Fascinating thread. Do you know if the same issues exist on low power, embedded CPUs like ESP32, or is this something that mostly affects high-end stuff?

@perpetuum_mobile @gabrielesvelto I used to even code in assembler on 8 bit platforms, for years I could not quite get my head round how modern CPUs worked until this thread (and now I know a bit more)

@gabrielesvelto

I don’t cut any slack for Intel producing two whole generations of CPUs with manufacturing flaws then trying to cover it up and never really offering full restitution to any customers.

@gabrielesvelto It was a very rich, exciting, interesting, and useful post! Thank you very much!

@vfrmedia @perpetuum_mobile if you have some free time this is a good deep dive: https://cseweb.ucsd.edu/classes/fa14/cse240A-a/pdf/04/Gonzalez_Processor_Microarchitecture_2010_Claypool.pdf

While it doesn't cover some of the most recent advancement it captures 90% of what you need to know.

If you have a lot of free time and want to dive deeper there's this: https://www.agner.org/optimize/microarchitecture.pdf

@gabrielesvelto The book 'Silicon' by the Italian who designed the 4004, 8080 and Z80 is a most splendid read. Fascinating that he had to add reverse engineering optical confusions to minimise cloning by rivals.

@vfrmedia @gabrielesvelto I did code a little bit in x86 asm when I was a teen. It was the only way to turn on SVGA modes in Turbo Pascal and I wanted to make games back then I did a program which simulated a flame in real time, doing per pixel average of surrounding pixels and adding random 255 sparks on the bottom to make the flame move and look real

@x0 hardware design already involves various forms of fuzzing, but the amount of state involved is so large that no amount of testing can be truly exhaustive

@gabrielesvelto Makes sense. Especially when something like voltage/temperature fluctuations are involved.

@grumble209 @gabrielesvelto I remember that the UltraSPARC-II (Blackbird) CPU, over it's lifetime (and to date, to boot) only had a single errata, and that was an extremely unlikely timing issue, not a logic bug. Unfortunately, that overall goodness was offset by the widespread off-chip L2 cache issue circa y2k.

@gabrielesvelto haha for sure

@gabrielesvelto I actually keep meaning to find a decent reference text on FET construction and modelling. I've got plenty on SI/EMI, power delivery, etc. but everything I've found for FETs has been the sort of thing that presumes you're either someone with a deep background in semiconductor physics or a professional semiconductor/ASIC engineer just looking for a reference text. very little out there for EE folks who are coming at it from the practical side.

@burnitdown @gabrielesvelto in the 70's, most issues were largely logic bugs, nowadays there are a larger proportion of timing/analogue issues.

@gabrielesvelto wow, and where does it get the microcode from? Another computer within the computer? (turtles and all that

@gabrielesvelto I was just thinking about those bugs and something crept up my neck when i thought "now add hallucinating AIs to all that". Pretty sure they're already used in CPU development to deal with the increasing complexity.
So the problem will become much worse than it already is.

for the article. Loved it.

@gsuberland @gabrielesvelto You mean you don’t want my college device physics textbook that starts with solving Schrödinger's equation for a hydrogen atom? They should not have allowed that class at 7am.

For the practical side, I really like Jacob Baker’s books.

@gabrielesvelto As someone involved in CPU design in those days I would frame it slightly differently. The bugs were there but we did a better job of providing work arounds, usually through compiler changes to avoid code sequences that would trigger the bug. The FDIV bug was memorable because of how many Pentium chips were in customer hands before it was discovered.

@mdione @gabrielesvelto in the same flash that contains UEFI. There's a set of headers that describe what is in the flash. That typically includes microcode for the chip generations supported by the motherboard. For example, a board that supports Zen2 and Zen3 will have two microcodes in the flash and the one that matches the CPU installed will be used

@gabrielesvelto @eniko I shouldn’t have read that while sick. Now every time I’m between sleep and wake, I have one of these feverish hallucinations that I’m a little worker inside a CPU core, waiting for a branch prediction to resolve, my hand on the button that dumps everything that was wrongly preloaded.

That’s a very boring job.

@gabrielesvelto #getafuckingblog