Show HN: I486SX_soft_FPU – Software FPU Emulator for NetBSD 10 on 486SX

github.com

75 points by mezantrop 13 hours ago

First Release is Here!

I'm excited to announce the first release of i486SX_soft_FPU — a software FPU emulator for the classic Intel 486SX CPU, running on NetBSD 10!

This project brings floating-point support back to life for 486SX machines, even though modern NetBSD versions no longer natively support processors without a hardware FPU. If you're into retrocomputing, operating system hacking, or just love old-school hardware, check it out!

Project page: https://github.com/mezantrop/i486SX_soft_FPU Contributions, feedback, and testing are all very welcome!

Let's keep these vintage machines alive!

#retrocomputing #NetBSD #486SX #opensource

evmar 4 hours ago

I noticed math_emulate.c comes from Linux (it even has a " * (C) 1991 Linus Torvalds" bit on it). I was wondering what the license on that code is. It looks like Linux adopted GPL in 1992 so maybe this copy predates that, but it was under some other non-BSD license before that.

  • einr 2 hours ago

    The full license for Linux prior to 0.12 was:

      This kernel is (C) 1991 Linus Torvalds, but all or part of it may be
  redistributed provided you do the following:

  - Full source must be available (and free), if not with the
    distribution then at least on asking for it.

  - Copyright notices must be intact. (In fact, if you distribute
    only parts of it you may have to add copyrights, as there aren't
    (C)'s in all files.) Small partial excerpts may be copied
    without bothering with copyrights.

  - You may not distibute this for a fee, not even "handling"
    costs.
    This is clearly written by someone who has no business writing software licenses ;) but does not appear to be incompatible with the BSD license and in fact, the code in question originates from 386BSD (https://github.com/386bsd/386bsd/blob/2.0/usr/src/kernel/mat...) and made it from there into the NetBSD mainline.
    • nikanj 2 hours ago

      I don't think the BSD license is compatible with "you may not distribute this for a fee"

      • einr an hour ago

        True! And if so, that license has clearly been broken many times by everyone selling 386BSD, NetBSD and Linux <0.12 on CD-ROMs etc ;)

        Then again -- and IANAL -- the license is worded so vaguely that I doubt any of it is enforcible. "You may not distibute this for a fee" -- what is "this"? Is it the entire kernel or does it apply to small excerpts of it? Because apparently "small partial excerpts may be copied without bothering with copyrights". But do you mean copyright attribution or are you rescinding your copyright entirely if I only copy "small partial excerpts"? But what is a small partial excerpt? And so on and so forth...

        • wkat4242 an hour ago

          I think nikanj means that this is not a stipulation of the BSD license at all, that's why it's not compatible with what Linus made there.

accrual 10 hours ago

This is super cool and I'm very glad to see such work! Does NetBSD natively support the 486DX CPUs which still have the FPU enabled?

I know OpenBSD dropped support for i486 in 6.8, making 6.7 the last that runs natively on them (FPU or not). The support was lost when moving to LLVM 9 and enabling `-march=i586 on i386`. However, old Socket 3 boards can still run the latest version of OpenBSD if one drops a Pentium Overdrive in. :)

  • dcassett 5 hours ago

    > Does NetBSD natively support the 486DX CPUs which still have the FPU enabled?

    According to [1] they do

    "Any i486 or better CPU should work - genuine Intel or a compatible such as Cyrix, AMD, or NexGen."

    [1] https://wiki.netbsd.org/ports/i386/

natas 8 hours ago

I wonder if this means we will soon be able to run Netbsd-10.1 (and above) on MiSTer with ao486 core.

rzzzt 11 hours ago

That .jpg in the readme file loaded from right to left. What is this sorcery?

  • quotemstr 9 hours ago

        Z$ identify -verbose weird-jpeg.jpg
    Image:
      Filename: weird-jpeg.jpg
      ...
      Orientation: RightTop
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
      ...
      Properties:
        ...
        unknown: iPhone 12 Pro
  • adzm 11 hours ago

    Pretty sure it's orientation in the jpeg itself

  • actionfromafar 11 hours ago

    Apparently¹ :

        <img align="right">
    I'm also impressed. :-D

    1: https://github.com/mezantrop/i486SX_soft_FPU/blob/main/READM...

    • accrual 11 hours ago

      The align tag isn't related, the image will load right-to-left even if opened by itself in a new tab.

      In JPEGSnoop I see Raw Image Orientation = Landscape. Maybe it was taken and encoded horizontally and the image itself is stored in a right-to-left scan order.

      • accrual 10 hours ago

        I am also just noticing the laptop display seems to be held together with K'nex... truly a hacker setup.

        • akadruid1 3 hours ago

          I believe at least some of it is LEGO Technic

userbinator 6 hours ago

Not only the 486SX but a lot of alternative x86 SoCs aimed at embedded systems have no FPU, including the original Vortex86.

queenkjuul 11 hours ago

This is awesome. I'm working on packaging a Linux distro for my 386 and 486, but I've stuck to just using very old kernels.

hsnewman 9 hours ago

I tried it under the Misterfpga ao486 core and it failed, not finding a partition to boot from. :(

dlachausse 12 hours ago

These used to be very common before Pentium processors. Many (most?) compiler tool chains included a software FPU emulator.

Very cool project!

  • rasz 5 hours ago

    Duke Nukem 3D is compiled with FPU emulation. Game engine is 99.9% fixed point math, only requires FPU for rarely used slopes (setupslopevlin_ and slopevlin_).

    It just so happens first room when starting the game - rooftop - has sloped roof vents and later walls with sloped edge. Even on fast FPUless 90MHz NexGen Nx586 (AMD K6 father) FPS drops down to 10-14fps on that roof https://www.youtube.com/watch?v=41O2bNG2qKA&t=234s while staying above 30 when facing away from slopes.

    • anthk an hour ago

      Could a table help there?

  • actionfromafar 11 hours ago

    I think GCC didn't have the emulating library, but the operating system had. (Such as Debian.) So you could compile and link your programs against a soft-float library. ( Something like this https://github.com/ant6n/ieeelib )

    What this NetBSD project does is not exactly like that though, it lets programs use regular 487 float instructions, which are trapped by the kernel, which steps in and emulates what the hardware float instruction would have done.

    It worked very well for regular program, because most programs would not use float instructions to any significant degree.

    If you however were going to use floats a lot for long calculations, a soft-float library would be much faster.

    • queenkjuul 11 hours ago

      Funny enough, I've been compiling a lot of stuff for 386 Linux lately. You can build a kernel with built in software floating point, at which point it doesn't matter what your library/compiler do. If your kernel isn't built to handle that, you can build glibc with floating point emulation.

      At least this is my impression, working with 2.2.x/2.4.x kernels, gcc 2.7~3.3, and glibc ~2.2

      • anyfoo 11 hours ago

        Yap. But as hinted by the comments before you, if you have CPUs without FPUs, you probably want to enable userspace (e.g. glibc) soft math support anyway, since it doesn’t have the overhead of trapping the instructions and context switching into the kernel and back.

        The benefit of OP’s solution in the kernel is that it works for everything out of the box, including pre-compiled binaries, and those that you can’t rebuild for whatever reason to begin with.

esafak 7 hours ago

Help me understand why you'd emulate an artificially crippled chip?!

  • LukeShu 7 hours ago

    It's not emulating a crippled chip, it runs on the i486SX (which has no FPU, by artificial limitation or not), and emulates an x87 FPU.

    • dcassett 6 hours ago

      > It's not emulating a crippled chip

      "Initial reviews of the i486SX chip were generally poor among technology publications and the buying public, who deemed it an example of crippleware." [1]

      [1] https://en.wikipedia.org/wiki/486sx

      EDIT: I get your point but this is what OP was referring to