CPUID Command - Different Response at Boot

[PDeme]

I have a system based on an Intel Apollo Lake processor. At boot up, the CPUID instruction is used by the BIOS to determine what features are supported. The values returned for a CPUID(1) are different during boot than when they are run in DOS.

I built a debug BIOS and the CPUID(1) command returns the following data:

  Features.Eax = 000506ca

  Features.Ebx = 00200800

  Features.Ecx = 47f8ebb7

  Features.Edx = bfebfbff

Bit 3 of the Ecx output is the bit used to determine if C-States are supported by the processor. It is clear here and no C-States menu is displayed in the BIOS setup screen.

If I run the same CPUID command in DOS debug, I get the same data except that bit 3 is now set, showing C-States are supported.

This issue is not seen on every board, only a select few.

The BIOS is acting appropriately, but why does the CPUID command return the wrong data during boot?

[그래요ITech]

Hello there @PDeme...

The discrepancy in the CPUID(1) command results between the boot process and running in DOS could be due to several factors. It's possible that during the boot process, certain features like C-States may be temporarily disabled by the BIOS based on the system configuration or other criteria. When you run the CPUID command in DOS, the operating system may have enabled these features, hence the difference in the Ecx bit 3 status.

Another possibility is that the BIOS version or settings on the affected boards differ from those where the issue isn't observed, leading to different CPUID readings. It's also worth considering that there might be a bug in the BIOS that's causing incorrect feature reporting during the boot process.

To troubleshoot this issue, you could:
• Compare the BIOS versions and settings between boards that exhibit this behavior and those that don't.

• Check for any BIOS updates that might address this inconsistency.

• Consult with the motherboard manufacturer for insights into this behavior and potential solutions.

Hope this helps.

Cheers,

Max