For all of these form factors, the number following “socket” refers to the number of CPU pins or connectors. Socket 478 is a format specifically designed for Intel’s Celeron, Pentium 4, and mobile Celeron/Pentium processors.

This socket supports processors with frontside buses of 400 MHz (100 MHz x 4), 533 MHz (133 MHz x 4), and 800 MHz (200 MHz x 4), and provides top speeds from below 1 GHz to higher than 3 GHz. The Intel naming system used for the Pentium 4 processors in this class uses letters to represent the frontside bus speeds present. An “A” means 400 MHz, “B” means 533 MHz, and “C” means 800 MHz. So, a Pentium 4 2.4C would offer greater performance than a 2.4B or a 2.4A, despite them all having the same 2.4 GHz clock speed.

The Pentium class of processors, like many others from AMD and Intel, also apply names to the “cores” present on the processor. The core is the processing portion of the CPU, generally found at the center of the silicon wafer. As the architecture within cores of the same type of processor change, new names are given to signify the different levels of performance that users may experience. The Northwood, Gallatin, and Prescott cores can be found on Pentium 4 processors of the same speed, and a main difference between the three is that the first two are fabricated on a 0.13 micron (one one-millionth of a meter) process and the Prescott is fabricated on a 0.09 micron process. A lower number implies a more tightly-packed core that generally requires less power to perform the same operations.

Later generation Socket 478 Pentium 4 processors were the first to implement Intel’s Hyper-Threading Technology, which allowed for applications to be run in parallel, thus improving the performance of the system. Although not nearly as strong, the concept is similar to having a dual processor system or dual-core processor, as systems with Hyper-Threading enabled processors can handle intensive applications much easier than the same system without Hyper-Threading.

Socket 478 was Intel’s flagship format for several years, and continues to be a popular choice among Pentium 4 motherboards. Socket 478 motherboards were first compatible with SDRAM memory, then RAMBUS memory, and as it became more popular DDR memory became the format of choice. But, Intel’s decision to embrace DDR2 (among other developments) has resulted in a totally new CPU socket for Pentium 4 processors… Socket T.

These CPUs may not be as common as the others, but are worthy of inclusion on this list anyway. Socket 603 and Socket 604 are the home to Intel’s modern Xeon processors, which are more likely to be found in a high-end server than in a desktop computer.

The Xeon can come with frontside buses ranging from 400 MHz (100 MHz x 4) to 800 MHz (200 MHz x 4), and provide top speeds that now reach 3.6 GHz. Socket 603 is for older, slower Xeons capable of a 400 MHz frontside bus, and Socket 604 adds the one pin simply to identify the faster bus speeds. Socket 603 processors will fit and function in a Socket 604 motherboard, but the opposite will not work.

Xeons are powerful processors all on their own, but the architecture of the CPU and supporting motherboards allows for serious systems to be configured with dual processors, or perhaps even four processors, to really make light work of even the most intensive application.

Socket A

Socket A was the staple format in AMD’s line-up for years, carrying the brand through the Athlon, Thunderbird, Duron, Athlon XP, Athlon MP, and Sempron classes of processors. The number of pins found on the bottom of a Socket A CPU totals 462, and this is one of the few formats that is not named for the number of pins found on the processor.

Socket A processors came with frontside bus speeds from 200 MHz (100 MHz x 2) up to 400 MHz (200 MHz x 2). Socket A processors ranged in top speeds from under 1 GHz to just over 2 GHz, but the identification of a Socket A processor’s true speed is difficult due to AMD’s use of PR (Performance Rating) grades instead of true processor speed as a sales tool. For example, an AMD Athlon XP 3000+ doesn’t have a speed of 3.0 GHz, but rather 2.167 GHz. AMD contends that their architecture provides performance equivalent to the PR grade, despite the actual speed being much lower. Early Socket A systems supported SDRAM memory, but as the technology advanced and DDR memory became available, these processors took advantage of the increased performance. DDR memory offered greater overall performance at the same clock speeds as SDRAM, as well as the potential for much higher clock speeds, so it was only a matter of time before it became the standard memory for use with these processors. Just like Intel, AMD cores in this series also have names to distinguish different levels of performance within the same class of processor. For example, Duron cores carry names such as Morgan and Applebred, while Athlon XP processor cores have names such as Palomino, Thoroughbred, and Barton (listed in order from weakest to strongest). The long-lived success of Socket A is winding down, as the last date for orders to be placed for production has just passed. As we say goodbye to Socket A, AMD intends to focus more attention on similar processors for Socket 754, as well as the higher-end Socket 939/940 formats.

Socket 754

Socket 754 picks up where Socket A left off, offering support for CPUs including the Sempron and Athlon 64. As the name implies, the processor features 754 tiny pins that interface with the socket on the motherboard. As processors develop, the number of pins manufacturers can fit on the back becomes more and more amazing. The Sempron class of processor is more of a budget-conscious offering, with current PR grades available from 2600+ to 3300+, while the Athlon 64 is the more powerful 64-bit offering with PR grades from 2800+ and climbing past 3700+. All Socket 754 processors support a frontside bus of 400 MHz (200 MHz x 2), and presently all systems utilize DDR memory.

Socket 939/940

Socket 939 and 940 support the current top-of-the-line offerings from AMD and, as the names imply, they are separated by one pin. Like their Socket 754 cousins, this format of AMD processors utilizes DDR memory and provides a frontside bus of 400 MHz (200 MHz x 2). Socket 939 supports Athlon 64 processors with PR grades currently up to 4000+, as well as the Athlon 64 FX series which takes the performance to new levels for game play and other intensive applications. Socket 940 is more the business side of this pair of sockets, offering support for both Athlon 64 FX series CPUs and the AMD Opteron class of server processors. The Opteron series of processors include a feature called “dual-core,” where one processor is effectively seen as two, and given the possibilities of multi-processor motherboards, the computing power can be incredible. A recent release from AMD is the Athlon 64 X2, which brings high-performance dual-core technology to a Socket 939 desktop environment, but has yet to become available to the public for purchase. Where many AMD processors used a 4 digit PR grade for marketing, the FX series of Athlon 64 processors uses a two digit code (i.e., FX-51, FX-55) and the Opteron uses a three digit code (i.e., Opteron 244, Opteron 252). All of these designations can sure make things confusing, and given the varied nomenclature that AMD alone currently uses, it can be tricky figuring out how fast a CPU really is.

Final Words

As CPU technology advances, the sockets that correspond to them are forced to change as well. The new physical sizes, internal architecture, and thermal properties require that the ‘packaging’ evolve. A negative side effect of this development is that motherboards wind up with limited “upgrade paths” when it comes to adding a faster processor. Although this can occasionally be remedied with socket adaptors, as AMD and Intel continue down their parallel paths of development, this situation can be expected to continue. With that said, it looks like AMD’s next CPU socket isn’t that far off.