Causticos

An operating system written entirely in the Caustic programming language. Boots via Limine into 64-bit long mode on x86_64.

Building

Requires the Caustic toolchain (caustic, caustic-as, caustic-ld, caustic-mk), Limine bootloader, xorriso, and QEMU.

caustic-mk build kernel
caustic-mk run iso
GDK_BACKEND=x11 qemu-system-x86_64 -cdrom build/causticos.iso -m 128M -serial file:build/serial.log

Architecture

kernel/
  main.cst        Entry point (_kernel_start)
  limine.cst       Limine boot protocol (framebuffer, HHDM, memory map)
  gdt.cst          Global Descriptor Table
  idt.cst          IDT, PIC, exception/IRQ handlers
  port.cst         x86 port I/O (in/out via raw opcodes)
  serial.cst       COM1 serial debug output
  pmm.cst          Physical Memory Manager (buddy allocator)
  heap.cst         Slab allocator + kmalloc
  vmm.cst          Virtual Memory Manager (paging, kvalloc, DMA)
  timer.cst        PIT timer (early boot) + monotonic clock
  lapic.cst        Local APIC timer (one-shot, PIT-calibrated)
  sched.cst        Scheduler (EEVDF, deadline EDF+CBS, anti-abuse)
  rbtree.cst       Red-black tree (used by scheduler)
  util.cst         Shared utilities (print_num, print_hex)
  fb.cst           Framebuffer rendering (32bpp)
font/
  font8x8.cst      8x8 bitmap font

Subsystems

Memory Management

• PMM: Buddy allocator (orders 0-10, 4KB-4MB), section-based sparse model (128 MiB sections, 12 bytes/page metadata), DMA32/NORMAL zones with reserves, IRQ-safe
• Heap: Slab allocator with 14 kmalloc size classes (16-4096), kcache API for typed objects, poison + redzone debug, shrinker, slab ID recycling
• VMM: 4-level x86_64 paging, deep-cloned kernel page tables, address space create/destroy/switch, kvalloc (bitmap range manager with guard pages), DMA allocator

Scheduler

• EEVDF within 6 QoS bands (input, render, active, normal, batch, lazy) with strict inter-band priority
• Deadline class (EDF + CBS): admission test, budget enforcement, period replenishment
• Anti-abuse: capability-gated bands (CAP_SCHED_INPUT, CAP_SCHED_RENDER, CAP_SCHED_REALTIME), per-thread budget measurement, demotion/promotion
• LAPIC timer: one-shot mode, PIT-calibrated, program_next_tick for precise event scheduling
• Red-black trees for both deadline list and per-band runqueues (O(log n))
• Per-CPU runqueue struct (single CPU in V1, SMP-ready layout)
• Dynamic threads via kcache + hash table TID lookup (unbounded thread count)
• Lazy FPU: CR0 TS bit + #NM handler, fxsave/fxrstor
• Preemption: preempt_count, cond_resched, IRQ-return preemption check

Hardware

• Limine boot protocol (framebuffer, HHDM, memory map)
• GDT with kernel code/data segments (raw opcode segment register loads)
• IDT with 19 exception + 16 IRQ + LAPIC timer handlers
• PIC 8259 remapping (vectors 32-47)
• Local APIC (MMIO-mapped, timer calibration)
• COM1 serial output (115200 8N1)
• PS/2 keyboard (scancode via IRQ1)
• Framebuffer text rendering (8x8 bitmap font)

Caustic Language Workarounds

The Caustic assembler (caustic-as) silently drops several x86_64 instructions. Workarounds used throughout:

Instruction	Workaround
mov ss/ds/es/fs/gs, ax	.byte 0x8E, 0xD0/D8/C0/E0/E8
push imm8	.byte 0x6A, N
lgdt/lidt/iretq/retfq	.byte raw opcodes
pushfq/popfq	.byte 0x9C/0x9D
cli/sti/hlt	.byte 0xFA/0xFB/0xF4
in/out	.byte 0xEC/0xEE
mov cr0/cr3	.byte 0x0F, 0x20/0x22, ...
invlpg/clts/fxsave/fxrstor	.byte sequences
imul rax, rax, imm	Use Caustic code + global (immediate dropped)
Function pointers (&fn)	lea rax, [rip+symbol] via asm
inb return value	Global shuttle (compiler overwrites rax)

License

MIT