Boot and Initialization

The init module is the kernel's sequential startup orchestrator. It is entered once, runs each subsystem check in order, prints a status line for each step, then hands off to the scheduler — at which point it never runs again.

The single entry point is init::check::init(m2_ptr: u32), called from main.rs with the Multiboot2 info pointer left in a register by the bootloader.

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Boot Sequence

Ordered groups of initialization steps. INIT.RC runs later in the scheduled init_rc task; it is not one of the 18 sequential checks. The final branches show the session each boot exposes; the text shell and clock already exist when the script runs.

Steps execute in this exact order:

#	Call	Description
1	vga::init_writer()	Create the VGA text writer at 0xB8000
2	clear_screen!()	Blank the text buffer
3	cpu::check()	Enable SSE and set SYSCALL/SYSRET MSRs
4	idt::idt_isrs_init()	Install ISRs, reload GDT, initialize TSS and load IDT
5	parser::parse_info(m2_ptr, ...)	Read framebuffer, usable RAM, command line and ACPI tags
6	usb::take_all()	With usb=os only: take controllers from firmware before PS/2 setup
7	mouse::init()	Initialize the PS/2 mouse
8	boot::usable_covers(...)	Check that all 32 process frames, 0x1000000..0x5000000, lie in usable RAM
9	heap::pmm_heap_init()	Initialize the kernel heap and run its smoke test
10	video::print_result(...)	Select the video path
11	video::wc::map_write_combining(...)	For an RGB framebuffer, map its identity-mapped pages write-combining
12	fs::floppy_check_init()	Probe the floppy and set the working directory to /
13	fs::vfs_init(fat12)	Mount the root, writable FAT volumes, CD and boot archive when available
14	color::color_demo()	Print the console colour swatch
15	ascii::ascii_art()	Print the splash
16	process::init_processes()	Save CR3, initialize the shared user heap and create initial tasks
17	debug::dump_debug_log_to_memdisk()	Write /mnt/tmp/KERNDBG.LOG
18	pit::pic_pit_init()	Remap the PIC, start the PIT at 1000 Hz and enable interrupts

Before this sequence, kernel_main calls fs::ustar::module::adopt to keep the GRUB-loaded archive clear of fixed process stacks and frames. See Boot Medium Archive. After step 18, timer interrupts can schedule the initial tasks; kernel_main then enters its idle path, or runs the self-tests with kernel_test enabled.

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Debug Log

debug!, debugn! and debugln! (src/debug.rs) append to DEBUG_LOG, an 8 KiB buffer in memory; what does not fit once it is full is dropped. The kernel writes it out at three points:

When	Where
The Multiboot2 framebuffer tag is parsed (step 5)	DEBUG.TXT in the floppy's root, and the serial port (COM1)
End of init (step 17)	/mnt/tmp/KERNDBG.LOG on the RAM disk
The shell's debug command	DEBUG.TXT, KERNDBG.LOG and serial, each file replaced

KERNDBG.LOG is there for a machine with neither a floppy nor a serial line, such as one booted from a USB stick: read /mnt/tmp/KERNDBG.LOG shows it. It is written before the timer starts, while no process can be writing to the RAM disk, so it holds everything up to that point and nothing logged afterwards until debug is run.

The write at step 5 comes before the floppy probe, and on a machine without a floppy it prints ERR: Could not find the FAT12 label, floppy may not be present on the boot screen. It is harmless: the log still reaches serial and, later, the RAM disk.

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Global State Set During Init

Symbol	Type	Set by step	Description
FRAMEBUFFER_PTR	boot::FramebufferTag	5	VESA framebuffer address, pitch, dimensions, bpp
VIDEO_MODE	Option<VideoMode>	10	Active video path (Framebuffer or TextMode)
SYSTEM_CONFIG	Mutex<SystemConfig>	12	hostname, user, cwd, version, IP, MAC
KERNEL_CR3	u64	16	Boot-time page table snapshot for process cloning
Userland heap P2[6/7]	page table	16	0xC00_000–0xFFF_FFF mapped USER+WRITE
SCHEDULER	Mutex<Scheduler>	16	Initial process slots populated

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Module Table

File	Purpose
check.rs	Entry point init(), FRAMEBUFFER_PTR global
boot.rs	Multiboot2 tag structs, parse_multiboot2_info
parser.rs	Thin Result-returning wrapper around boot::parse_multiboot2_info
cpu.rs	SSE enable, SYSCALL/SYSRET MSR setup
idt.rs	GDT reload, TSS init, IDT load
pit.rs	8259A PIC remap, PIT init at TICKS_PER_SECOND (1000 Hz)
heap.rs	Kernel heap init + smoke test
fs.rs	Floppy probe, VFS mount table init
video.rs	init_video(), optional VESA P1 mapping
process.rs	Initial task creation (kmain, init_rc, kclock, kshell; the last two not on a framebuffer)
config.rs	SYSTEM_CONFIG global, get_prompt()
font.rs	PSF1/PSF2 font parser, PSF_FONT static, glyph renderer
ascii.rs	Splash screen text
color.rs	16-color palette swatch

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Related documentation: Kernel Internals, Subsystem Setup, Build and Run.