Fiwix is an operating system kernel based on the UNIX architecture and fully focused on being POSIX compatible. It is designed and developed mainly as a hobbyist operating system, but it also serves for educational purposes. It runs on the i386^[1] hardware platform and is compatible with a good base of existing GNU applications. It follows the UNIX System V application binary interface and is also mostly Linux 2.0 system call ABI compatible.

The FiwixOS 3.4 operating system is a Fiwix distribution. It uses the Fiwix kernel, includes the GNU toolchain (GCC, Binutils, Make), it uses Newlib v4.4.0 as its C standard library, and Ext2 as its primary file system.

Between October 2022 and the whole 2023^[2] the Fiwix kernel accepted a series of patches that were necessary to be able to be compiled with TCC. This was a necessary step into the whole bootstrapping process^[3] to build a complete Linux distribution from scratch,^[4] with Fiwix being currently a crucial part^[5] of it.

Features according to the official website include:

• Written in ANSI C language (Assembly used only in the needed parts).
• GRUB Multiboot Specification v1 compliant.
• Full 32bit protected mode non-preemptive kernel.
• POSIX compliant (mostly).
• For i386 processors and higher.
• Process groups, sessions and job control.
• Interprocess communication with pipes, signals and UNIX-domain sockets.
• UNIX System V IPC (semaphores, message queues and shared memory).
• BSD file locking mechanism (POSIX restricted to whole file and advisory only).
• Virtual memory splits (user/kernel): 3GB/1GB and 2GB/2GB.
• Linux 2.0 ABI system calls compatibility (mostly).
• ELF-386 executable format support (statically and dynamically linked).
• Round Robin based scheduler algorithm (no priorities yet).
• VFS abstraction layer.
• Kexec support.
• Ext2 filesystem support with 1KB, 2KB and 4KB block sizes.
• Minix v1 and v2 filesystem support.
• Linux-like Proc filesystem support (read only).
• ISO9660 filesystem support with Rock Ridge extensions.
• RAMdisk device support.
• Initial RAMdisk (initrd) image support.
• SVGALib based applications support.
• PCI local bus support.
• Virtual consoles support (up to 12).
• Keyboard driver with Linux keymaps support.
• PS/2 mouse support.
• Frame buffer device support for VESA VBE 2.0+ compliant graphic cards.
• Framebuffer console (fbcon) support.
• Serial port RS-232 driver support.
• Remote serial console support.
• QEMU Bochs-style debug console support.
• Basic implementation of a Pseudo-Random Number Generator.
• Floppy disk device driver and DMA management.
• IDE/ATA hard disk device driver.
• IDE/ATA ATAPI CD-ROM device driver.

• Sanfeliu, Jordi (2003-10-16). "The GNU/Fiwix Operating System". osnews.com.
• "Fiwix: Unix-like kernel for the i386 architecture". Hacker News. 2024-07-05.
• "Fiwix: Small Unix-Like Kernel". Hacker News. 2025-02-05.

• Official website