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Suppose that a motherboard doesn't ship with a BIOS that supports your CPU. Can you still get into BIOS and flash it without the correct CPU? For example, does it automatically downgrade from 6-core to 4-core so you can at least get into BIOS and flash it?

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You will have to check the motherboard manufacturer's user manual, and BIOS upgrade release notes.

If the CPU is "similar enough", so it can be ran as a supported device (for example if a 6-core unit could run as if it was a 4-core unit) then you can likely do a normal install of the Operating System, and use the typical user application software or process to upgrade the BIOS to recognize and utilize the CPU fully.

IF the CPU is an entirely new series, (say the first 25nm chips or a new codename) then you may have to either upgrade the motherboard BIOS using an older but supported CPU, or send the motherboard back to the dealer / manufacturer to be upgraded, or if the motherboard has a fail-safe BIOS recovery / upgrade process that does not boot into an OS or use the main CPU as it does normally (limited 8086 real mode only perhaps for the hardcore readers) from media (floppy, cdrom, I don't know about USB). I don't know how common this process is, but I have seen Asus and at least one other manufacturers offer this capability.

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You can't get into the BIOS without a CPU (at least not the homeuser). First of all the CPU socket has to be the same. If your 4-core cpu is socket compatible and (second) being listed on the manufacturer's site as a supported CPU than you can power on the system with that CPU and flash the BIOS.

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Ok and you mean that if it says that BIOS version 2 supports a 6-core cpu and I got BIOS version 1, i can still get in and flash with the 6-core though unsupported with that BIOS version? – Chris_45 Apr 20 '10 at 13:10
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Yes, you can do it. I did it before. Tested and approved :) – Deniz Apr 20 '10 at 13:16
Ok what motherboard and so did you have? – Chris_45 Apr 20 '10 at 13:18
Can't remember the older ones but my recent "in advance CPU support flash" was with a AMD 770 chipset Gigabyte board. As I can remember we bought a very recent Phenom CPU for this board which was only listed as supported with the latest BIOS and the board didn't come with that BIOS version. So we just put the unsupported Phenom into the socket and finished the system installation after which we flashed the BIOS from the OS. – Deniz Apr 20 '10 at 13:26
I have heard of a couple cases where people could not access the bios with a "not yet supported" CPU installed, but more often than not, they were successful. Both of those cases were with older AM2 boards. – BBlake Apr 20 '10 at 13:46
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FYI, there's a new line of Asus motherboards that can be flashed from a USB flash drive without a CPU present at all.

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