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One of the most important parts of a computer is the Central Processing Unit (CPU). In a spider web of electronic highways from and to other parts of the computer the CPU carries out all kinds of instructions. The real CPU is a very small piece of silicon referred to as the core which is embedded in a larger casing we call the CPU. Very thin wires attached to the core connect to larger pins on the packaging. And finally, the pins on the packaging interface with the motherboard through a Socket.

The first CPU's, like the 8080, 8085, 8086 and 8088 had a packaging called DIP, Dual In Line Package. The Socket for this type of packaging is called DIL, Dual In Line. Compared to todays Sockets the DIL looks very basic. To insert the CPU it had to be placed on the Socket and then pressed into it. Much like, for example a graphics card in an AGP slot. There were only 40 pins, but quite some force was already needed to press the CPU into the socket.

When the 80286 came, the packaging was changed to the Pin Grid Array, PGA. This packaging type was also used for the 80386. The CPU still had to be placed on the socket and then pressed into it. With the 132 pins of the 386 this was a though and potentially dangerous job, 132 pins gave quite some resistance before slipping into the socket. Bending pins was not unusual even breaking pins off was possible!

In 1989 Intel introduced the 486, this CPU had even more pins than a 386 and it was clear the socket had to change. There had to be a way to install a CPU safely into a socket, no more bent or broken pins! The result of the search was the ZIF socket, Zero Insertion Force Socket. This Socket has three parts, the first part is soldered on the motherboard and has the connectors for the pins of the CPU. The second part moves over the first part, it will hold the CPU in its place and is connected to the third part, a lever. Place the CPU in the socket, then push the lever down. The second part will push the pins firmly against the contacts of the first part et voila! Almost without force the CPU can connect with the motherboard.

From the 486 on, all sockets had a name and what better way than to start with the number 1. The first Socket for the 486 was the SOCKET 1.

Socket 1 169 pins 486 SX/DX
Socket 2 238 pins 486 SX/DX/DX2
Socket 3 237 pins 486 SX/DX/DX2/DX4
Socket 6 235 pins 486 DX4

After the 486 came the Pentium. The first version which ran at speeds of 60 and 66MHz used the Socket 4. The ZIF construction was of course used, how else could a 273 pin CPU be installed?

Socket 4 273 pins Pentium 60 en 66MHz
Socket 5 320 pins Pentium 75 t/m 200MHz
Socket 7 321 pins Pentium, AMD K6/K6-2/K6-3, Cyrix M1/M2

Developments in the CPU business go fast and when Intel worked on their next generation CPU, a new socket was designed. The Pentium Pro was introduced in 1995 and used the Socket 8. This CPU was big, bigger than any CPU Intel had designed until then. The Pentium Pro had the core and secondary cache integrated into one package.

Socket 8 387 pins Pentium Pro 150 - 200MHz

After the Pentium came the Pentium II, the CPU that would be the king of performance. In order to have the best performance of all CPU at that time large amounts of cache was needed. Intel had planned to include the secondary cache into the CPU core, but production techniques at the time were not advanced enough to do this reliably and cost effective. Another solution had to be found and Intel came up with this idea : Put the CPU package on a small PCB together with some cache chips and use the PCB to interface with the motherboard! Slot 1 was born, best described as a ridge with a total of 242 contact points where the Pentium II package had to be pushed into. No Zero insertion force with a Slot interface!. The Server market was not forgotten by Intel and the Xeon, a server CPU, used Slot 2.

Slot 1 242 pins PII, PIII en Celeron
Slot 2 330 pins PII Xeon en P III Xeon

Production techniques developed further and by the time Intel was ready to introduce their next generation Pentium III it was possible to include the secondary cache onto the core.
The Slot 1-2 solution was not necessary anymore and the Socket was reinstated.

Socket 370 370 pins PIII, Celeron, Cyrix III
Socket 423/478 423/478 pins Pentium4
Socket 603 603 pins Pentium4 Xeon

But what about AMD? They have their own line of CPU´s, what sort of sockets did they use? AMD used the Socket 7 for a long time, the K6, K6-2 and K6-3 all used it. But with the CPU after the K6 family, the K7 also known as the Athlon (Classic), AMD introduced their own Slot interface. AMD ran into the same problems as Intel with the secondary cache and used the same solution, put the CPU and the secondary cach on a PCB. The Athlon used Slot A which was a reversed Slot 1 with 242 contact points.

Slot A 242 pins Athlon

After some time AMD´s production facilities were able to include the secondary cache onto the core and AMD switched back to the Socket. After all it is cheaper to manufacture a CPU than a CPU and some cache on a PCB. AMD used Socket A with 453 pins for their socket Athlons.

Socket A 453 pins Athlon, Athlon XP, Athlon MP and Duron

Next generation CPU´s from AMD and Intel like the Opteron and Itanium will use new sockets with more pins for increased speed and performance. It looks like the socket will be used for some time to come but developments won´t stop and who knows, maybe we will see a return of the Slot!

Socket 754 754 pins AMD Athlon 64
Socket 940 940 pins AMD Opteron

PAC418 418 pins Intel Itanium
PAC611 611 pins Intel Itanium 2



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