mirror of
https://github.com/michaelcmartin/Ophis.git
synced 2024-11-08 07:04:39 +00:00
138 lines
6.5 KiB
Plaintext
138 lines
6.5 KiB
Plaintext
<preface>
|
|
<title>Preface</title>
|
|
<para>
|
|
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 the
|
|
Commodore 64.
|
|
</para>
|
|
<para>
|
|
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.
|
|
</para>
|
|
<para>
|
|
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.
|
|
</para>
|
|
<section>
|
|
<title>History of the project</title>
|
|
<para>
|
|
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.
|
|
</para>
|
|
<para>
|
|
The Perl one—uncreatively
|
|
dubbed <quote>Perl65</quote>—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. <quote>Ophis</quote> is
|
|
Greek for <quote>snake</quote>, and a number of traditions also
|
|
use it as the actual <emphasis>name</emphasis> of the serpent in
|
|
the Garden of Eden. So, Pythons, snakes, and stories involving
|
|
really old Apples all combined to name the
|
|
assembler.<footnote><para>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.</para></footnote>
|
|
</para>
|
|
<para>
|
|
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.
|
|
</para>
|
|
<para>
|
|
This is an updated edition of <emphasis>Programming With
|
|
Ophis</emphasis>, 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 <emphasis>To HLL
|
|
and Back</emphasis> essays I wrote using Ophis and Perl65 as
|
|
example languages.
|
|
</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Getting a copy of Ophis</title>
|
|
<para>
|
|
As of this writing, the Ophis assembler is hosted at Github. The
|
|
latest downloads and documentation will be available
|
|
at <ulink url="http://github.com/michaelcmartin/Ophis"></ulink>. If
|
|
this is out-of-date, a Web search on <quote>Ophis 6502
|
|
assembler</quote> (without the quotation marks) should yield its
|
|
page.
|
|
</para>
|
|
<para>
|
|
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 <literal>Ophis</literal> package somewhere in your Python
|
|
package path, and then put the <command>ophis</command> script
|
|
somewhere in your path.
|
|
</para>
|
|
<para>
|
|
For Windows users, a prepackaged system made
|
|
with <command>py2exe</command> is also available. The default
|
|
Windows installer will use this. In this case, all you need to
|
|
do is have <command>ophis.exe</command> in your path.
|
|
</para>
|
|
<para>
|
|
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.
|
|
</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>About the examples</title>
|
|
<para>
|
|
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.
|
|
</para>
|
|
<para>
|
|
The code in this book is available in
|
|
the <literal>examples/</literal> subdirectory, while extra
|
|
examples will be in subdirectories of their own with brief
|
|
descriptions. They are largely all simple <quote>Hello
|
|
world</quote> 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.
|
|
</para>
|
|
<para>
|
|
Most examples will require use of <emphasis>platform
|
|
headers</emphasis>—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 <literal>platform/</literal> subdirectory.
|
|
</para>
|
|
</section>
|
|
</preface>
|