Processor SocketsSlots

Processor Sockets/Slots

The CPU is installed in either a socket or a slot, depending on the type of chip.

Starting with the 486 processors, Intel designed the processor to be a user-installable and replaceable part and developed standards for CPU sockets and slots that would allow different models of the same basic processor to plug in. One key was to use a zero insertion force (ZIF) socket design, which meant that the processor could be easily installed or removed with no tools. ZIF sockets use a lever to engage or release the grip on the chip, and with the lever released, the chip can be easily inserted or removed. The ZIF sockets were given a designation that was usually imprinted or embossed on the socket indicating what type it was. Different socket types accepted different families of processors. If you know the type of socket or slot on your motherboard, you essentially know which types of processors are designed to plug in.

See "Processor Socket and Slot Types," p. 87.

Sockets for processors prior to the 486 were not ZIF designs and, as such, were not designed for easy processor installation or removal. In addition, interchangeability was limited. Table 4.9 shows the designations for the various 486 and newer processor sockets/slots and lists the chips designed to plug into them.

Table 4.9. CPU Socket Specifications

	Socket	Pins	Layout	Voltage	Supported Processors
Intel/AMD 486 class	Socket 1	169	17x17 PGA	5V	486 SX/SX2, DX/DX2, DX4 OD
	Socket 2	238	19x19 PGA	5V	486 SX/SX2, DX/DX2, DX4 OD, 486 Pentium OD
	Socket 3	237	19x19 PGA	5V/3.3V	486 SX/SX2, DX/DX2, DX4, 486 Pentium OD, AMD 5x86
	Socket 6[1]	235	19x19 PGA	3.3V	486 DX4, 486 Pentium OD
Intel/AMD 586 (Pentium) class	Socket 4 Socket 5	273 320	21x21 PGA 37x37 SPGA	5V 3.3V/ 3.5V	Pentium 60/66, OD Pentium 75-133, OD
	Socket 7	321	37x37 SPGA	VRM	Pentium 75-233+, MMX, OD, AMD K5/K6, Cyrix M1/II
Intel 686 (Pentium II/III) class	Socket 8	387	Dual-pattern SPGA	Auto VRM	Pentium Pro, OD
	Slot 1 (SC242)	242	Slot	Auto VRM	Pentium II/III, Celeron SECC
	Socket 370	370	37x37 SPGA	Auto VRM	Celeron/Pentium III PPGA/FC-PGA
Pentium 4 class	Socket 423 Socket 478	423 478	39x39 SPGA 26x26 mPGA	Auto VRM Auto VRM	Pentium 4 FC-PGA Pentium 4/Celeron FC-PGA2
	Socket T (LGA775)	775	30x33 LGA	Auto VRM	Pentium 4/Celeron LGA775
AMD K7 class	Slot A Socket A (462)	242 462	Slot 37x37 SPGA	Auto VRM Auto VRM	AMD Athlon SECC AMD Athlon/Athlon XP/Duron PGA/FC-PGA
AMD K8 class[2]	Socket 754 Socket 939 Socket 940	754 939 940	29x29 mPGA 31x31 mPGA 31x31 mPGA	Auto VRM Auto VRM Auto VRM	AMD Athlon 64, AMD Athlon 64 v.2, AMD Athlon 64FX, Opteron
Intel/AMD Server and Workstation class	Slot 2 (SC330) Socket 603 Socket PAC418 Socket PAC611 Socket 940	330 603 418 611 940	Slot 31x25 mPGA 38x22 split SPGA 25x28 mPGA 31x31 mPGA	Auto VRM Auto VRM Auto VRM Auto VRM Auto VRM	Pentium II/III Xeon Xeon (P4) Itanium Itanium 2 AMD Athlon 64FX, Opteron
FC-PGA = Flip-chip pin grid array
FC-PGA2 = FC-PGA with an integrated heat spreader (IHS)
OD = OverDrive (retail upgrade processors)
PAC = Pin array cartridge
PGA = Pin grid array
PPGA = Plastic pin grid array
SC242 = Slot connector, 242 pins
SC330 = Slot connector, 330 pins
SECC = Single edge contact cartridge
SPGA = Staggered pin grid array
mPGA = Micro pin grid array
VRM = Voltage regulator module with variable voltage output determined by module type or manual jumpers
Auto VRM = Voltage regulator module with automatic voltage selection determined by processor voltage ID (VID) pins

^[1] Socket 6 was never actually implemented in any systems.

^[2] During 2006, AMD is transitioning its current Athlon 64 family of processors to the new Socket M2.

Originally, all processors were mounted in sockets (or soldered directly to the motherboard). With the advent of the Pentium II and original Athlon processors, both Intel and AMD temporarily shifted to a slot-based approach for their processors because the processors now incorporated built-in L2 cache, purchased as separate chips from third-party Static RAM (SRAM) memory chip manufacturers. Therefore, the processor then consisted not of one but of several chips, all mounted on a daughterboard that was then plugged into a slot in the motherboard. This worked well, but there were additional expenses in the extra cache chips, the daughterboard itself, the slot, optional casings or packaging, and the support mechanisms and physical stands and latches for the processor and heatsink. All in all, slot-based processors were expensive to produce compared to the previous socketed versions.

With the advent of the second-generation Celeron, Intel integrated the L2 cache directly into the processor die, meaning within the main CPU chip circuits with no extra chips required. The second-generation (code named Coppermine) Pentium III also received on-die L2 cache, as did the K6-3, Duron (code named Spitfire), and second-generation Athlon (code named Thunderbird) processors from AMD (some early Thunderbird Athlon CPUs were also made in the Slot A configuration). With on-die L2, the processor was back to being a single chip again, which also meant that mounting it on a separate board plugged into a slot was expensive and unnecessary. Because of on-die integrated L2 cache, processor packaging shifted back to sockets and will continue that way for the foreseeable future. All modern processors now have integrated L2 cache (some also have integrated L3 cache) and use the socket form. Besides allowing a return to socketed packaging, the on-die L2 cache runs at full processor speed, instead of the one-half or one-third speed of the previous integrated (but not on-die) L2 cache.