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; Declare constants for the multiboot header.
MBALIGN equ 1 << 0 ; align loaded modules on page boundaries
MEMINFO equ 1 << 1 ; provide memory map
FLAGS equ MBALIGN | MEMINFO ; this is the Multiboot 'flag' field
MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header
CHECKSUM equ -(MAGIC + FLAGS) ; checksum of above, to prove we are multiboot
; Declare a multiboot header that marks the program as a kernel. These are magic
; values that are documented in the multiboot standard. The bootloader will
; search for this signature in the first 8 KiB of the kernel file, aligned at a
; 32-bit boundary. The signature is in its own section so the header can be
; forced to be within the first 8 KiB of the kernel file.
section .multiboot
align 4
dd MAGIC
dd FLAGS
dd CHECKSUM
; The multiboot standard does not define the value of the stack pointer register
; (esp) and it is up to the kernel to provide a stack. This allocates room for a
; small stack by creating a symbol at the bottom of it, then allocating 16384
; bytes for it, and finally creating a symbol at the top. The stack grows
; downwards on x86. The stack is in its own section so it can be marked nobits,
; which means the kernel file is smaller because it does not contain an
; uninitialized stack. The stack on x86 must be 16-byte aligned according to the
; System V ABI standard and de-facto extensions. The compiler will assume the
; stack is properly aligned and failure to align the stack will result in
; undefined behavior.
section .bss
align 16
stack_bottom:
resb 16384 ; 16 KiB
stack_top:
; The linker script specifies _start as the entry point to the kernel and the
; bootloader will jump to this position once the kernel has been loaded. It
; doesn't make sense to return from this function as the bootloader is gone.
; Declare _start as a function symbol with the given symbol size.
section .text
global _start:function (_start.end - _start)
_start:
; The bootloader has loaded us into 32-bit protected mode on a x86
; machine. Interrupts are disabled. Paging is disabled. The processor
; state is as defined in the multiboot standard. The kernel has full
; control of the CPU. The kernel can only make use of hardware features
; and any code it provides as part of itself. There's no printf
; function, unless the kernel provides its own <stdio.h> header and a
; printf implementation. There are no security restrictions, no
; safeguards, no debugging mechanisms, only what the kernel provides
; itself. It has absolute and complete power over the
; machine.
; To set up a stack, we set the esp register to point to the top of our
; stack (as it grows downwards on x86 systems). This is necessarily done
; in assembly as languages such as C cannot function without a stack.
mov esp, stack_top
; This is a good place to initialize crucial processor state before the
; high-level kernel is entered. It's best to minimize the early
; environment where crucial features are offline. Note that the
; processor is not fully initialized yet: Features such as floating
; point instructions and instruction set extensions are not initialized
; yet. The GDT should be loaded here. Paging should be enabled here.
; C++ features such as global constructors and exceptions will require
; runtime support to work as well.
; Enter the high-level kernel. The ABI requires the stack is 16-byte
; aligned at the time of the call instruction (which afterwards pushes
; the return pointer of size 4 bytes). The stack was originally 16-byte
; aligned above and we've since pushed a multiple of 16 bytes to the
; stack since (pushed 0 bytes so far) and the alignment is thus
; preserved and the call is well defined.
; note, that if you are building on Windows, C functions may have "_" prefix in assembly: _kernel_main
extern kernel_main
call kernel_main
; If the system has nothing more to do, put the computer into an
; infinite loop. To do that:
; 1) Disable interrupts with cli (clear interrupt enable in eflags).
; They are already disabled by the bootloader, so this is not needed.
; Mind that you might later enable interrupts and return from
; kernel_main (which is sort of nonsensical to do).
; 2) Wait for the next interrupt to arrive with hlt (halt instruction).
; Since they are disabled, this will lock up the computer.
; 3) Jump to the hlt instruction if it ever wakes up due to a
; non-maskable interrupt occurring or due to system management mode.
cli
.hang: hlt
jmp .hang
.end:
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