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
  AccessByte = uint8
  Flags = uint8
  GDT{.packed.} = object
    descriptor: GDTPointer
    entries: array[0..2, GDTEntry]

const
  gdtAddress: uint32 = 0x00000800
  gdt = cast[ptr GDT](gdtAddress)

{.push stackTrace:off.}
proc gdtFlush(){.inline,asmNoStackFrame.} =
  asm """
    cli
    lgdtl [`0x00000800`]
    movw $0x10, %ax
    movw %ax, %ds
    movw %ax, %es
    movw %ax, %fs
    movw %ax, %gs
    ljmp $0x08, $next
    next:
  """
{.pop.}

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

  # Setup the descriptor limits
  gdt.entries[num].limit_low = (limit and 0xFFFF)
  gdt.entries[num].flags_and_limit_mid = ((limit shr 16) and 0x0F) or flags
  gdt.entries[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*() =
  gdt.descriptor.limit = uint16(sizeof(GDTEntry) * gdt.entries.len) - 1;
  gdt.descriptor.base = cast[uint32](gdt.entries.addr)
  # 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()