// vim: et:sw=12:ts=12:sts=12

            .global     _start
            .text
            .code16

_start:     cli
            cld

            mov         %cs, %ax
            mov         %ax, %ds

            mov         $message, %si
            call        print

            call        enable_a20

            lea         message, %si
            call        print

            lgdt        gdt_ptr

            call        enter_prot

1:          hlt
            jmp         1b

enable_a20: // TODO more thorough implementation
            inb         $0x92, %al
            or          $2, %al
            outb        %al, $0x92
            ret

enter_prot: mov         %cr0, %eax
            or          $1, %al
            mov         %eax, %cr0
            ret
            
print:      xor         %bx, %bx

1:          lodsb
            or          %al, %al
            jz          2f
            mov         $0x0E, %ah
            int         $0x10
            jmp         1b

2:          ret

gdt:        // entry 0: null descriptor
            .word       0
            .word       0
            .byte       0
            .byte       0
            .byte       0
            .byte       0
            // entry 1: code segment
            .word       0xFFFF
            .word       0
            .byte       0
            .byte       0b10011010
            .byte       0x8F
            .byte       0
            // entry 2: data segment
            .word       0xFFFF
            .word       0
            .byte       0
            .byte       0b10010010
            .byte       0x8F
            .byte       0
            .set        gdt_size, .-gdt
gdt_ptr:    .word       gdt_size-1
            .long       gdt

message:    .asciz     "Hello, World!\r\n"