Re: Does reboot clear RAM?

[Joachim Durchholz]

Am 11.11.19 um 15:35 schrieb Jakob Bohm:
> On physical machines, the following mechanisms are common:
>
> 1. DRAM chips physically loose their contents after a few seconds of power
>    off,

I am by no way an expert, but the forensic experts tell me that data can 
persist for *minutes*.
Of course, the first bits flip after a few seconds. But you don't get a 
guarantee that everything is zeroed.
I also hear that temperature plays a really big role here.

> 3. On x86 and x86_64 PCs, the IBM compatible BIOS typically does a memory
>    test and wipe during actual boot, but not upon a software initiated 
> boot.
>     This PC BIOS rule exists for the following two purposes:
>
> 3.1 Older guest operating systems use a software reset to switch the CPU
>    from "protected mode" to "real mode" because the historical 80286 CPU
>    chip had no other way to return to real mode and returning to real mode
>    was needed to invoke BIOS APIs.
>
> 3.2 Signalling if such a non-wiping boot is desired (for speed or other
>    reasons) is officially done by writing a magic value in one of the
>    well-known BIOS global addresses, if this global address has not been
>    set to one of those magic values, and the global RTC register with
>    related semantics have not been so set either, the BIOS (in qemu's
>    case SEABIOS) should do the wipe as part of the POST 
> (Power-On-Self-Test),
>    otherwise it should skip that and most other parts of the POST.

How does the BIOS do the wipe? Because zeroing out all memory should 
take some noteiceable time on today's large RAM, even with GHz machines 
(RAM bus speed has increased far more slowly than typical RAM size, so 
the time should have been increasing).
I can imagine some fast PCI burst transfers to quickly zero memory, but 
does anybody have realistic data, or information about how it's being 
done in practice?

Regards,
Jo