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x86low-levelbiososdev

Is there place in BIOS which can survive warm reset?

Is there any place (except HDD, Floppy and etc.) where I can store a few bytes of information and read them back after warm CPU reset?

I've tried to use BIOS Data Area (in particular: 0x0040:0x00F0 - Inter-application communication area), but this area is filled with zeroes upon reset, so my information is lost.

Is there any other place which is not cleared during reset?

Solution

  • The machine state is undefined after a reset (cold or warm), so the short answer is no.

    However, you may abuse some hardware, although not reliable, to store values when reset.

    • Display ram, some card however do clear the memory.
    • MSRs, this however is model specific
    • PCI registers - the BAR seems to be unchanged upon reset for some card.

    Again, all of the above is not reliable. Not surprise, to store value persistent across reset, you need persistent storage, i.e. disk, remote servers, etc.