Extremely CISC instructions

Guy Dunphy guykd at optusnet.com.au
Mon Aug 23 21:21:27 CDT 2021

>For the sake of illustration to folks who are not necessarily used to
>thinking about what computers do at the machine code level, I'm interested
>in collecting examples of single instructions for any CPU architecture that
>are unusually prolific in one way or another. This request is highly
>underconstrained, so I have to rely on peoples' good taste to determine
>what counts as "interesting" here.

This is perhaps outside even the vague bounds you were thinking of, but it
probably wins the 'unusually prolific' prize by a gigabyte-mile.
Behold, the hidden, secret and heinous X86 2-byte 'launch instruction' 0x0F, 0x3F.

See this talk about the discovery:

DEF CON 26 - Christopher Domas - GOD MODE UNLOCKED Hardware Backdoors in redacted x86      46:03
DEFCON Conference    Oct 23, 2018
Complexity is increasing. Trust eroding. In the wake of Spectre and Meltdown, when it seems that things cannot get any darker for processor security, the last light goes out. This talk will demonstrate what everyone has long feared but never proven: there are hardware backdoors in some x86 processors, and they're buried deeper than we ever imagined possible. While this research specifically examines a third-party processor, we use this as a stepping stone to explore the feasibility of more widespread hardware backdoors.

After which you will never trust your Intel-based PC, ever again.


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