boot.asm

boot.asm

[BITS 16]       ; We need 16-bit intructions for Real mode

[ORG 0x7C00]    ; The BIOS loads the boot sector into memory location 0x7C00

;jmp word load
global _start
jmp _start
drive db 0 
;       jmp reset_drive
;       jmp enter_pm
_start:
        mov ah, 0               ; RESET-command
        int 13h                 ; Call interrupt 13h
        mov [drive], dl
        or ah, ah               ; Check for error code
        jnz _start              ; Try again if ah != 0
        
        mov ax, 0
        mov es, ax
        mov bx, 0x1000          ; Destination address = 0000:1000
        
        mov ah, 02h             ; READ SECTOR-command
        mov al, 02h             ; Number of sectors to read = 1
        mov dl, [drive]
        mov ch, 0               ; Cylinder = 0
        mov cl, 2               ; Sector = 2
        mov dh, 0               ; Head = 0
        int 13h                 ; Call interrupt 13h
        
        or ah, ah               ; Check for error code
        jnz _start               ; Try again if ah != 0
        ;jc _start
        jmp enter_pm
        
        cli                     ; Disable interrupts, we want to be alone        
       
enter_pm:
        xor ax, ax
        mov ds, ax              ; Set DS-register to 0 - used by lgdt

        lgdt [gdt_desc]         ; Load the GDT descriptor        

        mov eax, cr0            ; Copy the contents of CR0 into EAX
        or eax, 1               ; Set bit 0
        mov cr0, eax            ; Copy the contents of EAX into CR0        

        jmp 08h:clear_pipe      ; Jump to code segment, offset clear_pipe

[BITS 32]                       ; We now need 32-bit instructions
clear_pipe:
        mov ax, 10h             ; Save data segment identifyer
        mov ds, ax              ; Move a valid data segment into the data segment register
        mov ss, ax              ; Move a valid data segment into the stack segment register
        mov esp, 090000h        ; Move the stack pointer to 090000h

        jmp 08h:01000h          ; Jump to section 08h (code), offset 01000h


gdt:                    ; Address for the GDT

gdt_null:               ; Null Segment
        dd 0
        dd 0

gdt_code:               ; Code segment, read/execute, nonconforming
        dw 0FFFFh
        dw 0
        db 0
        db 10011010b
        db 11001111b
        db 0

gdt_data:               ; Data segment, read/write, expand down
        dw 0FFFFh
        dw 0
        db 0
        db 10010010b
        db 11001111b
        db 0

gdt_end:                ; Used to calculate the size of the GDT



gdt_desc:                       ; The GDT descriptor
        dw gdt_end - gdt - 1    ; Limit (size)
        dd gdt                  ; Address of the GDT




times 510-($-$$) db 0           ; Fill up the file with zeros

        dw 0xAA55                ; Boot sector identifyer