My Mastodon thread about the bug was on HN a few weeks ago, so this might seem familiar, but now I've finished a detailed blog post. The previous HN post has a bunch of comments: https://news.ycombinator.com/item?id=42391079
For the Pentium, I'm curious about the FPU value stack (or whatever the correct term is) rework they did. It's been a long time, but didn't they do some kind of early "register renaming" thing that had you had to manually manage doing careful fxchg's?
IIRC fadd and fmul were both 3/1 (three cycles latency, one cycle throughput), so you'd start an operation, use the free fxch to get something else to the top, and then do two other operations while you were waiting for the operation to finish. That way, you could get long strings of FPU operations at effectively 1 op/cycle if you planned things well.
IIRC, MSVC did a pretty good job of it, too. GCC didn't, really (and thus Pentium GCC was born).
Compilers got pretty good at optimizing straight line math but were not as good at cases where variables needed to be kept in the stack during a loop, like a running sum. You had to get the order of exchanges just right to preserve stack order across loop iterations. The compilers at the time often had to spill to memory or use multiple FXCHs at the end of the loop.
Huh, are you sure? Do you have any documentation that clarifies the rules for this? I was under the impression that something like `FMUL st, st(2) ; FXCH st(1), FMUL st, st(2)` would kick off two muls in two cycles, with no stall.
You can immediately overlap with a FADD.
How hard is it to "dump" the microcode into a bitstream? Could it be done programatically from high resolution die photographs? Of course, I appreciate that's probably the easy part in comparison to reverse engineering what the bitstream means.
> By carefully examining the PLA under a microscope
Do you do this stuff at home? What kind of equipment do you have in your lab? How did you develop the skills to do all this?
I do this stuff at home. I have an AmScope metallurgical microscope; a metallurgical microscope shines light down through the lens, rather than shining the light from underneath like a biological microscope. Thus, the metallurgical microscope works for opaque chips. The Pentium is reaching the limits of my microscope, since the feature size is about the wavelength of light. I don't have any training in this; I learned through reading and experimentation.
I suppose you might be able to get slightly better resolution using a shorter wavelength, but at that point, it requires a lot of technical skill and environmental conditions and time and money, Just getting to the point you've reached (and knowing what the limitations are) can be satisfying in itself.
I never realised this is how floating point division can be implemented. Actually funny how I didn't realise that multiple integer division steps are required to implement floating point division :-)
In hindsight one could wonder why the unused parts of the lookup table were not filled with 2 and -2 in the first place.