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Show HN: I486SX_soft_FPU – Software FPU Emulator for NetBSD 10 on 486SX
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 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
- _mlbt 1y agoThese used to be very common before Pentium processors. Many (most?) compiler tool chains included a software FPU emulator. Very cool project!
- actionfromafar 1y agoI 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 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 1y agoFunny 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 1y agoYap. 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.
- rasz 1y agoDuke 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 https://www.youtube.com/watch?v=41O2bNG2qKA&t=234s while staying above 30 when facing away from slopes.
- anthk 1y agoCould a table help there?
- theragra 1y agoI recently encountered this when developing for atmel attiny13a, that has 1KB memory for code. I found that my code somehow cannot fit. Then found that just one line takes around 100 bytes. Insane! After some experiments, I remembered university course and was able to understand what is going on. When using float types or operations, fpu emulation is used, which takes huge amount of memory for anything. I think atmel is using gcc, but not sure if you emulation is part of gcc or atmel libraries.
- rzzzt 1y agoThat .jpg in the readme file loaded from right to left. What is this sorcery?
- actionfromafar 1y agoApparently¹ : <img align="right"> I'm also impressed. :-D 1: https://github.com/mezantrop/i486SX_soft_FPU/blob/main/README.md https://github.com/mezantrop/i486SX_soft_FPU/blob/main/READM...
- accrual 1y agoThe 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 1y agoI am also just noticing the laptop display seems to be held together with K'nex... truly a hacker setup.
- adzm 1y agoPretty sure it's orientation in the jpeg itself
- 1y ago
- queenkjuul 1y agoThis 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.
- LeFantome 1y agoI am sure you are aware of this: https://github.com/marmolak/gray386linux https://github.com/marmolak/gray386linux It uses kernel 3.7 For 486, there was a release just two months ago using kernel 6.13: https://github.com/marmolak/gray486linux https://github.com/marmolak/gray486linux
- queenkjuul 1y agoI actually wasn't familiar with those, they're cool but don't quite fit all my goals, though could still be useful to me
- accrual 1y agoThis 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 1y 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/ https://wiki.netbsd.org/ports/i386/
- PrivacyAI 1y agoThis is super cool!
- hsnewman 1y agoI tried it under the Misterfpga ao486 core and it failed, not finding a partition to boot from. :(
- karlgkk 1y agoSo you just consoomed something. Thanks for the contribution
- natas 1y agoI wonder if this means we will soon be able to run Netbsd-10.1 (and above) on MiSTer with ao486 core.
- esafak 1y ago[flagged]
- LukeShu 1y agoIt'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 1y 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 https://en.wikipedia.org/wiki/486sx EDIT: I get your point but this is what OP was referring to
- userbinator 1y agoNot only the 486SX but a lot of alternative x86 SoCs aimed at embedded systems have no FPU, including the original Vortex86.
- jabl 1y agoJust a few days ago patches were posted proposing to drop Linux kernel support for x86 cpus lacking a FPU: https://lore.kernel.org/lkml/20250425084216.3913608-1-mingo@kernel.org/ https://lore.kernel.org/lkml/20250425084216.3913608-1-mingo@...
- remlov 1y agoGood thing this has nothing to do with NetBSD.
- ahartmetz 1y agoFrom an outside POV, Linux supports quite unreasonably old hardware, so it's always weird and surprising when someone decides that "now it's TOO unreasonable to keep supporting it".
- karlgkk 1y agoIt often shows up when someone needs to touch an area of code, where archaic compatibility makes everyone’s life harder.
- evmar 1y agoI 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 1y agoThe 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/math/math_emulate.c https://github.com/386bsd/386bsd/blob/2.0/usr/src/kernel/mat...) and made it from there into the NetBSD mainline.
- nikanj 1y agoI don't think the BSD license is compatible with "you may not distribute this for a fee"
- einr 1y agoTrue! 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...
- Turboblack 1y agofinally something really useful, I wish you creative success, you are doing the right thing
- bezko 1y agoWhat are some examples of an OS needing to do floating point numbers calculations?