# Ophis Command Reference

## Command Modes

These mostly follow the MOS Technology 6500 Microprocessor Family Programming Manual, except for the Accumulator mode. Accumulator instructions are written and interpreted identically to Implied mode instructions.

- Implied: `RTS`

- Accumulator: `LSR`

- Immediate: `LDA #$06`

- Zero Page: `LDA $7C`

- Zero Page, X: `LDA $7C,X`

- Zero Page, Y: `LDA $7C,Y`

- Absolute: `LDA $D020`

- Absolute, X: `LDA $D000,X`

- Absolute, Y: `LDA $D000,Y`

- (Zero Page Indirect, X): `LDA ($80, X)`

- (Zero Page Indirect), Y: `LDA ($80), Y`

- (Absolute Indirect): `JMP ($A000)`

- Relative: `BNE loop`

- (Absolute Indirect, X): `JMP ($A000, X)` — Only available with 65C02 extensions

- (Zero Page Indirect): `LDX ($80)` — Only available with 65C02 extensions

## Basic arguments

Most arguments are just a number or label. The formats for these are below.

### Numeric types

- Hex: `$41` (Prefixed with $)

- Decimal: `65` (No markings)

- Octal: `0101` (Prefixed with zero)

- Binary: `%01000001` (Prefixed with %)

- Character: `'A` (Prefixed with single quote)

### Label types

Normal labels are simply referred to by name. Anonymous labels may be referenced with strings of - or + signs (the label `-` refers to the immediate previous anonymous label, `--` the one before that, etc., while `+` refers to the next anonymous label), and the special label `^` refers to the program counter at the start of the current instruction or directive.

Normal labels are defined by prefixing a line with the label name and then a colon (e.g., `label:`). Anonymous labels are defined by prefixing a line with an asterisk (e.g., `*`).

Temporary labels are only reachable from inside the innermost enclosing `.scope` statement. They are identical to normal labels in every way, except that they start with an underscore.

### String types

Strings are enclosed in double quotation marks. Backslashed characters (including backslashes and double quotes) are treated literally, so the string `"The man said, \"The \\ character is the backslash.\""` produces the ASCII sequence for `The man said, "The \ character is the backslash."`

Strings are generally only used as arguments to assembler directives—usually for filenames (e.g., `.include`) but also for string data (in association with `.byte`).

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.

## Compound Arguments

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.

Use brackets [ ] instead of parentheses ( ) when grouping arithmetic operations, as the parentheses are needed for the indirect addressing modes.

Examples:

- `$D000` evaluates to $D000

- `$D000+32` evaluates to $D020

- `$D000+$20` also evaluates to $D020

- `<$D000+32` evaluates to $20

- `>$D000+32` evaluates to $F0

- `>[$D000+32]` evaluates to $D0

- `>[$D000-275]` evaluates to $CE

## Memory Model

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.

### Basic PC tracking

The primary technique Ophis uses is program counter tracking. As it assembles the code, it keeps track of a virtual program counter, and uses that to determine where the labels should go.

In the absence of an `.org` directive, it assumes a starting PC of zero. `.org` is a simple directive, setting the PC to the value that `.org` specifies. In the simplest case, one `.org` directive appears at the beginning of the code and sets the location for the rest of the code, which is one contiguous block.

### Basic Segmentation simulation

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.

It is possible, but inconvenient, to do this with `.alias`, assigning a specific memory location to each variable. This requires careful coordination through your code, and makes creating reusable libraries all but impossible.

A better approach is to reserve a section at the beginning or end of your program, put an `.org` directive in, then use the `.space` 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.

The `.text` and `.data` 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 `.data` section (filled with `.space` commands) and a `.text` 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 `.included` by multiple projects without getting in anything's way.

However, any given program may have its own ideas about where data and code go, and it's good to ensure with a `.checkpc` at the end of your code that you haven't accidentally overwritten code with data or vice versa. If your `.data` segment did 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).

If you write code with no segment-defining statements in it, the default segment is `text`.

The `data` segment is designed only for organizing labels. As such, errors will be flagged if you attempt to actually output information into a `data` segment.

### General Segmentation Simulation

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 `.text` or `.data` produces a new segment with the specified name.

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:

    .data
    .org $200
    .data zp
    .org $2
    .text
    .org $800

And, to be safe, we would probably want to end our code with checks to make sure we aren't overwriting anything:

    .data
    .checkpc $800
    .data zp
    .checkpc $100

## Macros

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 macros to allow this. Ophis macros are very similar in form to function calls in higher level languages.

### Defining Macros

Macros are defined with the `.macro` and `.macend` commands. Here's a simple one that will clear the screen on a Commodore 64:

    .macro clr'screen
        lda #147
        jsr $FFD2
    .macend

### Invoking Macros

To invoke a macro, either use the `.invoke` 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:

    .invoke clr'screen

or

    `clr'screen

will work equally well.

### Passing Arguments to Macros

Macros may take arguments. The arguments to a macro are all of the word type, though byte values may be passed and used as bytes as well. The first argument in an invocation is bound to the label `_1`, the second to `_2`, and so on. Here's a macro for storing a 16-bit value into a word pointer:

    .macro store16   ; `store16 dest, src
            lda #<_2
            sta _1
            lda #>_2
            sta _1+1
    .macend

Macro arguments behave, for the most part, as if they were defined by `.alias` commands in the calling context. (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.)

### Features and Restrictions of the Ophis Macro Model

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.

The primary benefit of this expand-via-binding discipline is that there are no surprises in the semantics. The expression `_1+1` 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.

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 `LDA _1` and accept as arguments both `$C000` (which would put the value of memory location $C000 into the accumulator) and `#$40` (which would put the immediate value $40 into the accumulator). If you really need this kind of behavior, a run a C preprocessor over your Ophis source, and use `#define` to your heart's content.

## Assembler directives

Assembler directives are all instructions to the assembler that are not actual instructions. Ophis's set of directives follow.

- `.outfile` filename: 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 `ophis.bin`.

- `.advance` address [, filler]: Forces the program counter to be address. Unlike the `.org` directive, `.advance` outputs bytes (the value of filler, or zeroes if it is unspecified) until the program counter reaches a specified address. Attempting to `.advance` to a point behind the current program counter is an assemble-time error.

- `.alias` label value: 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).

- `.byte` arg [ , arg, ... ]: 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.

- `.cbmfloat` string [ , string, ... ]: Specifies a series of strings, which are interpreted as floating point constants, and then included in the 5-byte floating point format used by the Commodore BASICs. This format is 8 bits of exponent, followed by a sign bit and a 31-bit big-endian mantissa fraction. (The 1 in front of the binary point is presumed to be present.) An exponent of 0 specifies a constant of 0, and the exponent is shifted up by 129 before being stored.

  Because IEEE-754 doesn't perfectly match the Commodore's system, if you wish to precisely replicate individual constants that cannot be represented exactly you may have better luck with the following program, which will run on both the Commodore 64 and VIC-20:

      10 CLR:V=0:PV=PEEK(45)+256*PEEK(46)+2
      20 INPUT "NUMBER (0 TO QUIT)";V
      30 IF V=0 THEN END
      40 PRINT ".BYTE";
      50 FOR I=0 TO 4
      60 IF I>0 THEN PRINT CHR$(157);",";
      70 PRINT PEEK(PV+I);:NEXT I:PRINT:GOTO 20

  This program will print out a `.byte` directive for you to include in your program to represent that number.

- `.checkpc` address: Ensures that the program counter is less than or equal to the address specified, and emits an assemble-time error if it is not. 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.

- `.data` [label]: Sets the segment to the segment name specified and disallows output. If no label is given, switches to the default data segment.

- `.incbin` filename [, offset [, length]]: 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.

- `.include` filename: 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.

- `.org` address: Sets the program counter to the address specified. This does not emit any code in and of itself, nor does it overwrite anything that previously existed. If you wish to jump ahead in memory, use `.advance`.

- `.require` filename: Includes the entirety of the file specified at that point in the program. Unlike `.include`, however, code included with `.require` will only be inserted once. The `.require` 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.

- `.space` label size: 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 size steps ahead. No actual code is produced. This is equivalent to `label: .org ^+size`.

- `.text` [label]: Sets the segment to the segment name specified and allows output. If no label is given, switches to the default text segment.

- `.word` arg [ , arg, ... ]: Like `.byte`, 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.

- `.dword` arg [ , arg, ...]: Like `.word`, but for 32-bit values.

- `.wordbe` arg [ , arg, ...]: Like `.word`, but stores the value in a big-endian format (high byte first).

- `.dwordbe` arg [ , arg, ...]: Like `.dword`, but stores the value high byte first.

- `.scope`: Starts a new scope block. Labels that begin with an underscore are only reachable from within their innermost enclosing `.scope` statement.

- `.scend`: Ends a scope block. Makes the temporary labels defined since the last `.scope` statement unreachable, and permits them to be redefined in a new scope.

- `.macro` name: Begins a macro definition block. This is a scope block that can be inlined at arbitrary points with `.invoke`. Arguments to the macro will be bound to temporary labels with names like `_1`, `_2`, etc.

- `.macend`: Ends a macro definition block.

- `.invoke` label [argument [, argument ...]]: invokes (inlines) the specified macro, binding the values of the arguments to the ones the macro definition intends to read. A shorthand for `.invoke` is the name of the macro to invoke, backquoted.