What are the "compatibility classes" among LGA775 CPUs?

Question

I'm currently looking to upgrade a CPU for an MB made for Prescott LGA775 CPUs. More specifically, I'm interested in 800FSB one as this is the limit for the P5GD1-VM MB this system has.

But to increase the reuse value for this question, an overview of the whole family of CPUs for this socket would be welcome as my research shows this proves to be an extremily confusing area of hardware.

Intel site says there are a whole multitude of CPU families with LGA775 socket:

• Prescott Celerons and Pentium 4's, including Hyper-Threading ones
• Cedar Mill Celerons and Pentium 4's
• Extreme Editions of the previous two
• Dual-core Pentium D's and Celeron D's
• Some Pentiums and Celerons with model names starting with "E"
• Core 2 Solo, Duo and Quad (plus their extreme editions)

Some hints on compatibility that I dug up but which didn't provide any hard facts and only testified about the utter chaos in this area:

• MBs for Prescotts claim to support CPUs with "Platform Compatibility Guide" ratings "04A","04B","05A" or "05B". But these appear to be mere shorthands for the maximum allowed CPU TDP (and - indirectly - the amount of power it's allowed to draw from MB, so non-standard cooling won't help here): "IIRC the 04A equates to a maximum TDP of 84W, 04B 85-115W, 05A 116-130W, and 05B 131-145W. Don't quote me on the latter two though."
  • To add insult to injury, Intel doesn't specify these ratings in CPU specs at its site, appearing to have abandoned them shortly after introduction.
• Cedar Mill CPUs appear to be powered by lower voltage than Prescotts but it's unclear how compatible this is from MB standpoint (MBs are known to have been able to adjust CPU voltage for quite some time)
• MBs designed for single core CPUs are rumored to not support dual-core CPUs. But Hyper-Threading is dual-core from external standpoint - it still handles two execution pipelines at once (thus requires bus locks and related circuitry), just not as effectively.
• The tested CPU list for my MB shows Prescotts and, surprisingly, Celeron D's - which are supposed to be two steps apart in the evolution chain!
• Pentium 4's and Pentium D's are said to be very much alike and an MB may be able to run a Pentium D even if its tested list doesn't include it.

Answer 1

When it comes to Socket 775 CPU support, there are four things you need to have in order for your CPU to work flawlessly in any given motherboard:

1. Sufficient or higher TDP Limit
2. Sufficient or higher BIOS Revision
3. Correct Chipset
4. Sufficient or higher FSB speed

Your motherboard supports at least a TDP of 115W, given it lists the P4-672 on ASUS's supported CPUs page. Conservatively, we will use this as our upper bound on acceptable TDP. We need to be careful here because while it may be possible to put a CPU with a higher TDP than this into your board and have it run, it may also do damage to the board, ultimately rendering your system inoperable.

Only you can determine your BIOS revision, but luckily this is not hard to do - you can check was BIOS revision you have, then go through ASUS' approved method of flashing the BIOS (if necessary) to the most recent revision: 1015.

The chipset of your motherboard is, per ASUS, the Intel 915G Express. As @harrymc stated in the comments, www.cpu-upgrade.com is a wonderful, inerrant source on this sort of thing, and that site provides us with a much-expanded support list for that chipset. This site operates quite a bit on experimentally verified information, using a wiki-like approach; if you see a green checkmark next to a CPU listing, that means it was actually physically attempted and succeeded. Experiment like this does leave lacuna, but overall, the lists are very complete. Additionally, because the site exists for people looking to upgrade, you can bet that people will be trying all the potential CPUs which are possibly the best on a given chipset, because that would be the best thing to find.

The support list also resolves our last problem - only 800mhz FSB or lower CPUs are listed. The last BIOS udpate, while not listed on that series of supported CPUs, is from 2007, which pretty well puts a limit on what might work on your system - and the few Allendale dual core pentiums that might in theory work on your socket probably won't due to large architecture differences which would prevent them from playing nice with your chipset.

The most performative CPU for you, without taking into consideration overclocking, would therefore be the Intel Pentium 4 672. At 3.8ghz with HT, 64-bit OS support, and 2mb of L2 cache, it seems to me the best possible option.

Anecdote Time: Pentium D's may work, because they are basically two P4's bolted together, but I don't think it is likely. In any case, they wouldn't offer much additional performance over the best proven option. They might also run at a reduced capacity; I have had a situation on 775 before where I was able to kinda-sorta run a Core 2 Quad 8200 on a system that previously had an older Core 2 Duo in it, but because the BIOS microcode didn't know about that chip it wasn't able to fully utilize its capabilities, and the system ran slower than it should have. More specifically, I didn't even get to the point of checking how things ran in an OS - the POST was so painfully slow that I cancelled all attempts to move forward with the install and simply replaced it with an older core 2. I believe it was running at a lower than usual frequency and not all of its cache was recognized, or something along those lines. I also got an annoying little message from the BIOS it every time I booted.

Regarding your specific inquiry about Cedar Mill. Cedar Mill is actually just a die-shrink of the same Netburst architecture. If you use the resources I gave you, you can see that while your chipset supports those chips, your motherboard doesn't. This is most likely due to microcode updates needing to be made to the BIOS which were not made by Asus. To me that says in all likelihood they should run in your computer. You may get issues about it during POST, though.