SiMPLE OS

A from-scratch 32-bit hobby operating system written in C and x86 assembly.

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What it is

SiMPLE OS boots via the Stivale2 protocol on Limine, runs on i686, and provides a graphical desktop with a real kernel/userspace split, preemptive multitasking, and a syscall interface.

It started as a framebuffer experiment and grew into a small but complete OS kernel with working process isolation, ELF loading, FAT16 storage, and a GUI desktop.

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Architecture

Component	Details
Boot	Limine → Stivale2 → kernel_main()
Target	i686, 32-bit, freestanding C + x86 asm
Display	800×600 32bpp framebuffer
Memory	Bump allocator, per-process paging
Scheduling	Preemptive round-robin, 100Hz PIT, 100ms timeslice
Processes	Up to 4 concurrent, ring 3 isolation
Filesystem	FAT16 over ATA PIO (LBA28)
Syscalls	int 0x80, 12 calls

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Syscalls

ABI: eax = number, ecx = arg0, edx = arg1, ebx = arg2, return in eax.

#	Name	Args	Description
1	SYS_WRITE	buf, len	Write to VGA terminal
2	SYS_EXIT	code	Terminate process
3	SYS_READ	buf, max_len	Blocking keyboard input
4	SYS_YIELD	—	Cooperative yield
5	SYS_OPEN	path, flags	Open/create file
6	SYS_CLOSE	fd	Release file descriptor
7	SYS_FREAD	fd, buf, len	Read from file
8	SYS_FWRITE	fd, buf, len	Write to file
9	SYS_SEEK	fd, offset, whence	Seek in file
10	SYS_EXEC	path	Replace process with ELF
11	SYS_FORK	—	Fork current process
12	SYS_WAIT	—	Wait for child to exit

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Desktop

• Framebuffer window manager
• Mouse cursor + window dragging, focus, z-order
• Alt+Tab switching, Alt+Arrow movement
• App launcher

Built-in apps:

• STerm — graphical terminal with independent shell sessions
• SText — text editor with scrolling and cursor movement
• Calculator — GUI calculator with mouse + keyboard input

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Process Model

• Up to 4 concurrent processes
• Ring 3 userspace, ring 0 kernel
• Per-process page directories (user memory isolated)
• Preemptive scheduler via PIT at 100Hz
• proc_fork() duplicates page directory + register state
• Process states: RUNNING, RUNNABLE, BLOCKED, ZOMBIE, DEAD

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Building

Requirements:

• i686-elf-gcc (cross compiler)
• nasm
• qemu-system-x86_64
• mtools
• make

make

Produces simple.img.

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Running

qemu-system-x86_64 \
  -m 512M \
  -drive format=raw,file=simple.img \
  -display sdl

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Shell Commands

help, clear, echo, ls, cd, open, edit, touch, mkdir, rm, cp, mv
run <file.elf>   — execute a userspace ELF binary
ps               — show process table
poweroff / reboot

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User Programs

Freestanding ELF binaries in user/ using int 0x80:

Program	Tests
hello.c	Basic write syscall
forkwait.c	SYS_FORK + SYS_WAIT
forktest.c	Fork behavior
exectest.c	SYS_EXEC
fwritetest.c	File I/O
seektest.c	SYS_SEEK
systest.c	Syscall coverage

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Roadmap

• SYS_SBRK + userspace malloc
• VFS layer
• EXT2 support
• Networking experiments
• Loadable kernel modules
• Compositing

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Philosophy

SiMPLE OS is not trying to be Linux or POSIX-compatible. It exists to be readable, hackable, and fun. Inspired by TempleOS, classic Macintosh internals, and BSD design philosophy.

Expect bugs, architectural rewrites, and ambitious features implemented at 3am.

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License

See LICENSE.