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Index > Heap > Multi-GHz C=64, anyone? (changing CPUID on AMDs)

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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
I bet you thought the information returned by CPUID was hardcoded into the CPU, right? Well, not entirely so... have a look at http://f0dder.reteam.org/amdp4.htm
Post 06 Apr 2005, 21:56
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THEWizardGenius



Joined: 14 Jan 2005
Posts: 382
Location: California, USA
THEWizardGenius
weird...
Post 17 May 2005, 14:47
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
pretty nasty that this is possible, I'd say. CPUID stuff should be hardcoded and read-only...
Post 17 May 2005, 15:21
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scientica
Retired moderator


Joined: 16 Jun 2003
Posts: 689
Location: Linköping, Sweden
scientica
Is the change 'permanent' or does it reset when the machine is restarted?
Post 18 May 2005, 22:24
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UCM



Joined: 25 Feb 2005
Posts: 285
Location: Canada
UCM
I think it's permanent cause its in the bios.

_________________
This calls for... Ultra CRUNCHY Man!
Ta da!! *crunch*
Post 18 May 2005, 22:25
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scientica
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Joined: 16 Jun 2003
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Location: Linköping, Sweden
scientica
uhm, BIOS? aren't most BIOS flashable (and some got functionality to restore the factory copy)
Post 18 May 2005, 22:35
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
I wonder what values the MSRs hold after a reset... but yes, because of the BIOS; BIOSes for AMD systems have to detect CPU type and program the MSRs to the appropriate string.
Post 19 May 2005, 13:12
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THEWizardGenius



Joined: 14 Jan 2005
Posts: 382
Location: California, USA
THEWizardGenius
So if you're writing your own OS you could theoretically write some sh!t to make it nasty for developers. That's stupid. The CPU should have CPUID built-in, since anything BIOS can easily be changed, especially if the BIOS is flashable (and most are). Who got such an idea as that?
Post 22 May 2005, 21:53
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
Obviously, AMD did Wink
Post 23 May 2005, 02:42
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Torrey



Joined: 12 Oct 2003
Posts: 78
Torrey
That's a really cool discovery, and I noticed you didn't code it in FASM, shame, shame, shame. Razz
Post 23 May 2005, 03:56
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
Well, the contents is more interesting than the "language" Smile

I originally wrote the code for people who don't deal with FASM, btw... that's the reason for the dialect.
Post 23 May 2005, 15:43
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THEWizardGenius



Joined: 14 Jan 2005
Posts: 382
Location: California, USA
THEWizardGenius
i mean who at AMD thought that is a good idea?
Post 24 May 2005, 17:15
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MCD



Joined: 21 Aug 2004
Posts: 604
Location: Germany
MCD
Thanks f0dder for that stunning news! I had discovered this feature in AMDs manuals some time ago, but suretainly overread it.

I just made a little DOS only tool that uses this behaviour! seehttp://board.flatassembler.net/topic.php?t=3521
Post 25 May 2005, 11:28
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