function my_pset(x, y, c)
  addr = 0x6000 + shr(x, 1) + shl(y, 6)
  old_val = peek(addr)

  if (band(1, x) == 0) then
    new_val = c + band(0xf0, old_val)
  else
    new_val = shl(c, 4) + band(0x0f, old_val)
  end

  poke(addr, new_val)
end