64-bit bootloader from scratch in AT&T assembly, Part-5

/* GDT */
/* This Data structure is needed for memory
.align 16
gdtp:
    .word gdt_end - gdt_start - 1
    /* .long (0x07C0 << 4) + gdt */
    .long gdt_start

gdt_start:
    .quad 0x0000000000000000    #   Null Descriptor
gdt_code_segment:
    #    Segment limit  Base address
    .word 0xffff,       0x0000
    .byte 0x00        # Base
    # 1st  flags: (present) 1    (privilege)   0                    0         (descriptor type)1 -> 1001b
    # Type flags: (code)    1    (conforming)  0        (readable)  1         (accessed)       0 -> 1010b
    # 10011010
    .byte 0x9a
    # 2nd  flags: (granularity) 1    (32-bit default) 1    (64-bit segment) 0    (available)   0 -> 1100b
    .byte 0xcf
    .byte 0x00        # Base

gdt_data_segment:
    #    Segment limit  Base address
    .word 0xffff,       0x0000
    .byte 0x00        # Base
    # 1st  flags: (present) 1    (privilege)   0                    0         (descriptor type)1 -> 1001b
    # Type flags: (code)    0    (conforming)  0        (readable)  1         (accessed)       0 -> 1010b
    # 10010010
    .byte 0x92
    # 2nd  flags: (granularity) 1    (32-bit default) 1    (64-bit segment) 0    (available)   0 -> 1100b
    .byte 0xcf
    .byte 0x00        # Base

gdt_end:

# Where docs for 64-bit GDT? IDK, just copy it
# 64-bit GDT
.align 16
gdt64:
    .quad 0                      # Null
    .quad 0x00209a0000000000     # Code: L-bit set, D-bit clear
    .quad 0x0000920000000000     # Data
gdt64_ptr:
    .word . - gdt64 - 1
    .quad gdt64                  # 64-bit base

/* IDT */
idt:
    .word 0
    .long 0

# Some messages
running_kernel: .asciz "Trying to run Kernel"
disk_err_msg:   .asciz "Can't load kernel from disk"

KERNEL_OFFSET: .long 0x2000
BOOT_DRIVE:    .byte 0

# This is the end we saw at part 1
.fill 510-(.-_start), 1, 0
.word 0xAA55