startup.asm

global loader           ; making entry point visible to linker
extern _kmain            ; main is defined elsewhere
extern start_ctors, end_ctors, start_dtors, end_dtors

; setting up the Multiboot header - see GRUB docs for details
MODULEALIGN equ  1<<0                   ; align loaded modules on page boundaries
MEMINFO     equ  1<<1                   ; provide memory map
FLAGS       equ  MODULEALIGN | MEMINFO  ; this is the Multiboot 'flag' field
MAGIC       equ    0x1BADB002           ; 'magic number' lets bootloader find the header
CHECKSUM    equ -(MAGIC + FLAGS)        ; checksum required

section .text
align 4
MultiBootHeader:
   dd MAGIC
   dd FLAGS
   dd CHECKSUM

; reserve initial kernel stack space
STACKSTART equ 0xA0000                  ; that's 16k.

loader:
   mov esp, STACKSTART             ; set up the stack
   push ebx                        ; Multiboot info structure
   push eax                        ; Multiboot magic number

static_ctors_loop:
   mov ebx, start_ctors
   jmp .test
.body:
   call [ebx]
   add ebx,4
.test:
   cmp ebx, end_ctors
   jb .body

   call _kmain                      ; call kernel proper

static_dtors_loop:
   mov ebx, start_dtors
   jmp .test
.body:
   call [ebx]
   add ebx,4
.test:
   cmp ebx, end_dtors
   jb .body

   hlt                             ; halt machine should kernel return