boot.s

    global loader                   ; the entry symbol for ELF
    extern main                 ; the function kmain is defined elsewhere
    
    KERNEL_STACK_SIZE equ 4096      ; size of stack in bytes
                
    MAGIC_NUMBER equ 0x1BADB002     ; define the magic number constant
    ALIGN_MODULES equ 0x00000001    ; tell GRUB to align modules
    CHECKSUM     equ -(MAGIC_NUMBER + ALIGN_MODULES )  
                         ; calculate the checksum
                                    ; (magic number + checksum + flags should equal 0)
    
    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            ; write the align modules instruction
        dd CHECKSUM                 ; write the checksum
    
    mov esp, kernel_stack + KERNEL_STACK_SIZE       ; point esp to the start of the stack (end of memory area)
    
    loader:                         ; the loader label (defined as entry point in linker script)
    
        push ebx                    ; multiboot info in ebx 
        call main                  ; call the function, the result will be in eax
    
    .loop:
        jmp .loop                   ; loop forever