bootloader

[BITS 16]
[ORG 0x7C00]
    jmp 0x0000:start_16  ; In case bootloader is at 0x07C0:0x0000
start_16:
    xor ax, ax
    mov ds, ax
    mov es, ax
    cli                             ; Disable interrupts
    mov ss, ax
    mov sp, 0x7C00
;    sti                             ; Enable interrupts

    ; Store the drive number
    call dumpregs
    mov byte [drive_number], dl
    call dumpregs

    ; PLEASE HALT
    jmp $
    ; No? Please?!... Ok...

    ; Print message(s)
    mov si, msg
    call print_string
    mov si, msg
    call print_string
    mov si, msg
    call print_string

    jmp $   ; HALT


; print_string
;       si      = string
print_string:
    pusha
    mov ah, 0x0E
.repeat:
    lodsb
    cmp al, 0x00
    je .done
    call dumpregs
    int 0x10
    call dumpregs
    jmp short .repeat
.done:
    popa
    ret


dumpregs:
    push    es
    pusha
    push    0xb800
    pop es
    mov di, [vidmem_ptr]
    mov bp, sp
    mov cx, 8
dump_loop:
    dec bp
    dec bp
    mov ax, [bp + 16]
    call    printhex2
    inc di
    inc di
    loop    dump_loop
    mov [vidmem_ptr], di
    popa
    pop es
    ret

printhex2:
    push    ax
    mov al, ah
    call    convhex1
    pop ax
convhex1:
    aam 16
    ; DB    0D4h, 16
    xchg    al, ah
    call    convhex
    mov al, ah
convhex:
    cmp al, 10
    jb  lessthan_10
    add al, 'A' - '0' - 10
lessthan_10:
    add al, '0'
    stosb
    mov al, 7
    stosb
    ret

vidmem_ptr dw 5 * 80 * 2 ; start at row 5

; Variables
drive_number db 0x00
msg db 'TEST',0x0D, 0x0A, 0x00
times 510-($-$$) db 0x00
db 0x55
db 0xAA