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I'm reversing the BIOS of my laptop, for fun and for learning something new.
Inside it, I just stepped into this piece of code:

mov     ecx, 13Ah
rdmsr
and     eax, 1
jnz     SkipCacheAsRAM

Looking on the Intel datasheet "Intel 64 and IA-32 Architectures Software Developer’s Manual", I couldn't find any clue about it, but seems like that in this BIOS release, its value is essential the determine if a key feature such as 'cache as RAM' have to be initialized using an approach or another. I checked also on the AMD equivalent datasheet, even though I'm sure the MotherBoard is only for Intel CPUs.
Does anyone indicate me an alternative source(s) where to search what this 0x13a register does?

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GitHub is a good place to search for such stuff, e.g.:

https://github.com/search?q=0x13A+IA32+MSR&type=Code (may require logging in)

Produces results like:

#define MSR_BOOT_GUARD_SACM_INFO                                      0x13A
#define B_NEM_INIT                                                    BIT0

There is no comment, but from the name it look like the bit 0 determines whether the processor is already in NEM (Non-evict mode, another name for Cache-as-RAM).

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  • 1
    I really appreciate you gave me the answer I was looking for. But the reason why I have to say thank you is that you indicated me a source I never considered as a target for my searches. – Alessandro Jan 10 '19 at 10:55

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