[lolsowrong]

Most of the time. But sometimes, no. See ATL thunk emulation (last I checked, still alive in the windows kernel) and ntvdm handling of the BOP pseudoinstruction.

See also: Jazelle DBX.

Hell, on modern x86 processors, many “native” instructions are actually a series of micro-ops for a mostly undocumented and mostly poorly understood microcode architecture that differs from the natively documented instruction set.

It’s turtles all the way down.

[fc417fc802]

Aren't all of them microcoded? Some years back root was achieved on a line of intel processors and new instructions implemented as proof of concept. There's an academic paper, citation not immediately to hand.

[trynumber9]

Some instructions are microcoded but others take the fast path and avoid the microcode sequencer. Can't patch the latter in microcode RAM.

[lolsowrong]

I saw the paper from Google last year and thought something in it aligned with not everything running through the microcode engine, though I could be wrong.

[fc417fc802]

Might well be the case. I don't think I'm familiar with the paper you're referring to; any chance of at least a vague description?

[lolsowrong]

Can’t find the pdf, but it’s all related to the zentool stuff:

https://github.com/google/security-research/blob/master/pocs...

Tavis spells it out there pretty quickly:

“ The simplest instructions (add, sub, mov, etc) are all implemented in hardware. The more complicated instructions like rdrand, fpatan and cmpxchg are microcoded. You can think of them as a bit like calling into a library of functions written in that RISC-like code.”

[saagarjha]

Jazelle and micro-ops are not interpreters, they are executed in hardware.

[lolsowrong]

I believe only some parts of Jazelle are handled in hardware, though I don’t know if anybody has got their hands on any of the bits of the software side. I do know there’s documentation on handling unimplemented instructions.

I don’t know how I feel about micro-ops being executed in hardware - I mostly agree, but also, microcode updates exist…

[fc417fc802]

An interpreter implemented in hardware is still an interpreter. Hot take, all machine instruction sets are scripting languages and LLVM is a transpiler.