type
  VGA_Colour {.pure.} = enum
    black = 0,
    blue = 1,
    green = 2,
    cyan = 3,
    red = 4,
    magenta = 5,
    brown = 6,
    lightGrey = 7,
    darkGrey = 8,
    lightBlue = 9,
    lightGreen = 10,
    lightCyan = 11,
    lightRed = 12,
    lightMagenta = 13,
    lightBrown = 14,
    white = 15

proc vgaEntryColour(fg: VGA_Colour, bg: VGA_Colour): uint8 =
  result = uint8(ord(fg)) or (uint8(ord(bg)) shl 4)

proc vgaEntry(c: char, colour: uint8): uint16 =
  result = uint16(c) or (uint16(colour) shl 8)

const
  vgaWidth = 80
  vgaHeight = 25
  terminalBufferBaseAddress = 0xB8000

var
  terminalRow, terminalColumn = 0
  terminalColour = vgaEntryColour(VGA_Colour.lightGrey, VGA_Colour.black)

proc terminalWriteAtPoint(writeChar: char, colour: uint8, xPos: int, yPos: int) =
  let index = terminalBufferBaseAddress + (yPos * vgaWidth + xPos)
  cast[ptr uint16](index)[] = vgaEntry(writeChar, colour) # Write directly to display memory

proc terminalInitialize() =
  for y in 0..<vgaWidth:
    for x in 0..<vgaHeight:
      terminalWriteAtPoint(' ', 0, x, y)

proc setTerminalColour(newColour: uint8) =
  terminalColour = newColour

proc terminalWriteChar(writeChar: char) =
  if(writeChar == '\r'):
    inc(terminalRow)
    if(terminalRow == vgaHeight): terminalRow = 0
    return
  terminalWriteAtPoint(writeChar, terminalColour, terminalColumn, terminalRow)
  inc(terminalColumn)
  if(terminalColumn == vgaWidth):
    terminalColumn = 0
    inc(terminalRow)
    if(terminalRow == vgaHeight): terminalRow = 0

proc terminalWrite(data: string) =
  for character in data:
    terminalWriteChar(character)

proc kernelMain() {.exportc: "kernel_main"}=
  terminalInitialize()
  terminalWrite("Hello World\rThis is Nim!")