global str screen_namespace = "/dev/screen/0"
global str mouse_namespace = "/dev/mouse/0"
global str terminal_namespace = "/dev/term/0"
global str new_line = "\n"
global byte WHITE = 255

function main ()
  // Open screen
  // use load immediate because it a pointer to a string, not a value

  plex screen $0
  plex mouse $1
  str tmp_str $2
  byte color $3
  byte left_down $4
	int mode $5
  nat offset_temp $6
  nat x $7
  nat y $8
  nat width $9
  nat screen_buffer $10
  nat buffer_size $11
  nat pixel_pos $12

  load_address screen_namespace -> tmp_str
  load_immediate 0 -> mode
  syscall OPEN tmp_str mode -> screen // open Plex screen, in namespace, in flags

  nat_to_string screen -> tmp_str
  call pln tmp_str

  load_offset_32 screen 8 -> width // load width
  nat_to_string width -> tmp_str
  call pln tmp_str

  load_offset_32 screen 12 -> buffer_size // load size   
  nat_to_string buffer_size -> tmp_str
  call pln tmp_str

  load_immediate 16 -> offset_temp // offset for screen buffer
  add_nat screen offset_temp -> screen_buffer

  nat_to_string screen_buffer -> tmp_str
  call pln tmp_str

  // open mouse
  load_address mouse_namespace -> tmp_str
  syscall OPEN tmp_str mode -> mouse // open Plex mouse, in namespace, in flags

  syscall WRITE screen screen_buffer buffer_size // redraw

  loop draw_loop
    // load mouse click data
    syscall STAT mouse

    load_offset_8 mouse 16 -> left_down // load btn1 pressed

    jump_eq_nat draw_loop left_down mode // mode 0 which an alias for false

    load_offset_32 mouse 8 -> x   // load x
    load_offset_32 mouse 12 -> y // load y

    // Compute start address: y*width + x
    mul_nat y width -> pixel_pos       // = y * width
    add_nat x pixel_pos -> pixel_pos   // += x
    add_nat screen_buffer pixel_pos -> pixel_pos // += pixel_offset
    load_immediate 4 -> fat_ptr_size // need to add offset for fat pointer size
    add_nat pixel_pos fat_ptr_size -> pixel_pos

    load_absolute_32 WHITE -> color
    store_absolute_8 pixel_pos color // draw color at screen [x,y]
    syscall WRITE screen screen_buffer buffer_size // redraw

    jump draw_loop
  exit 0

function pln (str message $0) 
	str term $1
	int msg_length $2
	str nl $3
	int nl_length $4
	int mode $5

	load_address terminal_namespace -> term // get terminal device
	load_immediate 0 -> mode
	syscall OPEN term mode -> term
	strlen message -> msg_length
	syscall WRITE term message msg_length	
	load_address new_line -> nl
	strlen nl -> nl_length
	syscall WRITE term nl nl_length
	return