fun write(is_cmd: Bool, cmd: Int)
	do
		en.write(false)
		rs.write(not is_cmd)

		# high byte
		var hb = once [16,32,64,128]
		for i in [0..4[ do
			var b = hb[i]
			var r = b & cmd != 0
			var d = ds[i]
			d.write(r)
		end

		pulse_enable

		if is_cmd then
			# wait 5ms
			5.bcm2835_delay
		else
			# wait 200us
			200.bcm2835_delay_micros
		end

		# low byte
		var lb = once [1,2,4,8]
		for i in [0..4[ do
			var b = lb[i]
			var r = b & cmd != 0
			var d = ds[i]
			d.write(r)
		end

		pulse_enable

		if is_cmd then
			# wait 5ms
			5.bcm2835_delay
		else
			# wait 200us
			200.bcm2835_delay_micros
		end
	end