Upgrading and Repairing PCs

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. These specifications were given a designation that is usually imprinted or embossed on the connector or board. If you know the type of socket or slot on your motherboard, you essentially know which type of processors are designed to plug in.

Sockets for processors prior to the 486 were not designated, and interchangeability was limited. Table 4.7 shows the designations for the various processor sockets/slots and lists the chips designed to plug into them.

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 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.

Table 4.7. CPU Socket Specifications

Socket Number	Pins	Pin Layout	Voltage	Supported Processors
Socket 1	169	17x17 PGA	5V	486 SX/SX2, DX/DX2[1], DX4 OD
Socket 2	238	19x19 PGA	5V	486 SX/SX2, DX/DX2[1], 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 4	273	21x21 PGA	5V	Pentium 60/66, OD
Socket 5	320	37x37 SPGA	3.3V/3.5V	Pentium 75-133, OD
Socket 6[2]	235	1x19 PGA	3.3V	486 DX4, 486 Pentium OD
Socket 7	321	37x37 SPGA	VRM	Pentium 75-233+, MMX, OD, AMD K5/K6, Cyrix M1/II
Socket 8	387	Dual-pattern SPGA	Auto VRM	Pentium Pro, OD
Socket 370	370	37x37 SPGA	Auto VRM	Celeron/Pentium III PPGA/FC-PGA
Socket PAC418	418	38x22 split SPGA	Auto VRM	Itanium, Itanium 2
Socket 423	423	39x39 SPGA	Auto VRM	Pentium 4 FC-PGA
Socket A (462)	462	37x37 SPGA	Auto VRM	AMD Athlon/Athlon XP/Duron PGA
Socket 478	478	26x26 mPGA	Auto VRM	Pentium 4 FC-PGA2, Celeron FC-PGA2
Socket 603	603	31x25 mPGA	Auto VRM	Xeon (P4)
Socket 754	754	29x29 mPGA	Auto VRM	AMD Athlon 64
Slot A	242	Slot	Auto VRM	AMD Athlon SECC
Slot 1 (SC242)	242	Slot	Auto VRM	Pentium II/III, Celeron SECC
Slot 2 (SC330)	330	Slot	Auto VRM	Pentium II/III Xeon SECC
Auto VRM = Voltage regulator module with automatic voltage selection determined by processor VID pins FC-PGA = Flip-chip pin grid array 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

^[1] Non-overdrive DX4 or AMD 5x86 also can be supported with the addition of an aftermarket 3.3V voltage regulator adapter.

^[2] Socket 6 was never actually implemented in any systems.

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, the trend for processor packaging has shifted back to sockets and will likely continue that way for the foreseeable future. All modern processors now 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.

The 64-bit Itanium processor from Intel, however, features a cartridge design that includes L3 cache and yet plugs into a socket rather than a slot.