.globl _start

.code16
_start:
    cli

    lgdt (gdtr_desc) # 3. Load the GDTR

    mov %cr0, %eax
    or $1, %al
    mov %eax, %cr0

    ljmp $0x8, $PModeMain

.align 8
gdt_start: # 1. Define the GDT table in memory
    .quad 0x0000000000000000 # Null descriptor (required)
gdt_code:
    .word 0xFFFF     # Limit (low)
    .word 0x0000     # Base (low)
    .byte 0x00       # Base (middle)
    .byte 0b10011010 # Access byte (code, ring 0)
    .byte 0b11001111 # Flags + Limit (high)
    .byte 0x00       # Base (high)
gdt_data:
    .word 0xFFFF
    .word 0x0000
    .byte 0x00
    .byte 0b10010010 # Access byte (data, ring 0)
    .byte 0b11001111
    .byte 0x00
gdt_end:

gdtr_desc: # 2. Define the GDTR descriptor (limit + base)
    .word gdt_end - gdt_start - 1 # Limit: Size of GDT - 1
    .long gdt_start  # Base: Linear address of GDT

.code32
PModeMain:
    # 5. Load Data Segment Selector (Offset 0x10 in GDT)
    mov $0x10, %ax
    mov %ax, %ds
    mov %ax, %ss
    mov %ax, %es

    movw $0x0F4A, 0xB8000       # Write 'J' to the top-left pixel

    hang:
        jmp hang

.fill 510 - (. - _start), 1, 0 
.word 0xAA55