import serial

type
  GDTEntry{.packed.} = object
    limit_low: uint16
    base_low: uint16
    base_middle: uint8
    access: uint8
    flags_and_limit_mid: uint8
    base_high: uint8
  GDTPointer{.packed.} = object
    limit: uint16 # Size of GDT
    base: uint32 # GDT memory location
  GDTEntries = array[0..2, GDTEntry]
  AccessByte = uint8
  Flags = uint8

var
  gdt: GDTEntries
  gp: GDTPointer
  gpAddr: uint32

{.push stackTrace:off.}
proc gdtFlush(){.inline.} =
  gpAddr = cast[uint32](gp.addr)
  serial.write("GDPointer Address: ")
  serial.write(gpAddr)
  serial.newLine()
  asm """
    cli
    lgdt [`gpAddr`]
    mov %ax, 0x10
    mov %ds, %ax
    mov %es, %ax
    mov %fs, %ax
    mov %gs, %ax
    mov %ss, %ax
  """
{.pop.}

proc gdtSetGate(num: int, base: uint16, limit: uint32, access: AccessByte, flags: Flags) =
  # Setup the descriptor base address
  gdt[num].base_low = (base and 0xFFFF)
  gdt[num].base_middle = (base shr 16) and 0xFF
  gdt[num].base_high = (base shr 24) and 0xFF

  # Setup the descriptor limits
  gdt[num].limit_low = (limit and 0xFFFF)
  gdt[num].flags_and_limit_mid = ((limit shr 16) and 0x0F) or flags
  gdt[num].access = access

proc createAccessByte(priv: range[0..3], directionConform: bool, readWrite: bool): AccessByte =
  result = 0b10011000'u8 # Present and executable bit
  result = result or (uint8(priv) shl 5)
  if directionConform:
    result = result or 0b00000100'u8
  if readWrite:
    result = result or 0b00000010'u8

proc createFlags(granularity: bool, size: bool): Flags =
  if granularity:
    result = result or 0b10000000'u8
  if size:
    result = result or 0b01000000'u8

proc gdtInstall*() =
  gp.limit = uint16(sizeof(GDTEntry) * gdt.len) - 1;
  gp.base = cast[uint32](gdt.addr)
  serial.write("GDT Address: ")
  serial.write(gp.base)
  serial.newLine()
  # NULL descriptor
  gdtSetGate(0, 0, 0, 0, 0);

  # The second entry is our Code Segment
  gdtSetGate(1, 0'u16, 0xFFFFFFFF'u32, createAccessByte(0, false, true), createFlags(true, true));

  # The third entry is our Data Segment
  gdtSetGate(2, 0'u16, 0xFFFFFFFF'u32, createAccessByte(0, false, true), createFlags(true, true));
  serial.write("Flushing GDT.\L")
  gdtFlush();