; This is the kernel's entry point. We could either call main here,
; or we can use this to setup the stack or other nice stuff, like
; perhaps setting up the GDT and segments. Please note that interrupts
; are disabled at this point: More on interrupts later!
[BITS 32]
global start
start:
    mov esp, _sys_stack     ; This points the stack to our new stack area
    jmp stublet

; This part MUST be 4byte aligned, so we solve that issue using 'ALIGN 4'
ALIGN 4
mboot:
    ; Multiboot macros to make a few lines later more readable
    MULTIBOOT_PAGE_ALIGN  equ 1<<0
    MULTIBOOT_MEMORY_INFO     equ 1<<1
    MULTIBOOT_AOUT_KLUDGE     equ 1<<16
    MULTIBOOT_HEADER_MAGIC   equ 0x1BADB002
    MULTIBOOT_HEADER_FLAGS    equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO | MULTIBOOT_AOUT_KLUDGE
    MULTIBOOT_CHECKSUM      equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)
    EXTERN code, bss, end

    ; This is the GRUB Multiboot header. A boot signature
    dd MULTIBOOT_HEADER_MAGIC
    dd MULTIBOOT_HEADER_FLAGS
    dd MULTIBOOT_CHECKSUM
    
    ; AOUT kludge - must be physical addresses. Make a note of these:
    ; The linker script fills in the data for these ones!
    dd mboot
    dd code
    dd bss
    dd end
    dd start

; This is an endless loop here. Make a note of this: Later on, we
; will insert an 'extern _main', followed by 'call _main', right
; before the 'jmp $'.
stublet:
    jmp $


; Shortly we will add code for loading the GDT right here!


; In just a few pages in this tutorial, we will add our Interrupt
; Service Routines (ISRs) right here!



; Here is the definition of our BSS section. Right now, we'll use
; it just to store the stack. Remember that a stack actually grows
; downwards, so we declare the size of the data before declaring
; the identifier '_sys_stack'
SECTION .bss
    resb 8192               ; This reserves 8KBytes of memory here
_sys_stack:
    

dd MULTIBOOT_CHECKSUM    

    ; Multiboot macros to make a few lines later more readable
    MULTIBOOT_PAGE_ALIGN        equ (1 shl 0)
    MULTIBOOT_MEMORY_INFO       equ (1 shl 1)
    MULTIBOOT_AOUT_KLUDGE       equ (1 shl 16)
    MULTIBOOT_HEADER_MAGIC      equ 0x1BADB002
    MULTIBOOT_HEADER_FLAGS      equ MULTIBOOT_PAGE_ALIGN or MULTIBOOT_MEMORY_INFO or MULTIBOOT_AOUT_KLUDGE
    MULTIBOOT_CHECKSUM  equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)