147 lines
6.9 KiB
Plaintext
147 lines
6.9 KiB
Plaintext
<preface>
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<title>Preface</title>
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<para>
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Ophis is an assembler for the 6502 microprocessor - the famous
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chip used in the vast majority of the classic 8-bit computers and
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consoles. Its primary design goals are code readability and output
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flexibility - Ophis has successfully been used to create programs
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for the Nintendo Entertainment System, the Atari 2600, and various
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8-bit Commodore machines.
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</para>
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<para>
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Ophis's syntax is noticably different from the formats
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traditionally used for these chips; it draws its syntactic
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inspiration primarily from the assemblers for more modern chips,
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where the role of tokens is determined more by what they're made
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of and their grammatical location on a line rather than their
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absolute position on a line. It also borrows the sophisticated
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methods of tracking the location of labels when writing relinkable
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code—Ophis expects that the final output it produces will have
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only a vague resemblance to the memory image when loaded. Most of
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the alternatives when Ophis was first designed would place
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instructions and data into a memory map and then dump that map.
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</para>
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<para>
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That said, there remain many actively used 6502 assemblers out
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there. If you're already a seasoned 6502 assembly programmer, or
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want to get your old sources built again, Ophis is likely not for
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you—however, if you are writing new code, or are new to the
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chip while still having other experience, then Ophis is a tool
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built with you in mind.
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</para>
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<section>
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<title>History of the project</title>
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<para>
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The Ophis project started on a lark back in 2001. My graduate
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studies required me to learn Perl and Python, and I'd been
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playing around with Commodore 64 emulators in my spare time, so
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I decided to learn both languages by writing a simple
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cross-assembler for the 6502 chip the C64 used in both.
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</para>
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<para>
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The Perl one—uncreatively
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dubbed <quote>Perl65</quote>—was quickly abandoned, but
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the Python one saw more work. When it came time to name it, one
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of the things I had been hoping to do with the assembler was to
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produce working Apple II programs. <quote>Ophis</quote> is
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Greek for <quote>snake</quote>, and a number of traditions also
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use it as the actual <emphasis>name</emphasis> of the serpent in
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the Garden of Eden. So, Pythons, snakes, and stories involving
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really old Apples all combined to name the
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assembler.<footnote><para>Ironically, cross-platform development
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for the Apple II is extremely difficult, and while Ophis has
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been very successfully used to develop code for the Commodore
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64, Nintendo Entertainment System, and Atari 2600, it has yet to
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actually be deployed on any of the Apples which inspired its
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name.</para></footnote>
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</para>
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<para>
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Ophis slowly grew in scope and power over the years, and by 2005
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was a very powerful, flexible macro assembler that saw more use
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than I'd expect. In 2007 Ophis 1.0 was formally released.
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However, Ophis was written for Python 2.1 and this became more
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and more untenable as time has gone by. As I started receiving
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patches for parts of Ophis, and as I used it for some projects
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of my own, it became clear that Ophis needed to be modernized
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and to become better able to interoperate with other
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toolchains. It was this process that led to Ophis 2.
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</para>
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<para>
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After its release Ophis 2 was picked up by a number of
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developers work with actual hardware from the period, including
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prototype machines that never saw production. Some of their
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contributions have refined the code generators for version 2.1.
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</para>
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<para>
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This is an updated edition of <emphasis>Programming With
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Ophis</emphasis>, including documentation for all new features
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introduced and expanding the examples to include simple
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demonstration programs for platforms besides the Commodore
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64. It also includes updated versions of the <emphasis>To HLL
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and Back</emphasis> essays I wrote using Ophis and Perl65 as
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example languages.
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</para>
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</section>
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<section>
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<title>Getting a copy of Ophis</title>
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<para>
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As of this writing, the Ophis assembler is hosted at Github. The
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latest downloads and documentation will be available
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at <ulink url="http://github.com/michaelcmartin/Ophis"></ulink>. If
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this is out-of-date, a Web search on <quote>Ophis 6502
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assembler</quote> (without the quotation marks) should yield its
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page.
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</para>
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<para>
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Ophis is written entirely in Python and packaged using the
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distutils. The default installation script on Unix and Mac OS X
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systems should put the files where they need to go. If you are
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running it locally, you will need to install
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the <literal>Ophis</literal> package somewhere in your Python
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package path, and then put the <command>ophis</command> script
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somewhere in your path.
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</para>
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<para>
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For Windows users, a prepackaged system made
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with <command>py2exe</command> is also available. The default
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Windows installer will use this. In this case, all you need to
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do is have <command>ophis.exe</command> in your path.
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</para>
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<para>
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If you are working on a system with Python installed but to
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which you do not wish to install software, there is also a
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standalone pure-Python edition with an ophis.py script. This may
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be placed anywhere and running ophis.py will temporarily set the
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library path to point to your directory.
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</para>
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</section>
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<section>
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<title>About the examples</title>
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<para>
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Versions of the examples in this book are available from the
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Ophis site. Windows users will find them packaged with the
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distribution; all other users can get them as a separate
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download or pull them directly from github.
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</para>
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<para>
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The code in this book is available in
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the <literal>examples/</literal> subdirectory, while extra
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examples will be in subdirectories of their own with brief
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descriptions. They are largely all simple <quote>Hello
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world</quote> applications, designed mainly to demonstrate how
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to package assembled binaries into forms that emulators or ROM
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loaders can use. They are not primarily intended as tutorials
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for writing for the platforms themselves.
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</para>
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<para>
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Most examples will require use of <emphasis>platform
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headers</emphasis>—standardized header files that set
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useful constants for the target system and, if needed, contain
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small programs to allow the program to be loaded and run. These
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are stored in the <literal>platform/</literal> subdirectory.
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</para>
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</section>
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</preface>
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