os/loader.s at master · AlexSafatli/os · GitHub

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; =============================================================================
; os
; AlexSafatli
;
; Oct 2015
;
; Entry Point for Initializing Kernel
; =============================================================================

global loader                        ; entry symbol for ELF file

; =============================================================================

MAGIC_NUMBER equ 0x1BADB002          ; magic number constant for multiboot
BOOT_MEM_INFO equ 1 << 1             ; provides kernel with memory info
FLAGS equ BOOT_MEM_INFO              ; multiboot flags
CHECKSUM equ -(MAGIC_NUMBER + FLAGS) ; calculate checksum
                                     ; (magic number+checksum+flags should = 0)
KERNEL_STACK_SIZE equ 4096           ; size of stack for kernel in bytes (~4MB)

; =============================================================================

extern kernel_init                   ; kernel initialization entry point in C
                                     ;
section .text                        ; start of text (code)
align 4                              ; code must be 4 byte aligned
  dd MAGIC_NUMBER                    ; write magic number
  dd FLAGS                           ; write flags
  dd CHECKSUM                        ; checksum

; =============================================================================

loader:                              ; entry point
  cli                                ; stop all interrupts until initialized
  mov eax, 0xCAFEBABE                ; place 0xCAFEBABE into eax register
  mov esp, kernel_stack + KERNEL_STACK_SIZE
                                     ; make stack register point to stack
  call kernel_init                   ; initialize kernel
.loop:                               ;
  jmp .loop                          ; loop forever

; =============================================================================

section .bss                         ; start of bss
                                     ; (block started by symbol -
                                     ; statically allocated variables)
align 4                              ; code must be 4 byte aligned
kernel_stack:                        ; label points to beginning of memory
  resb KERNEL_STACK_SIZE             ; reserve the stack

; EOF =========================================================================