MiniKernel

Overview

MiniKernel started as a way to understand what actually happens before main() is called. It’s a minimal x86 kernel that handles the transition from real mode to protected mode, sets up the GDT/IDT, enables paging, and runs a simple round-robin scheduler.

Architecture

The kernel is structured around three core subsystems:

• Boot — GRUB multiboot, protected mode setup, GDT initialization
• Memory — Physical memory manager (bitmap allocator), virtual memory with 4KB pages
• Processes — Simple PCB, round-robin scheduler, context switch via TSS

Boot Sequence

The kernel follows a well-defined boot sequence: GRUB loads the multiboot image, the assembly stub sets up a minimal stack and calls the C entry point. From there the GDT is loaded, IDT configured, and paging enabled before the scheduler kicks in.

; boot.asm — minimal entry point
section .text
global _start
_start:
    mov esp, stack_top
    push ebx        ; multiboot info ptr
    call kmain
    hlt

Memory Layout

The kernel identity-maps the first 4MB for itself, then manages physical pages with a bitmap allocator. Each process gets its own page directory and 4KB page tables.

What I Learned

• How the CPU transitions between privilege levels (ring 0/3)
• What a page fault actually is at the hardware level
• How context switching works via the x86 TSS structure
• Why GRUB alignment matters for multiboot headers

Known Issues

The scheduler has no preemption yet — it’s cooperative only. Memory freeing is not fully implemented. SMP support is a future goal.