loader.s

global loader                                              ; the entry symbol for ELF

extern kmain                                               ; function kmain is defined elsewhere
extern kernel_virtual_start				;
extern kernel_virtual_end				;
extern kernel_physical_start				;
extern kernel_physical_end				;

MAGIC_NUMBER    equ 0x1BADB002                             ; define the magic number constant
ALIGN_MODULES   equ 0x00000001                             ; tell GRUB to align modules

                                                           ; calculate the checksum (all options + checksum should equal 0)
CHECKSUM        equ -(MAGIC_NUMBER + ALIGN_MODULES )       


KERNEL_STACK_SIZE equ 4096                                 ; size of stack in bytes

section .bss
align 4                                                    ; align at 4 bytes
kernel_stack:                                              ; label points to beginning of memory
    resb KERNEL_STACK_SIZE                                 ; reserve stack for the kernel

section .text                                              ; start of the text (code) section
align 4                                                    ; the code must be 4 byte aligned
    dd MAGIC_NUMBER                                        ; write the magic number to the machine code,
    dd ALIGN_MODULES                                       ;
    dd CHECKSUM                                            ; and the checksum


loader:                                                    ; the loader label (defined as entry point in linker script)
    mov esp, kernel_stack + KERNEL_STACK_SIZE              ; point esp to the start of the
                                                           ; stack (end of memory area)
    
    push ebx                                               ; multiboot info in ebx 
    push kernel_physical_end					;
    push kernel_physical_start				;
    push kernel_virtual_end					;
    push kernel_virtual_start					;
    
    call kmain                                             ; Calling the function kmain
   
.loop:
    jmp .loop                                              ; loop forever