Octo

• Dance following its arrow

• Free-Dancing

• Octo is a high-level assembler for the Chip8 virtual machine, complete with an environment for testing programs.
• The Chip8 keypad is different from your keyboard as follows:

	Chip8 Key   Keyboard
	---------   ---------
	 1 2 3 C     1 2 3 4
	 4 5 6 D     q w e r
	 7 8 9 E     a s d f
	 A 0 B F     z x c v

• Control Flow

The main control flow is if-then statement. The player can press the letter W to up and lift his arms up, letter S to down and lay down his arms, letter A to left and change his face to left and letter D and change his face to right, also moving by 2 pixels. Create a basic main loop and the subroutine is to execute movement. When again is encountered, Chip8 will skip back to the matchong loop and resume from that point. (Example of Free Dancing)

: main	
	i := facing-left 
	v0 := 30
	v1 := 20
	loop
		clear
		sprite v0 v1 12
		v2 := key
		if v2 == 7 then i := facing-left 
		if v2 == 7 then v0 += -2 
		if v2 == 9 then i := facing-right 
		if v2 == 9 then v0 +=  2
		if v2 == 5 then i := facing-up 
		if v2 == 5 then v1 +=  -2
		if v2 == 8 then i := facing-down 
		if v2 == 8 then v1 += 2 

	again

• Parsing

1.Register map

Chip8 has 16 v-registers, v0-vf, which we will use to store state. In my game, I use righta and rightb to keep track of the right-arrow position.

:alias righta v3
:alias rightb v4

2.Scope Scope refers to the visibility of variables. In chip8, it beigns with colon (:) and end with semi-colon(;) Move-arrow-right is starting by inserting subroutine calls into main loop.

: move-arrow-right
	va := righta
	vb := rightb 
	
	va += 5 
	
	i := arrow-right
	sprite righta rightb 5
	righta := va
	rightb := vb
	i := arrow-right
	sprite righta rightb 5
	sync
;

: main 
	righta := 0
	rightb := 0
	i := arrow-right
	sprite righta rightb 5
	loop 
		move-arrow-right
		#dancing
	again

Also anything after the hash symbol (#) is ignored by it.

• Others

1. Moving slow and fast In order to make objects move in whole numbers of pixels per second. This technique is based on fixed-point arithmetic and the flag(vf) register.

: sync
	loop
		vf := delay
		if vf != 0 then
	again

	# delay for up to 3/60th of a second
	# using the fixed-rate delay timer
	vf := 3
	delay := vf
;

2. Mutilple movement In main loop:

	righta := 0
	rightb := 0

	lefta := 0
	leftb := 0

	upa := 0
	upb := 0

	downa := 0
	downb := 0


	i := arrow-right
	sprite righta rightb 5
	
	i := arrow-left
	sprite lefta leftb 5

	i := arrow-up
	sprite upa upb 5

	i := arrow-down
	sprite downa downb 5

	i := facing-left 
	sprite leftx lefty 12
	
	loop 
		move-arrow-right
		move-arrow-left
		move-arrow-up
		move-arrow-down
	again

Reference

• Octo Chip-8 the most accurate Chip-8 reference online.
• Chip8