BIOS and CPU support

What chip to plug into your socket 

CPU sockets are named for the number of pin contacts. LGA775 has 775 pins. When a new socket comes out that has the same pin count as a previous socket, it gets some other name. AMD had a Socket 940, so when a new socket came out that also had 940 pins, it was called "AM2". 

If you get a current Intel mainboard with an LGA775 socket, you can plug into it almost any Intel CPU chip manufactured since early 2004. This includes Celeron, Pentium 4, Pentium 4 dual core, Core Duo, Core 2 Duo, and even Core 2 Quad processors. Unfortunately, while old CPUs can be dropped into a new board, a new CPU cannot always be dropped into an old board. The socket may be the same, but the BIOS power up logic has to recognize the CPU chip and provide the right amount of voltage. Newer CPUs require a lower core voltage than older processors. To avoid damaging the chip, a board will typically not power up if it discovers a CPU chip it cannot identify.

In this generation of Intel architecture, the CPU chip connects to a Northbridge chip on the mainboard, and the Northbridge chip connects to memory and video devices. So a Pentium 4 chip in the LGA775 socket could be used with DDR or DDR2 memory and AGP or PCI-e video depending on the Northbridge and mainboard.

AMD had just been through a long period of stability with its Socket A chips, but starting in 2003 it introduced the Opteron/Athlon family of CPU chips with a completely new architecture.  The CPU chip now contained a memory controller for one or two memory buses, and the system was designed inherently for multiple CPUs whether they were created by multiple chips or multiple cores inside a single chip. This provided a faster connection to memory, and multiprocessing was going to be the main direction of technology. AMD had a five year lead on Intel, which has only begun to adopt a mainboard architecture based on the same two key ideas.

However, there is a downside to the AMD design. The CPU chip and its socket now depend on a choice of DDR or DDR2 memory, with a single memory bus or two parallel buses, and in a single chip configuration or a server/workstation with multiple CPU chips. So while Intel could stick with a single LGA775 socket, AMD ran through Sockets 754 (DDR,1bus), 939 (DDR, 2bus), 940 (DDR, 2bus, 2chip), AM2 (DDR2), and now AM2+(DDR2, more power). During this period you had to compare the requirements of the CPU to the socket on the mainboard.

In Nov. 2008 Intel finally accepted the inevitable and started to distribute chipsets and CPUs supporting the Nehalem or Core i7 architecture. The LGA1366 is a really big socket that supports three parallel memory buses of DDR 3 chips. In mid 2009 it will introduce a chipset for a more reasonably priced LGA1066 socket with the same two memory buses used in most current chips.

Teach an old board new tricks

Nobody would expect that they could put an LGA1366 CPU chip into an LGA775 socket, but you can have a problem when you have a fairly new mainboard and a just announced CPU chip. While Intel does give mainboard builders some advance warning, a BIOS that is six or nine months old may not be programmed to recognize the newest CPU chip. Physically the mainboard will handle the chip, it just doesn't know that. Mainboard vendors release new versions of the BIOS to support new chips, but you cannot change the BIOS until you power up the system, and you cannot power up the system with an unrecognized CPU chip.

People who support a lot of systems prepare for this sort of problem by keeping an old CPU chip off some old system they have long since recycled. Remember that an LGA775 board made today will recognize and run an LGA775 Pentium or Celeron chip  that is four years old. Plug the old chip in, power up, flash the new BIOS from a USB drive, power down, and then plug in the new CPU chip.

This trick only works for mainboards that are fairly recent. An LGA775 board that is three years old may not be upgradable with just a BIOS change. However, today nobody would want to even bother with a mainboard that uses DDR1 memory or has SATA 1 disk controllers.

After a vendor ships a mainboard, they discover new problems. Some device does not allow the system to go to sleep. Some types of memory are not correctly identified. An updated BIOS will be shipped to correct the problems. With modern boards you just download the BIOS file, put it on a USB stick drive, and then during power up press DEL or some other key and select BIOS update from the menus. BIOS update is not without some danger. You must be absolutely sure that the BIOS file is correct for your mainboard. The system should not be interrupted in the middle of the BIOS change. So you only make the upgrade if you are experiencing problems that might be fixed. 

Power

CPU chips are rated by the maxium amount of power they will draw at full load. They range from the ultra low 4 watts of an Atom processor, up through 35, 45, 65, 90, 125, or even 140 watts. A Quad core draws more power than a Dual core, and generally in the same family of CPU chips a 3 GHz clock speed requires more power than a 2.5 GHz clock speed.

This turned out to be a problem for customers who did not read the fine print on AMD Phenom Quad core processors. Some boards had an AM2+ socket and supported certain models of Phenom processors, but they did not have the power distribution to support the top of the line fastest clock quad core CPU that needed 140 watts of power. That overloaded the board like putting 20 amps of load on a 15 amp fuse. The embarrased vendors explained that they never expected anyone would put a $600 CPU chip in a $50 mainboard. The moral to the story is to carefully check the mainboard documentation on the vendor's site to make sure your proposed CPU chip is in the list of supported processors.