Ophis is an assembler for the 6502 microprocessor - the famous chip used in the vast majority of the classic 8-bit computers and consoles. Its primary design goals are code readability and output flexibility - Ophis has successfully been used to create programs for the Nintendo Entertainment System, the Atari 2600, and various 8-bit Commodore machines. Ophis's syntax is noticably different from the formats traditionally used for these chips; it draws its syntactic inspiration primarily from the assemblers for more modern chips, where the role of tokens is determined more by what they're made of and their grammatical location on a line rather than their absolute position on a line. It also borrows the sophisticated methods of tracking the location of labels when writing relinkable code—Ophis expects that the final output it produces will have only a vague resemblance to the memory image when loaded. Most of the alternatives when Ophis was first designed would place instructions and data into a memory map and then dump that map. That said, there remain many actively used 6502 assemblers out there. If you're already a seasoned 6502 assembly programmer, or want to get your old sources built again, Ophis is likely not for you—however, if you are writing new code, or are new to the chip while still having other experience, then Ophis is a tool built with you in mind.

The Ophis project started on a lark back in 2001. My graduate studies required me to learn Perl and Python, and I'd been playing around with Commodore 64 emulators in my spare time, so I decided to learn both languages by writing a simple cross-assembler for the 6502 chip the C64 used in both. The Perl one—uncreatively dubbed Perl65—was quickly abandoned, but the Python one saw more work. When it came time to name it, one of the things I had been hoping to do with the assembler was to produce working Apple II programs. Ophis is Greek for snake, and a number of traditions also use it as the actual name of the serpent in the Garden of Eden. So, Pythons, snakes, and stories involving really old Apples all combined to name the assembler.Ironically, cross-platform development for the Apple II is extremely difficult, and while Ophis has been very successfully used to develop code for the Commodore 64, Nintendo Entertainment System, and Atari 2600, it has yet to actually be deployed on any of the Apples which inspired its name. Ophis slowly grew in scope and power over the years, and by 2005 was a very powerful, flexible macro assembler that saw more use than I'd expect. In 2007 Ophis 1.0 was formally released. However, Ophis was written for Python 2.1 and this became more and more untenable as time has gone by. As I started receiving patches for parts of Ophis, and as I used it for some projects of my own, it became clear that Ophis needed to be modernized and to become better able to interoperate with other toolchains. It was this process that led to Ophis 2. After its release Ophis 2 was picked up by a number of developers work with actual hardware from the period, including prototype machines that never saw production. Some of their contributions have refined the code generators for version 2.1. This is an updated edition of Programming With Ophis, including documentation for all new features introduced and expanding the examples to include simple demonstration programs for platforms besides the Commodore 64. It also includes updated versions of the To HLL and Back essays I wrote using Ophis and Perl65 as example languages.

As of this writing, the Ophis assembler is hosted at Github. The latest downloads and documentation will be available at . If this is out-of-date, a Web search on Ophis 6502 assembler (without the quotation marks) should yield its page. Ophis is written entirely in Python and packaged using the distutils. The default installation script on Unix and Mac OS X systems should put the files where they need to go. If you are running it locally, you will need to install the Ophis package somewhere in your Python package path, and then put the ophis script somewhere in your path. For Windows users, a prepackaged system made with py2exe is also available. The default Windows installer will use this. In this case, all you need to do is have ophis.exe in your path. If you are working on a system with Python installed but to which you do not wish to install software, there is also a standalone pure-Python edition with an ophis.py script. This may be placed anywhere and running ophis.py will temporarily set the library path to point to your directory.

Versions of the examples in this book are available from the Ophis site. Windows users will find them packaged with the distribution; all other users can get them as a separate download or pull them directly from github. The code in this book is available in the examples/ subdirectory, while extra examples will be in subdirectories of their own with brief descriptions. They are largely all simple Hello world applications, designed mainly to demonstrate how to package assembled binaries into forms that emulators or ROM loaders can use. They are not primarily intended as tutorials for writing for the platforms themselves. Most examples will require use of platform headers—standardized header files that set useful constants for the target system and, if needed, contain small programs to allow the program to be loaded and run. These are stored in the platform/ subdirectory.