/* Segment constants */
#define BOOT_SEG       0x7C0    /* Segment we reside on startup */
#define BOOT_RELOC_SEG 0x7A0    /* Relocation segment */

    .code16
_start:
    /* Normalize CS + other selectors */
    ljmp $BOOT_SEG,$1f
1:  xor %ax, %ax
    mov %ax, %ss
    mov %ax, %gs
    mov %ax, %fs

    /*
     * The second stage bootloader expects to be loaded to 0x7C00 but since we are
     * already executing at this address we'll need to relocate ourselves first before
     * we clobber it.
     */
    mov $BOOT_SEG, %ax
    mov %ax, %ds
    mov $BOOT_RELOC_SEG, %ax
    mov %ax, %es
    xor %si, %si
    xor %di, %di
    mov $0x200, %cx
    cld
    rep movsb

    /* Jump to our relocated self */
    ljmp $BOOT_RELOC_SEG,$reloc
reloc:
    /* Save the drive number */
    mov %dl, drive_num

    /* Set our stack */
    mov $0x7C00, %sp

    /* Write out our boot string */
    mov $boot_msg, %si
    call putstr

    cli
    hlt
    jmp reloc

/*
 * Write a string to the console
 *
 * %si:  String to write
 *
 * XXX: Clobbers %AL
 */
putstr:
    lodsb
    or %al, %al
    jz 1f
    call putchr
    jmp putstr
1:  ret

/*
 * Write a single character to the console
 *
 * %al: Character to write
 */
putchr:
    push %dx
    mov $0x3F8, %dx
    out %al, %dx
    pop %dx
    ret

drive_num: .byte 0x00
boot_msg: .ascii "booting\0"

    .org 510
    .byte 0x55
    .byte 0xAA