64-bit bootloader from scratch in AT&T assembly, Part-6

Now make a new file for kernel as kernel.asm

.code64
.text

.global _start
_start:
    mov $0xb8000, %rdx # VIDEO_MEMORY

    mov $0x40, %rsi
clear_start:
    cmp $0, %rsi
    je clear_end
    mov $0x0020, %ax
    mov %ax, (%rdx)
    add $2, %rdx
    sub $1, %rsi
    jmp clear_start
clear_end:
    mov $0xb8000, %rdx # VIDEO_MEMORY back to start

    mov $0x6f6c6c6548, %rdi    # "Hello"
    call print_vram

    mov $0x206d6f726620, %rdi  # " from "
    call print_vram

    mov $0x2d3436, %rdi        #  "64-"
    call print_vram

    mov $0x20746962, %rdi      # "bit "
    call print_vram

    mov $0x72656b, %rdi        # "ker"
    call print_vram

    mov $0x6c656e, %rdi        # "nel"
    call print_vram

out:
    hlt
    jmp out

print_vram:
    # string in rdi
print_vram_loop:
    mov %rdi, %rax
    cmp $0, %al
    je print_vram_end

    mov $0x2f, %ah  # add 2f color, green bg, whitw fg
    and $0x0000ffff, %rax
    mov %ax, (%rdx)

    shr $8, %rdi
    add $2, %rdx
    jmp print_vram_loop
print_vram_end:
    ret