hawks_os/boot.asm at main · HawksLab/hawks_os · GitHub

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[org 0x7c00]
[bits 16]

;Boot Loader
mov bx , 0x1000 ; Memory offset to which kernel will be loaded
mov ah , 0x02   ; Bios Read Sector Function
mov al , 30     ; No. of sectors to read(If your kernel won't fit into 30 sectors , you may need to provide the correct no. of sectors to read)
mov ch , 0x00   ; Select Cylinder 0 from harddisk
mov dh , 0x00   ; Select head 0 from hard disk
mov cl , 0x02   ; Start Reading from Second sector(Sector just after boot sector)

int 0x13        ; Bios Interrupt Relating to Disk functions


;Switch To Protected Mode
cli ; Turns Interrupts off
lgdt [GDT_DESC] ; Loads Our GDT

mov eax , cr0
or  eax , 0x1
mov cr0 , eax ; Switch To Protected Mode

jmp  CODE_SEG:INIT_PM ; Jumps To Our 32 bit Code
;Forces the cpu to flush out contents in cache memory

[bits 32]

INIT_PM:
mov ax , DATA_SEG
mov ds , ax
mov ss , ax
mov es , ax
mov fs , ax
mov gs , ax

mov ebp , 0x90000
mov esp , ebp ; Updates Stack Segment


call 0x1000
jmp $


GDT_BEGIN:

GDT_NULL_DESC:  ;The  Mandatory  Null  Descriptor
	dd 0x0
	dd 0x0

GDT_CODE_SEG:
	dw 0xffff		;Limit
	dw 0x0			;Base
	db 0x0			;Base
	db 10011010b	;Flags
	db 11001111b	;Flags
	db 0x0			;Base

GDT_DATA_SEG:
	dw 0xffff		;Limit
	dw 0x0			;Base
	db 0x0			;Base
	db 10010010b	;Flags
	db 11001111b	;Flags
	db 0x0			;Base

GDT_END:

GDT_DESC:
	dw GDT_END - GDT_BEGIN - 1
	dd GDT_BEGIN

CODE_SEG equ GDT_CODE_SEG - GDT_BEGIN
DATA_SEG equ GDT_DATA_SEG - GDT_BEGIN


times 510-($-$$) db 0
dw 0xaa55