OSDev.org

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; bootloader.asm -- the GigaDOS bootloader
bits 16
org 0x7C00

mov ax, 0
mov ds, ax
jmp 0x0:go

go:
cli
mov si, str_booting
call puts

; the area used for reading the kernel record and finally
; transferring control to the kernel
mov ax, 0x7E0
mov es, ax

; read the file header from floppy
mov ah, 0x02
mov al, 1
mov cx, 1
xor dx, dx
xor bx, bx
int 13h

; now read the file
mov ah, 0x02
mov al, [es:0x10]
mov cl, 2
int 13h

; reset the ES segment
mov ax, 0
mov es, ax

mov si, str_pmode
call puts

; load the GDT
lgdt [gdt_desc]

; enter Protected Mode
mov eax, cr0
or eax, 1
mov cr0, eax

jmp 0x08:clear_pipe

; put a string on the screen
; DS:SI -> NUL-terminated string
; AX = destroyed
puts:
	mov ah, 0x0E

	.loop:
	lodsb
	cmp al, 0
	je .end
	int 10h
	jmp .loop

	.end:
	ret

str_booting db 'GigaDOS booting up....', 0
str_pmode db 'Entering protected mode...', 0

; the default GDT
gdt:

.null dq 0
.code:
	dw 0xFFFF
	dw 0
	db 0
	db 0x9A
	db 0xCF
	db 0

.data:
	dw 0xFFFF
	dw 0
	db 0
	db 0x92
	db 0xCF
	db 0

.end:

; the GDT pointer
gdt_desc:
	db gdt.end - gdt
	dw gdt

[BITS 32]
clear_pipe:
	mov ax, 0x10
	mov ds, ax
	mov ss, ax

	mov ax, 0x4040
	mov [0xB8000], ax

	jmp 0x7E00

times 510 - ($ - $$) db 0
db 0x55, 0xAA

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; the GDT pointer
gdt_desc:
   db gdt.end - gdt
   dw gdt

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; the GDT pointer
gdt_desc:
   dw gdt.end - gdt - 1
   dd gdt

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xor bx, bx
mov es, bx ; seg 0
mov bx, 0x7e00 ; offset
xor dx, dx ; head 0 - drive 0
xor ch, ch ; cyl 0
mov cl, 2
mov ax, 0x201 ; function 2 - read 1 sector