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Linux on a 486SX
  A year or two ago, I came into posession of a 1993 Compaq Presario 425.  It's a Mac-ish all-in-one with a 14" color (S?)VGA screen capable of 800x600 in full 256-color glory.  On the inside it features a 25MHz i486SX (the sort without a floating-point unit), paired with 20MB of RAM - a significant upgrade over the stock 4MB.  When I received it it had a nearly-dead 1.6GB hard drive[1] in it, which I replaced with a far overkill 150GB drive[2].  It also has a single 3.5" floppy drive.  On the back it has two PS/2 ports for keyboard and mouse input, serial and parallel ports, and two ports for the built-in modem.  There is also what appears to be a sound card, along with a single 36-pin connector I don't recognize.

  The hard drive it came with had (as far as I can tell) the stock Windows for Workgroups 3.1 (not 3.11) installation, with Wolfenstein 3D and a couple other games.  It had clearly been heavily used and contained a large amount of personal data.

But can it run Linux?
  I've spent the past several months trying on and off to make Linux run on the Presario.  The 486SX is the oldest CPU Linux still supports!  I was quite hindered by my lack of any floppy disks - fortunately, I managed to get my hands on a few working ones for Christmas this year and made some headway, first getting MS-DOS 6.22 installed on the new hard disk, then messing with the Linux kernel configuration until I got it to work.

  And yesterday I finally got it!  Here are the steps for configuring a basic kernel with Linux 5.14.8.

  Begin with make tinyconfig.  Then run make menuconfig, and change at least the following options:
    - Under General setup, turn on Configure standard kernel features -> Enable support for printk, and change the kernel compression mode to LZO for faster decompression.
    - Under Processor type and features, change Processor family to 486SX.  Turn on Enable the LDT and Math emulation. Change Physical address where the kernel is loaded to 0xF00000 (only necessary for bigger kernels, but good to have). Change Alignment value to which the kernel should be aligned to 0x20000.
    - Under Bus options (PCI  etc.), enable ISA support.
    - Turn on Enable the block layer.
    - Under Executable file formats, turn on Kernel support for ELF binaries and Kernel support for scripts starting with #!.
    - Under Device drivers:
      - Turn on Block devices and Block devices -> Normal floppy disk support.
      - Under SCSI device support:
        - Turn on SCSI disk support.
        - Turn on SCSI generic support.
      - Under Serial ATA and Parallel ATA drivers (libata):
        - Turn on ATA SFF support.
        - Turn on Generic platform device PATA support.
        - Turn on Legacy ISA PATA support.
      - Make sure Input devices -> Keyboards -> AT keyboard is enabled.
      - Under Character devices:
        - Turn on Enable TTY.
        - Turn on TTY driver to output user messages via printk.
      - Make sure Graphics support -> Console display driver support -> VGA text console is enabled.
    - Under File systems, enable Second extended fs support, DOS/FAT/EXFAT/NT filesystems -> MSDOS fs support, and DOS/FAT/EXFAT/NT filesystems -> VFAT (Windows-95) fs support.
    - Under File systems -> Pseudo filesystems, enable /proc file system support and sysfs file system support.
  
  If you've done everything correctly you should have a working kernel.  Mine has some other convenience things enabled, but the above is the bare minimum for Linux to boot.

  For the init system and coreutils I chose BusyBox as it's extremely lightweight.  I built it with musl-cross-make.

  For loading the kernel I used Syslinux on a FAT partition with the following configuration:

    DEFAULT lin
    LABEL lin
      LINUX /linux
      APPEND rw loglevel=5 root=/dev/sda2

  Putting this all together the result is a system image of about 4MB in size (though the image I actually created is 128MB to give some extra space).  It's surprisingly responsive for a 30-year-old machine, and only takes around 50 seconds to do a cold boot - not much longer than Windows 95, and at least 30 seconds of that is the BIOS doing a memory test.

  You can download the resulting image for 486SX or 486DX.  You should be able to burn it directly to a hard drive[3] and boot it on just about any machine with at least 16MB of RAM.  I've included almost the full set of BusyBox utilities as well as Lua.  Pretty much all of the init setup comes from Krzysztof Krystian Jankowski's Floppinux tutorial.  The default credentials are root / root.

  Edit 2022-02-01: Slimmed down another MB of memory usage by moving the kernel load offset again; now only requires 8MB.

  Edit 2022-09-20: Increased the kernel load offset because a change somewhere, probably in GCC, made it mis-use screen data.  Added a sentence on the kernel alignment address.  Now requires 16MB again.

  Here is a video of the system booting.  (Edited 2022-01-01: added updated video)

  Here is a (slightly outdated) video of the system booting.  Since recording this video I've switched from LZMA to LZO for kernel compression, which made kernel decompression at least 200% faster.

Footnotes
  1.  The stock drive was 200MB.
  2.  The BIOS can only see 5GB or so of it.  I only put in such a large hard drive because that was what I found on Amazon.
  3.  After gzip -d-ing it, of course.