Boot

This document describes the Kyronix kernel boot process. It is the child of Kernel and parent of the Limine protocol reference.

Boot Sequence

The Kyronix kernel boots via the Limine v3 boot protocol. The bootloader loads the kernel ELF and provides the following information through Limine requests:

1. Memory map (LIMINE_MEMMAP_REQUEST) – Physical memory regions (usable, reserved, ACPI, framebuffer)
2. HHDM offset (LIMINE_HHDM_REQUEST) – Higher Half Direct Map base address
3. Framebuffer (LIMINE_FRAMEBUFFER_REQUEST) – Linear framebuffer for early display
4. Kernel address (LIMINE_KERNEL_ADDRESS_REQUEST) – Physical and virtual base addresses
5. RSDP (LIMINE_RSDP_REQUEST) – ACPI Root System Description Pointer
6. Modules (LIMINE_MODULE_REQUEST) – Bootloader modules (initrd)

All requests use LIMINE_BASE_REVISION(3).

Boot Phases

Phase 1: Early Hardware (BSP)

1. serial_init(COM1) – Initialize serial port for debug output
2. gdt_init() – Set up Global Descriptor Table and Task State Segment
3. idt_init() – Set up Interrupt Descriptor Table, remap PIC to vectors 32-47
4. kbd_init() – Initialize keyboard driver

Phase 2: Memory Setup

1. pmm_init() – Initialize physical memory manager from Limine memory map
2. vmm_init() – Enable NX bit, initialize kernel page tables
3. heap_init() – Initialize kernel heap allocator (64 KiB initial)

Phase 3: Per-CPU and SMP

1. Write MSR_GS_BASE and MSR_KERNEL_GS_BASE for BSP per-CPU data
2. smp_init() – Enumerate CPUs from Limine SMP response
3. SMEP detection and enable via CPUID leaf 7
4. syscall_init() – Configure SYSCALL/SYSRET MSRs, enable SSE

Phase 4: Device Drivers

1. pci_enumerate() – Scan PCI bus
2. acpi_init() – Parse ACPI tables from RSDP
3. ahci_init() – Initialize SATA/AHCI controllers
4. virtnet_init() – Initialize VirtIO network device
5. net_init() – Initialize lwIP network stack

Phase 5: Timer and Scheduling

1. pit_init() – Program PIT channel 0 at ~250 Hz
2. lapic_calibrate_timer() – Calibrate LAPIC timer against PIT
3. Create BSP idle process
4. smp_boot_aps() – Wake Application Processors

Phase 6: Filesystem and Init

1. ext2_init() – Register ext2 filesystem driver
2. Mount root filesystem from disk or load initrd via CPIO
3. process_exec("/init") – Load and execute init process

AP Boot Sequence

Application Processors are woken via the Limine SMP goto_address trampoline (ap_trampoline in kernel/proc/ap_trampoline.S). Each AP:

1. Loads its cpu_local_t from limine_smp_info.extra_argument
2. Loads the idle process kernel stack
3. Calls ap_init_cpu() which initializes GDT, IDT, MSRs, SSE, SYSCALL, LAPIC
4. Spins on g_kernel_ready until BSP signals completion
5. Starts 250 Hz LAPIC periodic timer
6. Enters ap_sched_loop() (idle/scheduling loop)

Status Output

During boot, each initialization step prints a status line:

* Initialising PMM ...                                              [ ok ]
* Initialising VMM ...                                              [ ok ]

The [ ok ] / [ !! ] indicator is right-aligned at column 72 using ANSI escape codes.

Last reviewed: 2026-07-22