Often when upgrading a CPU, people say to check that the BIOS/UEFI (going to refer to both as "BIOS") supports the new CPU before buying/installing it, but what are the specific reasons that the BIOS needs the support, given the following assumptions about the upgrade?

  • Same package/socket
  • Same TDP
  • Same voltages (since differing core voltage is one of the reasons a BIOS update might be needed)

As a concrete example, consider upgrading a system from a i5-6440HQ to a i7-6870HQ, assuming one has the necessary tools and experience to take care the hardware side of things (i.e., BGA rework).

Both chips have the same FCBGA1440 package and TDP of 45W, as well as being of the same "generation" of chips (Skylake). A quick glance through the datasheet seems to show that they both run on the same voltages (correct me if I'm wrong).

The main high level differences between the chips is the i7 has 8 threads, runs at a slightly higher clock rate, has a more powerful graphics chip.

Is there any reason why the BIOS has to explicitly support the new CPU for the system to work? Since surely if the new CPU's interface is similar enough, things should work just fine? Or is this a bad assumption?


Its a bad assumption. It could work, but it may not. Although they look similar at first blush - similar numbering, same generation, there are significant differences between the 2 processors including launch date, cache, number of threads, speeds, GRAPHICS ADAPTOR, graphics memory, Maximum resolution, device ID, Intel vPro support, Intel Hyperthreading Technology, Intel TSX-NI support, Intel Stable Image Platform Program, Intel Trusted Execution Technology.

A number of these functions require BIOS support to work (Intel Trusted Execution springs to mind as the most important, although I imagine many of these need BIOS support).

  • software.intel.com/en-us/blogs/2012/09/25/… also shows BIOS as being tied into Trusted Execution. – davidgo Mar 17 '16 at 9:35
  • If the new CPU adds extra features that require BIOS support, wouldn't a BIOS that doesn't recognise the features just ignore them or leave them disabled rather than failing to boot? – 小太郎 Mar 17 '16 at 9:51
  • You would need to ask the motherboard manufacturer this.... so it would be better off seeing if the processor is supported. (I would expect that Trusted Execution ties pretty deeply into the system, so its not a given) – davidgo Mar 17 '16 at 20:55

Generally the CPU itself works without the bios upgrade. The bios upgrades provide:

Chip identification: if the chip was not in existance at the time, the bios may not identify what it is seeing stuck in the socket. Usually it will work all the same, even the speeds.

Microcode upgrade: I dont quite know what this is, but it can include proper support for new features of the chip, or changes to the size of something. Even proper ignoring of a change that is not supported on the specific board.

Things they may not say: There has been more than once that a cpu or chipset chip needed a bit of code adjustment to insure stable operation of some aspect of it. Because the motherboard manufactures life is tied to the integration of the CPU into thier boards, there are probably times when a slight adjustment of the firmware assists in proper operation when a tiny bug exists in the design.

  • microcode updates the firmware on the processor (the BIOS is not always needed for this). Microcode are, I believe, low level instructions to control the processor - if you think about it, its unsurprising that systems with hundeds of millions of transistors have errors - Microcode can help hide these errors, and provide changes in functionality - as well as "binning" cpu's - binning is the process of taking a CPU, testing it, disabling or reducing the speed of parts of it which don't work properly or reducing the speed of it and selling it as a lower spec CPU. – davidgo Mar 17 '16 at 20:59
  • ahh thats right, it was the software doing the microcode thing, not the bios specific. I am going to have to fix something, uhh somehow. – Psycogeek Mar 18 '16 at 4:15

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