There is this huge confusion regarding Intel socket 478 / 479 / M / P. No doubt they are very similar in physical appearance and number of pin hole counts. Even places like Wikipedia, has made mistakes of identifying these sockets, no kidding!

Below is the most reasonable description of what these sockets are, based on my research. Please feel free to comment on any incorrect information or add any additional information.

Socket 478 (mPGA478B)
This is the socket for most of the Pentium 4 desktop CPUs. It has 478 pin holes. There are actually different versions (A, B, C) of mPGA478 sockets based on the positions of the 2 missing pins. The mPGA478B is the most widely used. It uses a lever mechanism to secure the CPU onto the socket.

Missing Pins: A1, B1
Diagram: http://www.x86-guide.com/en/articles/sockets/Socket%20478%20(mPGA478B)-no11.html
Document: http://download.intel.com/design/Pentium4/guides/24989002.pdf


Socket 479 (mPGA479M)
This is the socket first used for Pentium M mobile CPUs. It has 479 pin holes. It uses a screw mechanism to secure the CPU onto the socket.

Since there is only 1 pin missing, the socket can physically fit both socket 479 CPUs and Socket M CPUs. So, to add to the confusion, some motherboard manufacturers actually used this type of socket as an "universal dock" for both socket 479 CPUs and Socket M CPUs, which includes some of the Core 2 Duo mobile CPUs (Merom). This is accomplished by using chipsets that can accommodate both types of CPUs.

Missing Pin: A1
Diagram: http://www.x86-guide.com/en/articles/sockets/Socket%20479%20(mPGA479M)-no28.html
Document: ftp://download.intel.com/design/mobile/applnots/29852001.pdf


Socket M (mPGA478MT)
This is the socket first introduced for Core Duo mobile CPUs. It was continued to be used for early Merom C2D mobile CPUs. It has 478 pin holes. It uses a screw mechanism to secure the CPU onto the socket. For some of the laptop motherboards, this socket is in "Gold" color.

Missing Pins: A1, A2
Diagram: http://www.x86-guide.com/en/articles/sockets/Socket%20M%20(mPGA478MT)-no24.html
Article: http://www.pcstats.com/articleview.cfm?articleid=2062&page=5


Socket P (mPGA478MN)
This is the new socket type introduced when the Santa Rosa mobile platform was out. It is being used for some of the Merom C2D mobile CPUs and all of the new Penryn C2D mobile CPUs. It has 478 pin holes. It uses a screw mechanism to secure the CPU onto the socket. For some of the laptop motherboards, this socket is in Purple color. It is not backward compatible with Socket M.

Missing Pins: A1, B1
Diagram: http://www.x86-guide.com/en/articles/sockets/Socket%20P%20(mPGA478MN)-no25.html
Article: http://www.hardwarezone.com/articles/print.php?cid=7&id=2256


I couldn't find at this point Intel documents for Socket M and P. Here are some of the mistakes made even from creditable sources.

Wikipedia, a picture of socket P was shown for socket M. I was fooled myself, too. http://en.wikipedia.org/wiki/Socket_M
DailyTech, made mistakes in pointing out the missing pins positions. http://www.dailytech.com/article.aspx?newsid=3180


Edit:
1) Corrected minor spelling errors.
2) Socket M was introduced with Core Duo first, not Core 2 Duo. Sorry.

good stuff doc. we need more manuels/articles like this. should submitted, something for the front page.