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. The manual for Octo assembly language can be found here.
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.