF type PGA socket
A CPU socket or CPU slot is a connector on a computer's motherboard that accepts a CPU and forms an electrical interface with it. As of 2007, most desktop and server computers, particularly those based on the Intel x86 architecture, include socketed processors.
Most CPU-sockets interfaces are based on the pin grid array (PGA) architecture, in which short, stiff pins on the underside of the processor package mate with holes in the socket. To minimize the risk of bent pins, zero insertion force (ZIF) sockets allow the processor to be inserted without any resistance, then grip the pins firmly to ensure a reliable contact after a lever is flipped.
As of 2007, several current and upcoming socket designs use land grid array (LGA) technology instead. In this design, it is the socket which contains pins. The pins contact pads or lands on the bottom of the processor package.
In the late 1990s, many x86 processors fit into slots, rather than sockets. CPU slots are single-edged connectors similar to expansion slots, into which a PCB holding a processor is inserted. Slotted CPU packages offered two advantages: L2 cache memory could be upgraded by installing an additional chip onto the processor PCB, and processor insertion and removal was often easier. However, slotted packages require longer traces between the CPU and chipset, and therefore became unsuitable as clock speeds passed 500 MHz. Slots were abandoned with the introduction of AMD's Socket A and Intel's Socket 370.
Socket 462 (Socket A)
Socket 462 (also called Socket A) is a PGA socket designed for AMD K7 family of processors. This socket can be used with AMD Athlon and Duron processors ranging in speed from 600 MHz to 2200 MHz (3200+) and with bus frequences ranging from 100 MHz to 200 MHz (400 MHz DDR). For a full list of supported processors please see below.
Socket A has 462 pin holes (hence the name "Socket 462") with 11 pluged pin holes. Socket dimensions are 5.59 cm (5.24 cm without lever) x 6.55 cm or 2.2" (2.06" without lever) x 2.58", which is almost the same as Socket 7/Super Socket 7.
Socket A (also known as Socket 462) is the CPU socket used for AMD processors ranging from the Athlon Thunderbird to the Athlon XP/MP 3200+, and AMD budget processors including the Duron and Sempron. Socket A also supports the recent AMD Geode NX embedded processors (derived from the Mobile Athlon XP). The socket is a zero insertion force pin grid array type with 453 pins (nine pins are blocked in the socket to prevent accidental insertion of Socket 370 CPUs, hence the number 462). The front side bus frequencies supported for the AMD Athlon XP and Sempron are 133 MHz, 166 MHz, and 200 MHz.
AMD recommends that the mass of a Socket A CPU cooler not exceed 300 grams (10.6 ounces). Heavier coolers may result in damage to the die when the system is not properly handled.
Socket A has been discontinued in favor of Socket 754, Socket 939, and recently Socket AM2, except for its use with Geode NX processors. However, microprocessors and motherboards from many vendors are still available.
Socket 478 (mPGA478B)
Socket 478 (mPGA478B) is a Pin Grid Array (PGA) socket for microprocessors based on Intel NetBurst architecture. This socket was introduced in August 2001 as replacement for short-lived socket 423. The socket 478 supports desktop and mobile Pentium 4 and Celeron processors from 1.4 GHz to 3.46 GHz with effective front-side bus frequencies 400 MHz - 1066 MHz (100 MHz - 266 MHz QDR). This socket was phased out in favour of socket 775 (LGA775).
One major difference between the socket 478 and older sockets is the socket size. The distance between pin holes in socket 478 is twice smaller than the distance between pin holes in the socket 370, which was used by older Celeron, Pentium and Pentium III families, and in the socket 423 used by first generation of Pentium 4 microprocessors. This results in much smaller socket size - only 1.38" x 1.38" (3.5 cm x 3.5 cm). Another difference between socket 478 and earlier sockets is the arrangement of pins. Pentium and newer CPUs, including socket 423 Pentium 4 microprocessors, had pins arranged as two grids where one grid was diagonally shifted relative to another grid by 1/2 of the distance between pins. In mPGA478B the pins are arranged as one grid, that is the same way as in Intel 80486 and older microprocessors.
The mPGA478B has grid size 26 x 26 with a 14 x 14 section removed from the center of the grid. Two pin-holes in one corner of the socket are plugged, so there is only one way to install the CPU. The total number of pins is (26 x 26) - (14 x 14) - 2 = 478.
Socket 370 (PGA370)
Socket 370 (also called PGA370) is a PGA socket designed to work with Intel Celeron and Pentium III processors in Pin Grid Array (PGA) package. There are three different revisions of the socket 370:
Originally the Socket 370 (PGA370) was designed for Celeron Mendocino CPUs. These processors were manufactured in plastic PGA (PPGA) package, ran at speeds 300 MHz - 533 MHz, and had 66 MHz front-side bus and 2V core voltage. Original Socket 370 is not compatible with Coppermine and Tualatin Intel processors. It is possible to run Coppermine processors in this socket with the help of special PPGA-to-FC-PGA adapters, for example, PowerLeap PL-NeoS370.
To support newer
Coppermine Celeron and Pentium III processors Intel revised specifications for the socket 370. The revised socket was mechanically compatible with the original specification, but there were differences in electrical specifications. As a result, new Coppermine CPUs would not work in the original socket, even though they would fit into it. Revised Socket 370 supported Coppermine processors running at speeds 533 MHz - 1133 MHz with FSB 100 and 133 MHz and core voltages in the range from 1.5V to 1.8V. Tualatin Pentium III processors were not directly supported by the socket, though it was possible to run them in the socket with the help of Tualatin adapters (for example, PowerLeap Neo/T). Many motherboards with this PGA370 socket revision also supported Celeron processors in PPGA package.
With the introduction of new Tualatin Pentium III core the socket 370 was revised once again. Like the previous revision of the socket, the second revision was mechanically compatible with old sockets, but electrically incompatible with previous socket revisions. This socket revision supported Tualatin processors in the range from 1 GHz to 1.4 GHz with FSB 100 and 133 MHz and core voltages 1.45V and 1.5V. Majority of motherboards with this revision of the socket 370 were also compatible with older Coppermine Pentium III and Celeron processors. Visually it's not possible to distinguish between different revisions of the socket 370 (PGA370), they all look like the socket on the picture below: 
The socket has 370 pin-holes - hence the name Socket 370, and it has the same size as the Socket 7. There are two differences between these sockets:
Socket 370 processors don't have 2 corner pins while Socket 7 processors don't have only one corner pin. The extra corner pin doesn't allow Socket 7 processors to fit into PGA370 socket.
There are 6 rows of pins between the edge and the center of the processor for Socket 370 processors, and 5 rows for Socket 7 processors.
Socket 423 (PGA423) 
Socket 423, also called PGA423, is a Pin Grid Array (PGA) socket introduced together with Pentium 4 microprocessor family in November 2000. The socket supported only Pentium 4 CPUs with Willamette core and was replaced with socket 478 just in 9 months after its introduction.
The size of PGA423 socket is 2.1" x 2.1" (5.33 cm x 5.33 cm). The socket has 423 pin holes arranged as 39 x 39 matrix with 26 x 26 section of pins removed. No corner pins in the socket are plugged, but, because the 26 x 26 section is removed not exactly from the center of the pin grid, there is only one way to insert the CPU into the socket.
Socket 7 (Socket7)
Socket 7 was introduced by Intel for it's Pentium 133 - 200 MHz processors and for Pentium MMX processor family. The major feature of the new socket was support for dual plane voltage - the socket could supply different voltages to processor core and I/O logic. At the same time, the socket 7 was backward compatible with socket 5, and it was possible to run older (single voltage) processors in socket 7 motherboards. For their next generation of processors Intel chose different socket type - slot 1, and completely abandoned socket 7. Luckily, Intel competitors continued to support socket 7 architecture, and they even enhanced it by creating a "Super socket 7" specification by adding support for 100 MHz bus frequency, backside L2 cache and frontside L3 cache.
Socket 7 has 321 pin holes arranged as 37 x 37 pin matrix. The socket has the same size as the Socket 5, but the socket 5 has only 320 pin holes. The extra pin on socket 7 processors is not electrically connected and it's main purpose is to prevent socket 7 processors to be inserted into socket 5 motherboards.
Socket 775 (LGA775)
Socket 775 (LGA775) is a Land Grid Array (PGA) socket used for the most recent generations of desktop Intel microprocessors starting from Pentium 4 and up to Core 2 Quad / Core 2 Extreme families. The socket LGA775 supports Intel CPUs from 1.8 to 3.8 GHz with Front-Side Bus frequencies ranging from 533 MHz to 1066 MHz. This socket superseded socket 478.
The Socket 775 differs from all earlier x86 sockets in the sense that it doesn't have pin holes. Instead, the socket has 775 gold-plated contacts arranged as a grid 33 x 30 contacts with 15 x 14 section de-populated in the center, with one corner contact and 4 contacts on two sides on the socket removed. After the processor is placed into the socket it has to be secured by pushing socket's load lever down. The LGA775 socket is guaranteed to last at least 20 insert cycles. 
The size of LGA775 socket is 1.48" x 1.48" (3.75 cm x 3.75 cm), or about 15% larger than the size of socket 478. At the same time the socket 775 has 60% more contacts. The increase in the number of contacts without equivalent increase of socket size was achieved by reducing the distance between contacts. Another advantage of the new socket design is the lower resistance of the contacts, which reduces the amount of heat generated by socket and socket contacts.
LF-LGA775 socket is RoHS-compliant version of LGA775 socket.
Socket 939
Socket 939 is a PGA socket designed for desktop K8 microprocessors. The socket is primarily used with Athlon 64, Athlon 64 X2 and Athlon FX microprocessors. AMD also manufactured uni-processor single and dual-core Opteron processors for the socket 939, although these processors were just re-branded Athlon 64/64 X2 CPUs with larger level 2 cache. In addition to that small number (compared to the total number of processors for this socket) of socket 939 Sempron processors was produced for this socket.
The socket can be used with processors with internal frequencies from 1.8 GHz to 3 GHz, or with rated frequencies 3000+ - 4800+. All processors working in this socket have one HyperTransport link, dual-channel DDR memory controller, and support DDR-200 - DDR-400 unbuffered memory DIMMs.
