A 32-bit x86 operating system kernel written from scratch — no libc, no host runtime, no framework. Boots under GRUB, sets up its own descriptor tables, services its own hardware interrupts, manages its own memory, and drops you into an interactive shell.
=== Hermes OS Shell === Type 'help' for available commands. $ help Available commands: help Show this help message hello Print hello world echo Echo text to screen uptime Show system uptime ps List running tasks ls List files in filesystem cat Display file contents mem Show memory info reboot Reboot the system $ hello Hello, World! Welcome to Hermes OS. $ echo hermes os hermes os $ uptime Uptime: 15.7 seconds (1575 ticks) $ ps PID STATE NAME --- ----- ---- 0 READY demo1 1 READY demo2 2 READY demo3 $ ls NAME TYPE SIZE ---- ---- ---- README FILE 665 VERSION FILE 60 HELLO FILE 37 docs DIR 0 $ cat README ==================================== Hermes OS - x86 Kernel from Scratch ==================================== This is a minimal x86 operating system kernel built from scratch featuring: [BOOT] Multiboot-compliant bootloader [GDT] Protected mode with segmentation [IDT] Interrupt handling (32 IRQs + 16 exceptions) [PIC] Programmable Interrupt Controller [TIM] PIT timer with 100Hz scheduling tick [KBD] PS/2 keyboard driver [MMU] Physical memory manager + paging [SCH] Preemptive round-robin scheduler [FS] Simple in-memory filesystem [SH] Interactive command shell Commands: help, hello, echo, uptime, ps, ls, cat, mem, reboot $ mem Free memory blocks: 15784 (63136 KB) $
Captured from the serial port of a real qemu-system-i386 boot, with keystrokes
injected as genuine PS/2 scancodes through the QEMU monitor — the same path a physical
keyboard would take.
boot/boot.asmMultiboot header, GRUB-loadable, enters 32-bit protected mode at 1 MB
kernel/gdt.c5 descriptors — null, ring-0 code/data, ring-3 code/data
kernel/idt.c256 gates: 32 CPU exceptions + 16 hardware IRQs
kernel/irq.c8259A remapped to vectors 32–47, per-IRQ handler registry
kernel/timer.cPIT channel 0 at 100 Hz, tick counter driving uptime
kernel/keyboard.c8042 PS/2 init, scancode→ASCII, shift/caps, ring buffer
kernel/pmm.cBitmap physical allocator + x86 paging, 16 MB identity-mapped
kernel/scheduler.cRound-robin task table, TCBs, assembly context switch
kernel/fs.cIn-memory file table with read/list/stat
kernel/shell.c9 built-in commands, line editing, VGA + serial output
Every IRQ is dispatched as irq_handler(registers_t*), but the assembly stub called irq_handler without pushing the frame pointer. The C code then read “int_no” out of whatever happened to be on the stack and indexed the handler table with it — a page fault at 0xc014d3d0, recursing until the stack hit an unmapped page and triple-faulted. Fix: push esp before the call in both the ISR and IRQ stubs.
0xFB looks like “enable IRQ0 and IRQ1” but the 8259A mask is inverted — 1 means masked. That value actually disabled both the timer and the keyboard. Correct mask: 0xFC.
The idle loop used hlt, which only wakes on an interrupt. With IRQ0 and IRQ1 masked and the UART’s receive interrupt off, nothing ever woke it — so polled serial input was never read. Keeping the 100 Hz timer unmasked fixes it, and additionally required enabling the 8042 controller’s keyboard-interrupt bit (config byte bit 0) during init.
# toolchain: nasm, clang (freestanding), x86_64-elf-ld, qemu make # → oskernel.elf (~37 KB) qemu-system-i386 -kernel oskernel.elf -m 64 # headless, with the shell on your terminal: qemu-system-i386 -kernel oskernel.elf -m 64 -serial stdio -display none
| Command | What it does | Proves |
|---|---|---|
help | List all commands | Shell command table |
hello | Print a greeting | VGA + serial output |
echo … | Echo arguments back | Line parsing |
uptime | Ticks and seconds since boot | PIT + IRQ0 firing |
ps | Task table with state | Scheduler structures |
ls | Files with type and size | Filesystem directory scan |
cat <file> | Print file contents | Filesystem read path |
mem | Free blocks and KB | Bitmap allocator |
reboot | Triple-fault reset | CPU control |
~/Documents/projects/drafts/oskernel