Octo is a high-level assembler for the Chip8 virtual machine, complete with an environment for testing programs. You can try it out here.
While a program is running, you can press escape to return to the editor. The Chip8 keypad is represented on 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
To learn more about Chip8 programming techniques, have a look at the documentation section as well as the manual for Octo assembly language. Octo fully supports SuperChip extended instructions as well as a set of original extended instructions called XO-Chip.
The Octo assembler can also be used as a command-line tool via a Node.js fontend:
$ ./octo
usage: octo [--decompile] [--roundtrip] [--qshift]
[--qloadstore] <source> [<destination>]
$ cat simple.8o
: main
va := 1
vb := 2
$ ./octo simple.8o simple.ch8
$ hexdump simple.ch8
0000000 6a 01 6b 02
0000004
The --decompile option can be used to send an existing Chip8 binary through Octo's general-purpose decompiler.
Octo has a share feature which stores source code and configuration metadata in Github gists. Following one of these links will run the program and then you can back out into the Octo IDE. Alternatively, you can use these urls to embed a Chip8 emulator in an iframe on your websites:
<iframe
src="http://johnearnest.github.io/Octo/embed.html?scale=2&gist=f3685a75817cde6d5c0d"
width="256"
height="128"
></iframe>
(Special thanks to rmmh.)
The scale argument is optional and specifies the number of pixels which should make up a high-resolution mode Chip8 pixel. Low-resolution mode pixels will be twice this size.