; Declare constants used for creating a multiboot header.
HEADER_MAGIC    equ  0x1BADB002         ; 'magic number' lets bootloader find the header
HEADER_FLAGS    equ  0x00010003         ; this is the Multiboot 'flag' field
STACK_SIZE  equ  0x4000     ; the size of the stack is 16KB
CHECKSUM        equ  -(HEADER_MAGIC + HEADER_FLAGS) ; to prove we are multiboot

; Declare a header as in the Multiboot Standard.
section .multiboot
align 4
    dd HEADER_MAGIC
    dd HEADER_FLAGS
    dd CHECKSUM


; The stack is defined here
[section .init_stack nobits alloc noexec write align=4]
stack_bottom:
    resb STACK_SIZE
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.
section .text
global _start
_start:
    ; Set up a stack
    mov ebp, stack_top
    mov esp, ebp

    ; Set EFLAGS to 0
    push 0
    ; pop stack into the EFLAGS register
    popf

    ; Push the magic and the address on the stack, so that they
    ; will be the parameters of the C main function
    push ebx
    push eax

    ; Calling kernel's entry point.
    extern versatile_main
    call versatile_main

    ; In case the function returns, we'll want to put the computer into an
    ; infinite loop. To do that, we use the clear interrupt ('cli') instruction
    ; to disable interrupts, the halt instruction ('hlt') to stop the CPU until
    ; the next interrupt arrives, and jumping to the halt instruction if it ever
    ; continues execution, just to be safe.
    cli

.hang:
    hlt
    jmp .hang