Mystery Cpuid Bit | Hacker News Reader

Mystery Cpuid Bit

os2museum.com

39 pointsuserbinator3 comments

And people say "never read the comments":

CL says: April 20, 2026 at 9:18 pm

I updated sandpile.org to reflect that

  Bit 19 = MP-capable
  Bit 18 = ECC-capable

Those were two distinct capabilities.

[sources: K7 µcode + K8 XOS-based public simulator published in 2000 + I worked at AMD back then]

That’s the NSA backdoor /s

Unfortunately, there is no need to advertise separately in CPUID that a CPU is backdoor-capable, because other features implicitly specify whether the processor supports backdoors.

Since Intel has added the System Management Mode in 80386SL (October 1990), all Intel and AMD CPUs that support SMM implicitly enable the insertion of backdoors in the BIOS/UEFI firmware. ARM has followed the Intel example by introducing EL3 (Exception Level 3), which is equivalent with Intel SMM and it also enables the insertion of backdoors in the computer firmware.

Most modern CPU vendors have not remained content with enabling firmware backdoors, but they have added separate management processors, which may be shamelessly named "security processors", enabling backdoors that are even harder to detect and disable.

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