# bootblock.s

# .equ symbol, expression.
# This directive sets the value of symbol to expression
.equ    BOOT_SEGMENT,       0x7c00
.equ    DISPLAY_SEGMENT,    0xb800

.text                  # Code segment
.globl  _start         # The entry point must be global
.code16                # Real mode
#
# The first instruction to execute in a program is called the entry
# point. The linker expects to find the entry point in the "symbol" _start
# (with underscore).
#
_start:
    jmp over
#
# Do not add any instructions before or in os_size.
#

os_size:
    # area reserved for createimage to write the OS size
    .word   0
    .word   0
over:
    #xchg   %bx, %bx
    #print_string is now working, kernel isn't

    # set up segments
    mov  $0x0, %ax
    mov %ax, %ds
    mov %ax, %es
    mov %ax, %ss     # setup stack
    mov $BOOT_SEGMENT, %sp # stack grows downwards from 0x7C00

    #xchg   %bx, %bx
    mov $hellostring, %si
    add $BOOT_SEGMENT, %si
    call print_string

#INT 13h AH=00h: Reset Disk Drive
    #xchg   %bx, %bx
    movb    $0x00, %ah
    int     $0x13


#INT 13h AH=08h: Read Drive Parameters
    #xchg   %bx, %bx
    xor %ax, %ax
    mov %ax, %es
    mov %ax, %di

    mov $0x08, %ah
    mov $0x80, %dl

    int $0x13

#INT 13h AH=02h: Read Sectors From Drive
    #xchg   %bx, %bx

    xor %ax, %ax
    mov %ax, %es
    mov $0x0550, %bx

    movb    $0x02, %ah
    movb    $0x01, %al
    movb    $0x00, %ch
    movb    $0x01, %cl
    movb    $0x00, %dh
    movb    $0x80, %dl

    #xchg   %bx, %bx
    int $0x13

#INT 13h AH=02h: Read Sectors From Drive
    #xchg   %bx, %bx

    xor %ax, %ax
    mov %ax, %es
    mov $0x8000, %bx

    movb    $0x02, %ah
    movb    $0x01, %al
    movb    $0x00, %ch
    movb    $0x02, %cl
    movb    $0x00, %dh
    movb    $0x80, %dl

    int $0x13

    #xchg   %bx, %bx
    mov $0x00, %ax
    mov %ax, %ds
    #invoke kernel at 0x00008000
    ljmp $0x0, $0x8000

print_string:
    mov $0x0e, %ah
.repeat:
    lodsb

    cmp $0x00, %al
    je .done
    int $0x10
    jmp .repeat
.done:
    ret

forever:
    jmp     forever                 # Loop forever


hellostring:
    .asciz  "Hello\r\n"

. = _start + 510 #mov to 510th byte from 0 pos
.byte 0x55 #append boot signature
.byte 0xaa #append boot signature