alcedo/kernel.nim

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 (ord(bg) shl 4))

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

const
  vgaWidth = 80
  vgaHeight = 25
  terminalBufferBaseAddress = 0xB8000

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

proc terminalInitialize() =
  for y in 0..<vgaWidth:
    for x in 0..<vgaHeight:
      let index = terminalBufferBaseAddress + (y * vgaWidth + x)
      cast[ptr uint16](index)[] = vgaEntry(' ', terminalColour) # Write directly to display memory