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

var
  gdt: GDTEntries
  gp: GDTPointer

let gpAddr = cast[uint32](addr(gp))

{.push stackTrace:off.}
proc gdtFlush(){.asmNoStackFrame, inline.} =
  asm """
    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: uint8, granularity: uint8) =
  # 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].granularity = ((limit shr 16) and 0x0F)

  # Finally, set up the granularity and access flags
  gdt[num].granularity = gdt[num].granularity or (granularity and 0xF0)
  gdt[num].access = access

proc gdtInstall*() =
    gp.limit = uint16(sizeof(GDTEntry) * 3) - 1;
    gp.base = cast[uint32](addr(gdt));

    # NULL descriptor
    gdtSetGate(0, 0, 0, 0, 0);

    # The second entry is our Code Segment
    gdtSetGate(1, 0'u16, 0xFFFFFFFF'u32, 0x9A'u8, 0xCF'u8);

    # The third entry is our Data Segment
    gdtSetGate(2, 0'u16, 0xFFFFFFFF'u32, 0x92'u8, 0xCF'u8);

    gdtFlush();