Reinhard OS

A small 32-bit x86 operating system written in C and assembly.

Features

Area	What's implemented
Boot	Multiboot (GRUB or qemu -kernel), 16 KiB boot stack
CPU	GDT (flat model), IDT with all 32 exception vectors + 16 IRQs, PIC remap, spurious-IRQ handling
Drivers	VGA text mode (scrolling, hardware cursor, colours), PIT timer @ 100 Hz, PS/2 keyboard (shift, caps lock, ctrl, arrows, Home/End/Del/PgUp/PgDn)
Memory	Bitmap physical memory manager, two-level paging with recursive page-directory mapping, NULL-page guard, demand-paged kernel heap, first-fit allocator with split/coalesce
Filesystem	Hierarchical RAM filesystem (files, directories, ./.., absolute/relative paths)
Apps	Notepad full-screen editor; shell with history, quoting and > / >> redirection for echo

Layout

src/boot/      multiboot header + _start
src/arch/      gdt, idt, pic, isr dispatcher, interrupt stubs (asm)
src/drivers/   vga, timer, keyboard
src/mm/        pmm (frames), paging (page tables, fault handler), heap (kmalloc)
src/fs/        ramfs
src/lib/       string routines, kprintf/ksnprintf
src/shell/     shell loop + built-in commands
src/apps/      notepad
include/       one header per module

Build and run

Requirements: i686-elf-gcc (recommended) or a 32-bit-capable host gcc + ld, plus make and QEMU. For ISOs also grub-mkrescue and xorriso.

make          # build/reinhard_os.bin
make run      # boot straight in QEMU (multiboot, no ISO)
make iso      # reinhard_os.iso via GRUB
make run-iso
make clean

Shell commands

help about hello clear echo ls cd pwd mkdir touch cat rm notepad/edit meminfo pageinfo vmtest uptime crash reboot shutdown

Notepad keys: arrows, Home/End, PgUp/PgDn, Backspace/Delete, Tab (4 spaces), Ctrl+S save, Ctrl+Q quit (press twice to discard unsaved changes).

Try these to see paging in action:

vmtest 256            # touch 256 untouched heap pages -> 256 demand-paging faults
pageinfo 0xB8000      # walk the page tables for the VGA buffer
pageinfo 0xD0000000   # first heap page
crash                 # NULL write -> page-fault panic with register dump

Design notes

• Recursive paging. The last page-directory entry points at the directory itself, so the directory lives at 0xFFFFF000 and page table N at 0xFFC00000 + N*4096. Mappings can be edited for any frame without identity-mapping all of RAM.
• Demand paging. heap_grow() only moves the brk pointer. The first touch of a page raises a page fault; the handler allocates a frame, maps it, zeroes it and resumes. Faults outside the heap (or protection faults) panic with a decoded error code.
• Memory map. The PMM reads the Multiboot memory map; low memory and the kernel image (kernel_end from linker.ld) are reserved. The first 8 MiB are identity-mapped.
• No libgcc. The code avoids 64-bit division, so the link needs nothing beyond the kernel.
• string.c is built with -fno-tree-loop-distribute-patterns; otherwise GCC may compile memset into a call to itself.

Roadmap

Preemptive multitasking and a scheduler, user mode (ring 3 + TSS + syscalls), ELF loader, ATA/disk driver with a persistent filesystem, then networking (the original TCP/IP goal).