Does the job quite well if you're fine with just adding opcodes. Adding them is simple enough. However, if you want to alter the assembler in any way, be prepared to be met with some of the most atrocious code formatting you'll ever see. It looks like the author's keyboard didn't have a tab, space or return key. I like it aside from the (aesthetic) complaint above, and it does what it needs to do but before I begin expanding it for my needs I'm going to really have to clean up the code formatting. Otherwise, it's a nice little tool and I highly recommend it if you're trying to make a custom assembler for emulators or some other application requiring a cross compiler. Bottom line: I recommend this.