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<appendix id="ref-link">
<title>Ophis Command Reference</title>
<section>
<title>Command Modes</title>
<para>
These mostly follow the <emphasis>MOS Technology 6500
Microprocessor Family Programming Manual</emphasis>, except
for the Accumulator mode. Accumulator instructions are written
and interpreted identically to Implied mode instructions.
</para>
<itemizedlist>
<listitem><para><emphasis>Implied:</emphasis> <literal>RTS</literal></para></listitem>
<listitem><para><emphasis>Accumulator:</emphasis> <literal>LSR</literal></para></listitem>
<listitem><para><emphasis>Immediate:</emphasis> <literal>LDA #$06</literal></para></listitem>
<listitem><para><emphasis>Zero Page:</emphasis> <literal>LDA $7C</literal></para></listitem>
<listitem><para><emphasis>Zero Page, X:</emphasis> <literal>LDA $7C,X</literal></para></listitem>
<listitem><para><emphasis>Zero Page, Y:</emphasis> <literal>LDA $7C,Y</literal></para></listitem>
<listitem><para><emphasis>Absolute:</emphasis> <literal>LDA $D020</literal></para></listitem>
<listitem><para><emphasis>Absolute, X:</emphasis> <literal>LDA $D000,X</literal></para></listitem>
<listitem><para><emphasis>Absolute, Y:</emphasis> <literal>LDA $D000,Y</literal></para></listitem>
<listitem><para><emphasis>(Zero Page Indirect, X):</emphasis> <literal>LDA ($80, X)</literal></para></listitem>
<listitem><para><emphasis>(Zero Page Indirect), Y:</emphasis> <literal>LDA ($80), Y</literal></para></listitem>
<listitem><para><emphasis>(Absolute Indirect):</emphasis> <literal>JMP ($A000)</literal></para></listitem>
<listitem><para><emphasis>Relative:</emphasis> <literal>BNE loop</literal></para></listitem>
<listitem><para><emphasis>(Absolute Indirect, X):</emphasis> <literal>JMP ($A000, X)</literal> — Only available with 65C02 extensions</para></listitem>
<listitem><para><emphasis>(Zero Page Indirect):</emphasis> <literal>LDX ($80)</literal> — Only available with 65C02 extensions</para></listitem>
</itemizedlist>
</section>
<section>
<title>Basic arguments</title>
<para>
Most arguments are just a number or label. The formats for
these are below.
</para>
<section>
<title>Numeric types</title>
<itemizedlist>
<listitem><para><emphasis>Hex:</emphasis> <literal>$41</literal> (Prefixed with $)</para></listitem>
<listitem><para><emphasis>Decimal:</emphasis> <literal>65</literal> (No markings)</para></listitem>
<listitem><para><emphasis>Octal:</emphasis> <literal>0101</literal> (Prefixed with zero)</para></listitem>
<listitem><para><emphasis>Binary:</emphasis> <literal>%01000001</literal> (Prefixed with %)</para></listitem>
<listitem><para><emphasis>Character:</emphasis> <literal>'A</literal> (Prefixed with single quote)</para></listitem>
</itemizedlist>
</section>
<section>
<title>Label types</title>
<para>
Normal labels are simply referred to by name. Anonymous
labels may be referenced with strings of - or + signs (the
label <literal>-</literal> refers to the immediate
previous anonymous label, <literal>--</literal> the
one before that, etc., while <literal>+</literal>
refers to the next anonymous label), and the special
label <literal>^</literal> refers to the program
counter at the start of the current instruction or directive.
</para>
<para>
Normal labels are <emphasis>defined</emphasis> by
prefixing a line with the label name and then a colon
(e.g., <literal>label:</literal>). Anonymous labels
are defined by prefixing a line with an asterisk
(e.g., <literal>*</literal>).
</para>
<para>
Temporary labels are only reachable from inside the
innermost enclosing <literal>.scope</literal>
statement. They are identical to normal labels in every
way, except that they start with an underscore.
</para>
</section>
<section>
<title>String types</title>
<para>
Strings are enclosed in double quotation marks. Backslashed
characters (including backslashes and double quotes) are
treated literally, so the string <literal>"The man said,
\"The \\ character is the backslash.\""</literal> produces
the ASCII sequence for <literal>The man said, "The \
character is the backslash."</literal>
</para>
<para>
Strings are generally only used as arguments to assembler
directives—usually for filenames
(e.g., <literal>.include</literal>) but also for string
data (in association with <literal>.byte</literal>).
</para>
<para>
It is legal, though unusual, to attempt to pass a string to
the other data statements. This will produces a series of
words/dwords where all bytes that aren't least-significant
are zero. Endianness and size will match what the directive
itself indicated.
</para>
</section>
</section>
<section>
<title>Compound Arguments</title>
<para>
Compound arguments may be built up from simple ones, using the
standard +, -, *, and / operators, which carry the usual
precedence. Also, the unary operators > and <, which
bind more tightly than anything else, provide the high and low
bytes of 16-bit values, respectively.
</para>
<para>
Use brackets [ ] instead of parentheses ( ) when grouping
arithmetic operations, as the parentheses are needed for the
indirect addressing modes.
</para>
<para>
Examples:
</para>
<itemizedlist>
<listitem><para><literal>$D000</literal> evaluates to $D000</para></listitem>
<listitem><para><literal>$D000+32</literal> evaluates to $D020</para></listitem>
<listitem><para><literal>$D000+$20</literal> also evaluates to $D020</para></listitem>
<listitem><para><literal><$D000+32</literal> evaluates to $20</para></listitem>
<listitem><para><literal>>$D000+32</literal> evaluates to $F0</para></listitem>
<listitem><para><literal>>[$D000+32]</literal> evaluates to $D0</para></listitem>
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<listitem><para><literal>>[$D000-275]</literal> evaluates to $CE</para></listitem>
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</itemizedlist>
</section>
<section>
<title>Memory Model</title>
<para>
In order to properly compute the locations of labels and the
like, Ophis must keep track of where assembled code will
actually be sitting in memory, and it strives to do this in a
way that is independent both of the target file and of the
target machine.
</para>
<section>
<title>Basic PC tracking</title>
<para>
The primary technique Ophis uses is <emphasis>program counter
tracking</emphasis>. As it assembles the code, it keeps
track of a virtual program counter, and uses that to
determine where the labels should go.
</para>
<para>
In the absence of an <literal>.org</literal> directive, it
assumes a starting PC of zero. <literal>.org</literal>
is a simple directive, setting the PC to the value
that <literal>.org</literal> specifies. In the simplest
case, one <literal>.org</literal> directive appears at the
beginning of the code and sets the location for the rest of
the code, which is one contiguous block.
</para>
</section>
<section>
<title>Basic Segmentation simulation</title>
<para>
However, this isn't always practical. Often one wishes to
have a region of memory reserved for data without actually
mapping that memory to the file. On some systems (typically
cartridge-based systems where ROM and RAM are seperate, and
the target file only specifies the ROM image) this is
mandatory. In order to access these variables symbolically,
it's necessary to put the values into the label lookup
table.
</para>
<para>
It is possible, but inconvenient, to do this
with <literal>.alias</literal>, assigning a specific
memory location to each variable. This requires careful
coordination through your code, and makes creating reusable
libraries all but impossible.
</para>
<para>
A better approach is to reserve a section at the beginning
or end of your program, put an <literal>.org</literal>
directive in, then use the <literal>.space</literal>
directive to divide up the data area. This is still a bit
inconvenient, though, because all variables must be
assigned all at once. What we'd really like is to keep
multiple PC counters, one for data and one for code.
</para>
<para>
The <literal>.text</literal>
and <literal>.data</literal> directives do this. Each
has its own PC that starts at zero, and you can switch
between the two at any point without corrupting the other's
counter. In this way each function can have
a <literal>.data</literal> section (filled
with <literal>.space</literal> commands) and
a <literal>.text</literal> section (that contains the
actual code). This lets our library routines be almost
completely self-contained - we can have one source file
that could be <literal>.included</literal> by multiple
projects without getting in anything's way.
</para>
<para>
However, any given program may have its own ideas about
where data and code go, and it's good to ensure with
a <literal>.checkpc</literal> at the end of your code
that you haven't accidentally overwritten code with data or
vice versa. If your <literal>.data</literal>
segment <emphasis>did</emphasis> start at zero, it's
probably wise to make sure you aren't smashing the stack,
too (which is sitting in the region from $0100 to
$01FF).
</para>
<para>
If you write code with no segment-defining statements in
it, the default segment
is <literal>text</literal>.
</para>
<para>
The <literal>data</literal> segment is designed only
for organizing labels. As such, errors will be flagged if
you attempt to actually output information into
a <literal>data</literal> segment.
</para>
</section>
<section>
<title>General Segmentation Simulation</title>
<para>
One text and data segment each is usually sufficient, but
for the cases where it is not, Ophis allows for user-defined
segments. Putting a label
after <literal>.text</literal>
or <literal>.data</literal> produces a new segment with
the specified name.
</para>
<para>
Say, for example, that we have access to the RAM at the low
end of the address space, but want to reserve the zero page
for truly critical variables, and use the rest of RAM for
everything else. Let's also assume that this is a 6510
chip, and locations $00 and $01 are reserved for the I/O
port. We could start our program off with:
</para>
<programlisting>
.data
.org $200
.data zp
.org $2
.text
.org $800
</programlisting>
<para>
And, to be safe, we would probably want to end our code
with checks to make sure we aren't overwriting anything:
</para>
<programlisting>
.data
.checkpc $800
.data zp
.checkpc $100
</programlisting>
</section>
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</section>
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<section>
<title>Macros</title>
<para>
Assembly language is a powerful tool—however, there are
many tasks that need to be done repeatedly, and with
mind-numbing minor modifications. Ophis includes a facility
for <emphasis>macros</emphasis> to allow this. Ophis macros
are very similar in form to function calls in higher level
languages.
</para>
<section>
<title>Defining Macros</title>
<para>
Macros are defined with the <literal>.macro</literal>
and <literal>.macend</literal> commands. Here's a
simple one that will clear the screen on a Commodore
64:
</para>
<programlisting>
.macro clr'screen
lda #147
jsr $FFD2
.macend
</programlisting>
</section>
<section>
<title>Invoking Macros</title>
<para>
To invoke a macro, either use
the <literal>.invoke</literal> command or backquote the
name of the routine. The previous macro may be expanded
out in either of two ways, at any point in the
source:
</para>
<programlisting>.invoke clr'screen</programlisting>
<para>or</para>
<programlisting>`clr'screen</programlisting>
<para>will work equally well.</para>
</section>
<section>
<title>Passing Arguments to Macros</title>
<para>
Macros may take arguments. The arguments to a macro are
all of the <quote>word</quote> type, though byte values may
be passed and used as bytes as well. The first argument in
an invocation is bound to the label
<literal>_1</literal>, the second
to <literal>_2</literal>, and so on. Here's a macro
for storing a 16-bit value into a word pointer:
</para>
<programlisting>
.macro store16 ; `store16 dest, src
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lda #<_2
sta _1
lda #>_2
sta _1+1
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.macend
</programlisting>
<para>
Macro arguments behave, for the most part, as if they were
defined by <literal>.alias</literal>
commands <emphasis>in the calling context</emphasis>.
(They differ in that they will not produce duplicate-label
errors if those names already exist in the calling scope,
and in that they disappear after the call is
completed.)
</para>
</section>
<section>
<title>Features and Restrictions of the Ophis Macro Model</title>
<para>
Unlike most macro systems (which do textual replacement),
Ophis macros evaluate their arguments and bind them into the
symbol table as temporary labels. This produces some
benefits, but it also puts some restrictions on what kinds of
macros may be defined.
</para>
<para>
The primary benefit of this <quote>expand-via-binding</quote>
discipline is that there are no surprises in the semantics.
The expression <literal>_1+1</literal> in the macro above
will always evaluate to one more than the value that was
passed as the first argument, even if that first argument is
some immensely complex expression that an
expand-via-substitution method may accidentally
mangle.
</para>
<para>
The primary disadvantage of the expand-via-binding
discipline is that only fixed numbers of words and bytes
may be passed. A substitution-based system could define a
macro including the line <literal>LDA _1</literal> and
accept as arguments both <literal>$C000</literal>
(which would put the value of memory location $C000 into
the accumulator) and <literal>#$40</literal> (which
would put the immediate value $40 into the accumulator).
If you <emphasis>really</emphasis> need this kind of
behavior, a run a C preprocessor over your Ophis source,
and use <literal>#define</literal> to your heart's
content.
</para>
</section>
</section>
<section>
<title>Assembler directives</title>
<para>
Assembler directives are all instructions to the assembler
that are not actual instructions. Ophis's set of directives
follow.
</para>
<itemizedlist>
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<listitem>
<para>
<literal>.outfile</literal> <emphasis>filename</emphasis>:
Sets the filename for the output binary if one has not
already been set. If no name is ever set, the output will
be written to <literal>ophis.bin</literal>.
</para>
</listitem>
<listitem>
<para>
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<literal>.advance</literal> <emphasis>address</emphasis>
[, <emphasis>filler</emphasis>]: Forces the program
counter to be <emphasis>address</emphasis>. Unlike
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the <literal>.org</literal>
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directive, <literal>.advance</literal> outputs bytes (the
value of <emphasis>filler</emphasis>, or zeroes if it is
unspecified) until the program counter reaches a
specified address. Attempting
to <literal>.advance</literal> to a point behind the
current program counter is an assemble-time error.
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</para>
</listitem>
<listitem>
<para>
<literal>.alias</literal> <emphasis>label</emphasis> <emphasis>value</emphasis>:
The .alias directive assigns an arbitrary value to a
label. This value may be an arbitrary argument, but
cannot reference any label that has not already been
defined (this prevents recursive label
dependencies).
</para>
</listitem>
<listitem>
<para>
<literal>.byte</literal> <emphasis>arg</emphasis> [
, <emphasis>arg</emphasis>, ... ]: Specifies a series of
arguments, which are evaluated, and strings, which are
included as raw ASCII data. The final results of these
arguments must be one byte in size. Seperate constants
are seperated by comments.
</para>
</listitem>
<listitem>
<para>
<literal>.checkpc</literal> <emphasis>address</emphasis>:
Ensures that the program counter is less than or equal to
the address specified, and emits an assemble-time error
if it is not. <emphasis>This produces no code in the
final binary - it is there to ensure that linking a large
amount of data together does not overstep memory
boundaries.</emphasis>
</para>
</listitem>
<listitem>
<para>
<literal>.data</literal> <emphasis>[label]</emphasis>:
Sets the segment to the segment name specified and
disallows output. If no label is given, switches to the
default data segment.
</para>
</listitem>
<listitem>
<para>
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<literal>.incbin</literal> <emphasis>filename</emphasis>
[, <emphasis>offset</emphasis>
[, <emphasis>length</emphasis>]]: Inserts the contents of
the file specified as binary data. Use it to include
graphics information, precompiled code, or other
non-assembler data. You may also optionally specify an
index to start including from, or a length to only
include a subset.
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</para>
</listitem>
<listitem>
<para>
<literal>.include</literal> <emphasis>filename</emphasis>:
Includes the entirety of the file specified at that point
in the program. Use this to order your final sources, if
you aren't doing it via the command line.
</para>
</listitem>
<listitem>
<para>
<literal>.org</literal> <emphasis>address</emphasis>:
Sets the program counter to the address
specified. <emphasis>This does not emit any code in and
of itself, nor does it overwrite anything that previously
existed.</emphasis> If you wish to jump ahead in memory,
use <literal>.advance</literal>.
</para>
</listitem>
<listitem>
<para>
<literal>.require</literal> <emphasis>filename</emphasis>:
Includes the entirety of the file specified at that point
in the program. Unlike <literal>.include</literal>,
however, code included with <literal>.require</literal>
will only be inserted once.
The <literal>.require</literal> directive is useful for
ensuring that certain code libraries are somewhere in the
final binary. They are also very useful for guaranteeing
that macro libraries are available.
</para>
</listitem>
<listitem>
<para>
<literal>.space</literal> <emphasis>label</emphasis> <emphasis>size</emphasis>:
This directive is used to organize global variables. It
defines the label specified to be at the current location
of the program counter, and then advances the program
counter <emphasis>size</emphasis> steps ahead. No actual
code is produced. This is equivalent to <literal>label:
.org ^+size</literal>.
</para>
</listitem>
<listitem>
<para>
<literal>.text</literal> <emphasis>[label]</emphasis>:
Sets the segment to the segment name specified and allows
output. If no label is given, switches to the default
text segment.
</para>
</listitem>
<listitem>
<para>
<literal>.word</literal> <emphasis>arg</emphasis> [
, <emphasis>arg</emphasis>, ... ]:
Like <literal>.byte</literal>, but values are all treated
as two-byte values and stored low-end first (as is the
6502's wont). Use this to create jump tables (an
unadorned label will evaluate to that label's location)
or otherwise store 16-bit data.
</para>
</listitem>
<listitem>
<para>
<literal>.dword</literal> <emphasis>arg</emphasis> [
, <emphasis>arg</emphasis>, ...]:
Like <literal>.word</literal>, but for 32-bit
values.
</para>
</listitem>
<listitem>
<para>
<literal>.wordbe</literal> <emphasis>arg</emphasis> [
, <emphasis>arg</emphasis>, ...]:
Like <literal>.word</literal>, but stores the value in a
big-endian format (high byte first).
</para>
</listitem>
<listitem>
<para>
<literal>.dwordbe</literal> <emphasis>arg</emphasis> [
, <emphasis>arg</emphasis>, ...]:
Like <literal>.dword</literal>, but stores the value high
byte first.
</para>
</listitem>
<listitem>
<para>
<literal>.scope</literal>: Starts a new scope
block. Labels that begin with an underscore are only
reachable from within their innermost
enclosing <literal>.scope</literal>
statement.
</para>
</listitem>
<listitem>
<para>
<literal>.scend</literal>: Ends a scope block. Makes the
temporary labels defined since the
last <literal>.scope</literal> statement unreachable, and
permits them to be redefined in a new
scope.
</para>
</listitem>
<listitem>
<para>
<literal>.macro</literal> <emphasis>name</emphasis>:
Begins a macro definition block. This is a scope block
that can be inlined at arbitrary points
with <literal>.invoke</literal>. Arguments to the macro
will be bound to temporary labels with names like
<literal>_1</literal>, <literal>_2</literal>, etc.
</para>
</listitem>
<listitem>
<para>
<literal>.macend</literal>: Ends a macro definition block.
</para>
</listitem>
<listitem>
<para>
<literal>.invoke</literal> <emphasis>label</emphasis> [<emphasis>argument</emphasis> [,
<emphasis>argument</emphasis> ...]]: invokes (inlines) the
specified macro, binding the values of the arguments to the
ones the macro definition intends to read. A shorthand
for <literal>.invoke</literal> is the name of the macro to
invoke, backquoted.
</para>
</listitem>
</itemizedlist>
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</section>
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</appendix>
