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Processor Sockets and Slots

Intel and AMD have created a set of socket and slot designs for their processors. Each socket or slot is designed to support a different range of original and upgrade processors. Table 3.12 shows the specifications of these sockets.

Table 3.12 CPU Socket and Slot Types and Specifications

Socket Number


Pin Layout


Supported Processors

Socket 1


17x17 PGA


486 SX/SX2, DX/DX27, DX4 Overdrive

Socket 2


19x19 PGA


486 SX/SX2, DX/DX27, DX4 Overdrive, 486 Pentium Overdrive

Socket 3


19x19 PGA


486 SX/SX2, DX/DX2, DX4, 486 Pentium Overdrive, AMD 5x86

Socket 4


21x21 PGA


Pentium 60/66, Overdrive

Socket 5


37x37 SPGA


Pentium 75-133, Overdrive

Socket 68


19x19 PGA


486 DX4, 486 Pentium Overdrive

Socket 7


37x37 SPGA


Pentium 75-233+, MMX, Overdrive, AMD K5/K6, Cyrix M1/II

Socket Number


Pin Layout


Supported Processors

Socket 8


dual pattern SPGA

Auto VRM

Pentium Pro

Socket 370 (PGA370)


37x37 SPGA

Auto VRM

Celeron/Pentium III PPGA/FC-PGA

Slot A



Auto VRM

AMD Athlon PGA

Socket A (Socket 462)


PGA Socket

Auto VRM

AMD Athlon/Duron SECC

Slot 1 (SC242)



Auto VRM

Pentium II/III, Celeron SECC

Slot 2 (SC330)



Auto VRM

Pentium II/III Xeon

PGA = Pin Grid Array

PPGA = Plastic Pin Grid Array

FC-PGA = Flip Chip Pin Grid Array

SPGA = Staggered Pin Grid Array

SECC = Single Edge Contact Cartridge

VRM = Voltage Regulator Module

Sockets 1, 2, 3, and 6 are 486 processor sockets and are shown together in Figure 3.10 so you can see the overall size comparisons and pin arrangements between these sockets. Sockets 4, 5, 7, and 8 are Pentium and Pentium Pro processor sockets and are shown together in Figure 3.11 so you can see the overall size comparisons and pin arrangements between these sockets. More detailed drawings of each socket are included throughout the remainder of this section with the thorough descriptions of the sockets.

Figure 3.10 486 processor sockets.

Figure 3.11 Pentium and Pentium Pro processor sockets.

Socket 1

The original OverDrive socket, now officially called Socket 1, is a 169-pin PGA socket. Motherboards that have this socket can support any of the 486SX, DX, and DX2 processors, and the DX2/OverDrive versions. This type of socket is found on most 486 systems that originally were designed for OverDrive upgrades. Figure 3.12 shows the pinout of Socket 1.

Figure 3.12 Intel Socket 1 pinout.

The original DX processor draws a maximum 0.9 amps of 5v power in 33MHz form (4.5 watts) and a maximum 1 amp in 50MHz form (5 watts). The DX2 processor, or OverDrive processor, draws a maximum 1.2 amps at 66MHz (6 watts). This minor increase in power requires only a passive heat sink consisting of aluminum fins that are glued to the processor with thermal transfer epoxy. Passive heat sinks don't have any mechanical components like fans. Heat sinks with fans or other devices that use power are called active heat sinks. OverDrive processors rated at 40MHz or less do not have heat sinks.

Socket 2

When the DX2 processor was released, Intel was already working on the new Pentium processor. The company wanted to offer a 32-bit, scaled-down version of the Pentium as an upgrade for systems that originally came with a DX2 processor. Rather than just increasing the clock rate, Intel created an all new chip with enhanced capabilities derived from the Pentium.

The chip, called the Pentium OverDrive processor, plugs into a processor socket with the Socket 2 or Socket 3 design. These sockets will hold any 486 SX, DX, or DX2 processor, as well as the Pentium OverDrive. Because this chip is essentially a 32-bit version of the (normally 64-bit) Pentium chip, many have taken to calling it a Pentium-SX. It is available in 25/63MHz and 33/83MHz versions. The first number indicates the base motherboard speed; the second number indicates the actual operating speed of the Pentium OverDrive chip. As you can see, it is a clock-multiplied chip that runs at 2.5 times the motherboard speed. Figure 3.13 shows the pinout configuration of the official Socket 2 design.

Figure 3.13 238-pin Intel Socket 2 configuration.

Notice that although the new chip for Socket 2 is called Pentium OverDrive, it is not a full-scale (64-bit) Pentium. Intel released the design of Socket 2 a little prematurely and found that the chip ran too hot for many systems. The company solved this problem by adding a special active heat sink to the Pentium OverDrive processor. This active heat sink is a combination of a standard heat sink and a built-in electric fan. Unlike the aftermarket glue-on or clip-on fans for processors that you might have seen, this one actually draws 5v power directly from the socket to drive the fan. No external connection to disk drive cables or the power supply is required. The fan/heat sink assembly clips and plugs directly into the processor and provides for easy replacement if the fan fails.

Another requirement of the active heat sink is additional clearance—no obstructions for an area about 1.4 inches off the base of the existing socket to allow for heat-sink clearance. The Pentium OverDrive upgrade will be difficult or impossible in systems that were not designed with this feature.

Another problem with this particular upgrade is power consumption. The 5v Pentium OverDrive processor will draw up to 2.5 amps at 5v (including the fan) or 12.5 watts, which is more than double the 1.2 amps (6 watts) drawn by the DX2 66 processor.


See Intel's Web site (http://www.intel.com) for a comprehensive list of certified OverDrive-compatible systems.

Socket 3

Because of problems with the original Socket 2 specification and the enormous heat the 5v version of the Pentium OverDrive processor generates, Intel came up with an improved design. The new processor is the same as the previous Pentium OverDrive processor, except that it runs on 3.3v and draws a maximum 3.0 amps of 3.3v (9.9 watts) and 0.2 amp of 5v (1 watt) to run the fan—a total 10.9 watts. This configuration provides a slight margin over the 5v version of this processor. The fan will be easy to remove from the OverDrive processor for replacement, should it ever fail.

Intel had to create a new socket to support both the DX4 processor, which runs on 3.3v, and the 3.3v Pentium OverDrive processor. In addition to the new 3.3v chips, this new socket supports the older 5v SX, DX, DX2, and even the 5v Pentium OverDrive chip. The design, called Socket 3, is the most flexible upgradable 486 design. Figure 3.14 shows the pinout specification of Socket 3.

Figure 3.14 237-pin Intel Socket 3 configuration.

Notice that Socket 3 has one additional pin and several others plugged compared with Socket 2. Socket 3 provides for better keying, which prevents an end user from accidentally installing the processor in an improper orientation. However, one serious problem exists: This socket cannot automatically determine the type of voltage that will be provided to it. A jumper is likely to be added on the motherboard near the socket to enable the user to select 5v or 3.3v operation.


Because this jumper must be manually set, however, a user could install a 3.3v processor in this socket when it is configured for 5v operation. This installation will instantly destroy a very expensive chip when the system is powered on. So, it is up to the end user to make sure that this socket is properly configured for voltage, depending on which type of processor is installed. If the jumper is set in 3.3v configuration and a 5v processor is installed, no harm will occur, but the system will not operate properly unless the jumper is reset for 5v.

Socket 4

Socket 4 is a 273-pin socket that was designed for the original Pentium processors. The original Pentium 60MHz and 66MHz version processors had 273 pins and would plug into Socket 4—a 5v-only socket, because all the original Pentium processors run on 5v. This socket will accept the original Pentium 60MHz or 66MHz processor, and the OverDrive processor. Figure 3.15 shows the pinout specification of Socket 4.

Figure 3.15 273-pin Intel Socket 4 configuration.

Somewhat amazingly, the original Pentium 66MHz processor consumes up to 3.2 amps of 5v power (16 watts), not including power for a standard active heat sink (fan). The 66MHz OverDrive processor that replaced it consumes a maximum 2.7 amps (13.5 watts), including about one watt to drive the fan. Even the original 60MHz Pentium processor consumes up to 2.91 amps at 5v (14.55 watts). It might seem strange that the replacement processor, which is twice as fast, consumes less power than the original, but this has to do with the manufacturing processes used for the original and OverDrive processors.

Although both processors will run on 5v, the original Pentium processor was created with a circuit size of 0.8 micron, making that processor much more power-hungry than the newer 0.6 micron circuits used in the OverDrive and the other Pentium processors. Shrinking the circuit size is one of the best ways to decrease power consumption. Although the OverDrive processor for Pentium-based systems will draw less power than the original processor, additional clearance may have to be allowed for the active heat sink assembly that is mounted on top. As in other OverDrive processors with built-in fans, the power to run the fan will be drawn directly from the chip socket, so no separate power-supply connection is required. Also, the fan will be easy to replace should it ever fail.

Socket 5

When Intel redesigned the Pentium processor to run at 75, 90, and 100MHz, the company went to a 0.6 micron manufacturing process and 3.3v operation. This change resulted in lower power consumption: only 3.25 amps at 3.3v (10.725 watts). Therefore, the 100MHz Pentium processor can use far less power than even the original 60MHz version. The newest 120 and higher Pentium, Pentium Pro, and Pentium II chips use an even smaller die 0.35 micron process. This results in lower power consumption and allows the extremely high clock rates without over- heating.

The Pentium 75 and higher processors actually have 296 pins, although they plug into the official Intel Socket 5 design, which calls for a total 320 pins. The additional pins are used by the Pentium OverDrive for Pentium processors. This socket has the 320 pins configured in a staggered Pin Grid Array, in which the individual pins are staggered for tighter clearance.

Several OverDrive processors for existing Pentiums are currently available. If you have a first-generation Pentium 60 or 66 with a Socket 4, you can purchase a standard Pentium OverDrive chip that effectively doubles the speed of your old processor. An OverDrive chip with MMX technology is available for second-generation 75MHz, 90MHz, and 100MHz Pentiums using Socket 5 or Socket 7. Processor speeds after upgrade are 125MHz for the Pentium 75, 150MHz for the Pentium 90, and 166MHz for the Pentium 100. MMX greatly enhances processor performance, particularly under multimedia applications, and is discussed in the section "Pentium-MMX Processors," later in this chapter. Figure 3.16 shows the standard pinout for Socket 5.

Figure 3.16 320-pin Intel Socket 5 configuration.

The Pentium OverDrive for Pentium processors has an active heat sink (fan) assembly that draws power directly from the chip socket. The chip requires a maximum 4.33 amps of 3.3v to run the chip (14.289 watts) and 0.2 amp of 5v power to run the fan (one watt), which means total power consumption of 15.289 watts. This is less power than the original 66MHz Pentium processor requires, yet it runs a chip that is as much as four times faster!

Socket 6

The last 486 socket was designed for the 486 DX4 and the 486 Pentium OverDrive processor. Socket 6 was intended as a slightly redesigned version of Socket 3 and had an additional two pins plugged for proper chip keying. Socket 6 has 235 pins and would accept only 3.3v 486 or OverDrive processors. Although Intel went to the trouble of designing this socket, it never was built or implemented in any systems. Motherboard manufacturers instead stuck with Socket 3.

Socket 7 (and Super7)

Socket 7 is essentially the same as Socket 5 with one additional key pin in the opposite inside corner of the existing key pin. Socket 7, therefore, has 321 pins total in a 21x21 SPGA arrangement. The real difference with Socket 7 is not the socket but with the companion VRM (Voltage Regulator Module) or VRM circuitry on the motherboard that must accompany it.

The VRM is either a small circuit board or a group of circuitry embedded in the motherboard that supplies the proper voltage level and regulation of power to the processor.

The main reason for the VRM is that Intel and AMD wanted to drop the voltages the processors would use from the 3.3V or 5V supplied to the motherboard by the power supply. Rather than require custom power supplies for different processors, the VRM converts the 3.3V or 5V to the proper voltage for the particular CPU you are using. Intel has several different versions of the Pentium and Pentium-MMX processors that run on 3.3v (called VR), 3.465v (called VRE), or 2.8v, while AMD, Cyrix, and others use different variations of 3.3V, 3.2V, 2.9V, 2.4V, 2.3V, 2.2V, 2.1V, 2.0V, 1.9V, or 1.8V. Because of the variety of voltages that may be required to support different processors, most newer motherboard manufacturers are either including VRM sockets or building adaptable VRMs into the motherboard.

Figure 3.17 shows the Socket 7 pinout.

Figure 3.17 Socket 7 (Pentium) Pinout (top view).

AMD, along with Cyrix and several chipset manufacturers, pioneered an improvement or extension to the Intel Socket 7 design called Super Socket 7 (or Super7), taking it from 66MHz to 95MHz and 100MHz. This allows for faster Socket 7–type systems to be made, which are nearly as fast as the newer Slot 1 and Socket 370 type systems using Intel processors. Super7 systems also have support for the AGP video bus, as well as Ultra-DMA hard disk controllers, and advanced power management.

New chipsets are required for Super7 boards. Major third-party chipset suppliers, including Acer Laboratories Inc. (ALi), VIA Technologies, and SiS, are supporting the Super7 platform. ALi, VIA, and SiS all have chipsets for Super7 boards. Most of the major motherboard manufacturers are making Super7 boards in both Baby-AT and especially ATX form factors.

If you want to purchase a Pentium class board that can be upgraded to the next generation of even higher speed Socket 7 processors, look for a system with a Super7 socket and an integrated VRM that supports the different voltage selections required by your intended processor.

Socket 8

Socket 8 is a special SPGA (Staggered Pin Grid Array) socket featuring a whopping 387 pins! This was specifically designed for the Pentium Pro processor with the integrated L2 cache. The additional pins are to allow the chipset to control the L2 cache that is integrated in the same package as the processor. Figure 3.18 shows the Socket 8 pinout.

Figure 3.18 Socket 8 (Pentium Pro) pinout showing power pin locations.

Socket 370 (PGA-370)

In January 1999, Intel introduced a new socket for P6 class processors. The new socket is called Socket 370 or PGA-370, because it has 370 pins and was designed for lower-cost PGA (Pin Grid Array) versions of the Celeron and Pentium III processors. Socket 370 is designed to directly compete in the lower-end system market along with the Super7 platform supported by AMD and Cyrix. Socket 370 brings the low cost of a socketed design, with less expensive processors, mounting systems, heat sinks, and so on to the high-performance P6 line of processors.

Initially all the Celeron and Pentium III processors were made in SECC (Single Edge Contact Cartridge) or SEPP (Single Edge Processor Package) formats. These are essentially circuit boards containing the processor and separate L2 cache chips on a small board that plugs into the motherboard via Slot 1. This type of design was necessary when the L2 cache chips were made a part of the processor, but were not directly integrated into the processor die. Intel did make a multi-die chip package for the Pentium Pro, but this proved to be a very expensive way to package the chip, and a board with separate chips was cheaper, which is why the Pentium II looks different from the Pentium Pro.

Starting with the Celeron 300A processor introduced in August 1998, Intel began combining the L2 cache directly on the processor die; it was no longer in separate chips. With the cache fully integrated into the die, there was no longer a need for a board-mounted processor. Because it costs more to make a Slot 1 board or cartridge-type processor instead of a socketed type, Intel moved back to the socket design to reduce the manufacturing cost—especially with the Celeron, which competes on the low end with Socket 7 chips from AMD and Cyrix.

The Socket 370 (PGA-370) pinout is shown in Figure 3.19.

Figure 3.19 Socket 370 (PGA-370) Pentium III/Celeron pinout (top view).

The Celeron is gradually being shifted over to PGA-370, although for a time both were available. All Celeron processors at 333MHz and lower were only available in the Slot 1 version. Celeron processors from 366MHz to 433MHz were available in both Slot 1 and Socket 370 versions; all Celeron processors from 466MHz and up are only available in the Socket 370 version.

FC-PGA (Flip Chip Pin Grid Array)

Starting in October 1999, Intel also introduced Pentium III processors with integrated cache that plug into Socket 370. These use a packaging called FC-PGA (Flip Chip Pin Grid Array), which is a type of packaging where the raw die is mounted on the substrate upside down. It seems that all future Pentium III processors will continue in the socket version because the slot version is more expensive and no longer needed.

Note that due to some voltage changes and one pin change, many original Socket 370 motherboards will not accept the later FC-PGA (Flip Chip PGA) versions of the Pentium III and Celeron. Pentium III processors in the FC-PGA form have two RESET pins and require VRM 8.4 specifications. Prior motherboards designed only for the Celeron are referred to as legacy motherboards, and the newer motherboards supporting the second RESET pin and VRM 8.4 specification are referred to as flexible motherboards. Contact your motherboard or system manufacturer for information to see if your socket is the flexible version. Some motherboards, such as the Intel CA810, do support the VRM 8.4 specifications and supply proper voltage, but without Vtt support the Pentium III processor in the FC-PGA package will be held in RESET#.

Installing a Pentium III processor in the FC-PGA package into an older motherboard is unlikely to damage the motherboard. However, the processor itself could be damaged. Pentium III processors in the 0.18 micron process operate on either 1.60 or 1.65 volts, whereas the Intel Celeron processors operate at 2.00 volts. There is a chance that the motherboard could be damaged if the motherboard BIOS fails to recognize the voltage identification of the processor. Contact your PC or motherboard manufacturer before installation to ensure compatibility.

A motherboard with a Slot 1 can be designed to accept almost any Celeron, Pentium II, or Pentium III processor. To use the socketed Celerons and Pentium III processors, a low-cost adapter called a "slot-ket" has been made available by several manufacturers. This is essentially a Slot 1 board containing only a Socket 370, which allows you to use a PGA (Pin Grid Array) processor in any Slot 1 board. A typical slot-ket adapter is shown in the "Celeron" section, later in this chapter.

See "Celeron."

Socket A (Socket 462)

Socket A, also called Socket 462, was introduced in June 2000 by AMD to support the PGA (Pin Grid Array) versions of the Athlon and Duron processors. It is designed as a replacement for Slot A used by the original Athlon processor. Since the Athlon has now moved to incorporate L2 cache on-die, and the new Duron is available only in an on-die cache version, there was no longer a need for the expensive cartridge packaging used by the original Athlon processors.

Socket A has 462 pins and 11 plugs oriented in a SPGA (Staggered Pin Grid Array) form (see Figure 3.20). Socket A has the same physical dimensions and layout as Socket 370; however, the location and placement of the plugs prevent Socket 370 processors from being inserted. Socket A supports 32 different voltage levels from 1.100V to 1.850V in 0.025V increments, controlled by the VID0-VID4 pins on the processor. The automatic voltage regulator module circuitry will normally be embedded on the motherboard.

Figure 3.20 Socket A (Socket 462) Athlon/Duron layout.

There are 11 total plugged holes, including two of the outside pin holes at A1 and AN1. These are used to allow for keying to force the proper orientation of the processor in the socket. The pinout of Socket A is shown in Figure 3.21.

AMD has indicated that all future Athlon and Duron processors, at least for the time being, will be made in Socket A form, and the Slot A versions of the Athlon will be phased out.

Figure 3.21 Socket A (Socket 462) Athlon/Duron pinout (top view).

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