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546 lines
22 KiB
ReStructuredText
.. _programstructure:
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====================
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Programming in Prog8
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====================
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This chapter describes a high level overview of the elements that make up a program.
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Details about the syntax can be found in the :ref:`syntaxreference` chapter.
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Elements of a program
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---------------------
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Program
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Consists of one or more *modules*.
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Module
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A file on disk with the ``.p8`` suffix. It contains *directives* and *code blocks*.
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Whitespace and indentation in the source code are arbitrary and can be tabs or spaces or both.
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You can also add *comments* to the source code.
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One moudule file can *import* others, and also import *library modules*.
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Comments
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Everything after a semicolon ``;`` is a comment and is ignored by the compiler.
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If the whole line is just a comment, it will be copied into the resulting assembly source code.
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This makes it easier to understand and relate the generated code. Examples::
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A = 42 ; set the initial value to 42
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; next is the code that...
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Directive
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These are special instructions for the compiler, to change how it processes the code
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and what kind of program it creates. A directive is on its own line in the file, and
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starts with ``%``, optionally followed by some arguments.
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Code block
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A block of actual program code. It defines a *scope* (also known as 'namespace') and
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can contain Prog8 *code*, *variable declarations* and *subroutines*.
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More details about this below: :ref:`blocks`.
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Variable declarations
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The data that the code works on is stored in variables ('named values that can change').
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The compiler allocates the required memory for them.
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There is *no dynamic memory allocation*. The storage size of all variables
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is fixed and is determined at compile time.
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Variable declarations tend to appear at the top of the code block that uses them.
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They define the name and type of the variable, and its initial value.
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Prog8 supports a small list of data types, including special 'memory mapped' types
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that don't allocate storage but instead point to a fixed location in the address space.
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Code
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These are the instructions that make up the program's logic. There are different kinds of instructions
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('statements' is a better name):
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- value assignment
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- looping (for, while, repeat, unconditional jumps)
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- conditional execution (if - then - else, and conditional jumps)
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- subroutine calls
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- label definition
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Subroutine
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Defines a piece of code that can be called by its name from different locations in your code.
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It accepts parameters and can return result values.
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It can define its own variables, and it is even possible to define subroutines nested inside other subroutines.
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Their contents is scoped accordingly.
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Label
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This is a named position in your code where you can jump to from another place.
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You can jump to it with a jump statement elsewhere. It is also possible to use a
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subroutine call to a label (but without parameters and return value).
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Scope
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Also known as 'namespace', this is a named box around the symbols defined in it.
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This prevents name collisions (or 'namespace pollution'), because the name of the scope
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is needed as prefix to be able to access the symbols in it.
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Anything *inside* the scope can refer to symbols in the same scope without using a prefix.
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There are three scopes in Prog8:
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- global (no prefix)
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- code block
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- subroutine
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Modules are *not* a scope! Everything defined in a module is merged into the global scope.
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.. _blocks:
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Blocks, Scopes, and accessing Symbols
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-------------------------------------
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Blocks are the separate pieces of code and data of your program. They are combined
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into a single output program. No code or data can occur outside a block. Here's an example::
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~ main $c000 {
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; this is code inside the block...
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}
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The name of a block must be unique in your entire program.
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Also be careful when importing other modules; blocks in your own code cannot have
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the same name as a block defined in an imported module or library.
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If you omit both the name and address, the entire block is *ignored* by the compiler (and a warning is displayed).
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This is a way to quickly "comment out" a piece of code that is unfinshed or may contain errors that you
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want to work on later, because the contents of the ignored block are not fully parsed either.
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The address can be used to place a block at a specific location in memory.
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Usually it is omitted, and the compiler will automatically choose the location (usually immediately after
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the previous block in memory).
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The address must be >= ``$0200`` (because ``$00``--``$ff`` is the ZP and ``$100``--``$200`` is the cpu stack).
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A block is also a *scope* in your program so the symbols in the block don't clash with
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symbols of the same name defined elsewhere in the same file or in another file.
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You can refer to the symbols in a particular block by using a *dotted name*: ``blockname.symbolname``.
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Labels inside a subroutine are appended again to that; ``blockname.subroutinename.label``.
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A symbol name that's not a dotted name is searched for in the current scope, if it's not found there,
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one scope higher, and so on until it is found.
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Every symbol is 'public' and can be accessed from elsewhere given its dotted name.
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**The special "ZP" ZeroPage block**
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Blocks named "ZP" are treated a bit differently: they refer to the ZeroPage.
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The contents of every block with that name (this one may occur multiple times) are merged into one.
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Its start address is always set to ``$04``, because ``$00 - $01`` are used by the hardware
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and ``$02 - $03`` are reserved as general purpose scratch registers.
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Program Start and Entry Point
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-----------------------------
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Your program must have a single entry point where code execution begins.
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The compiler expects a ``start`` subroutine in the ``main`` block for this,
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taking no parameters and having no return value.
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As any subroutine, it has to end with a ``return`` statement (or a ``goto`` call)::
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~ main {
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sub start () -> () {
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; program entrypoint code here
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return
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}
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}
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The ``main`` module is always relocated to the start of your programs
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address space, and the ``start`` subroutine (the entrypoint) will be on the
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first address. This will also be the address that the BASIC loader program (if generated)
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calls with the SYS statement.
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Variables and values
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--------------------
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Variables are named values that can change during the execution of the program.
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When declaring a variable it is required to specify the initial value it should get.
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Values will usually be part of an expression or assignment statement::
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12345 ; integer number
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$aa43 ; hex integer number
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%100101 ; binary integer number
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"Hi, I am a string" ; text string
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-33.456e52 ; floating point number
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byte counter = 42 ; variable of size 8 bits, with initial value 42
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Array and Matrix (2-dimensional array) types are also supported like this::
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byte[4] array = [1, 2, 3, 4] ; initialize the array
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byte[99] array = 255 ; initialize array with all 255's [255, 255, 255, 255, ...]
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byte[100] array = 100 to 199 ; initialize array with [100, 101, ..., 198, 199]
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byte[2,3] matrix = 1 ; a matrix of 2*3=6 bytes all with value 1
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byte[2,3] matrix = [1,2,3,4,5,6] ; a 2*3 matrix with value |(1,2) (3,4) (5,6)|
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Note that the various keywords for the data type and variable type (``byte``, ``word``, ``const``, etc.)
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cannot be used as *identifiers* elsewhere. You can't make a variable, block or subroutine with the name ``byte``
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for instance.
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.. todo::
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There must be a way to tell the compiler which variables you require to be in Zeropage:
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``zeropage`` modifier keyword on vardecl perhaps?
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Variables that represent CPU hardware registers
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The following variables are reserved
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and map directly (read/write) to a CPU hardware register: ``A``, ``X``, ``Y``, ``AX``, ``AY``, ``XY`` (the 2-letter ones
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are a pseudo 16-bit 'register' by pairing two 8-bit registers).
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Special types: const and memory-mapped
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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When using ``const``, the value of the 'variable' can no longer be changed.
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You'll have to specify the initial value expression. This value is then used
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by the compiler everywhere you refer to the constant (and no storage is allocated
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for the constant itself).
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When using ``memory``, the variable will point to specific location in memory,
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rather than being newly allocated. The initial value (mandatory) must be a valid
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memory address. Reading the variable will read the given data type from the
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address you specified, and setting the varible will directly modify that memory location(s)::
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const byte max_age = 2000 - 1974 ; max_age will be the constant value 26
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memory word SCREENCOLORS = $d020 ; a 16-bit word at the addres $d020-$d021
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.. note::
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Directly accessing random memory locations is not yet supported without the
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intermediate step of declaring a memory-mapped variable for the memory location.
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The advantages of this however, is that it's clearer what the memory location
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stands for, and the compiler also knows the data type.
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Integers
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^^^^^^^^
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Integers are 8 or 16 bit numbers and can be written in normal decimal notation,
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in hexadecimal and in binary notation.
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.. todo::
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Right now only unsinged integers are supported (0-255 for byte types, 0-65535 for word types)
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@todo maybe signed integers (-128..127 and -32768..32767) will be added later
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Strings
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^^^^^^^
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Strings are a sequence of characters enclosed in ``"`` quotes. The length is limited to 255 characters.
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They're stored and treated much the same as a byte array,
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but they have some special properties because they are considered to be *text*.
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Strings in your source code files will be encoded (translated from ASCII/UTF-8) into either CBM PETSCII or C-64 screencodes.
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PETSCII is the default choice. If you need screencodes (also called 'poke' codes) instead,
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you have to use the ``str_s`` variants of the string type identifier.
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If you assign a string literal of length 1 to a non-string variable, it is treated as a *byte* value instead
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with has the PETSCII value of that single character.
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.. caution::
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It's probably best that you don't change strings after they're created.
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This is because if your program exits and is restarted (without loading it again),
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it will then operate on the changed strings instead of the original ones.
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The same is true for arrays and matrixes by the way.
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Floating point numbers
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^^^^^^^^^^^^^^^^^^^^^^
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Floats are stored in the 5-byte 'MFLPT' format that is used on CBM machines,
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and also most float operations are specific to the Commodore-64.
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This is because routines in the C-64 BASIC and KERNAL ROMs are used for that.
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So floating point operations will only work if the C-64 BASIC ROM (and KERNAL ROM)
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are banked in (and your code imports the ``c64lib.p8``)
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The largest 5-byte MFLPT float that can be stored is: **1.7014118345e+38** (negative: **-1.7014118345e+38**)
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Initial values across multiple runs of the program
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The initial values of your variables will be restored automatically when the program is (re)started,
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*except for string variables, arrays and matrices*. It is assumed these are left unchanged by the program.
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If you do modify them in-place, you should take care yourself that they work as
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expected when the program is restarted.
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Indirect addressing and address-of
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----------------------------------
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The ``#`` operator is used to take the address of the symbol following it.
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It can be used for example to work with the *address* of a memory mapped variable rather than
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the value it holds. You could take the address of a string as well, but that is redundant:
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the compiler already treats those as a value that you manipulate via its address.
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For most other types this prefix is not supported and will result in a compilation error.
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The resulting value is simply a 16 bit word. Example::
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AX = #somevar
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.. todo::
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This is not yet implemented.
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Indirect addressing, Indirect addressing in jumps (jmp/jsr indirect)
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Loops
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-----
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The *for*-loop is used to iterate over a range of values. Iteration is done in steps of 1, but you can change this.
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The *while*-loop is used to repeat a piece of code while a certain condition is still true.
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The *repeat--until* loop is used to repeat a piece of code until a certain condition is true.
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You can also create loops by using the ``goto`` statement, but this should usually be avoided.
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Conditional Execution
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---------------------
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Conditional execution means that the flow of execution changes based on certiain conditions,
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rather than having fixed gotos or subroutine calls::
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if (A > 4) goto overflow
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if (X == 3) Y = 4
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if (X == 3) Y = 4 else A = 2
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if (X == 5) {
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Y = 99
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} else {
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A = 3
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}
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Conditional jumps (``if (condition) goto label``) are compiled using 6502's branching instructions (such as ``bne`` and ``bcc``) so
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the rather strict limit on how *far* it can jump applies. The compiler itself can't figure this
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out unfortunately, so it is entirely possible to create code that cannot be assembled successfully.
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You'll have to restructure your gotos in the code (place target labels closer to the branch)
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if you run into this type of assembler error.
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There is a special form of the if-statement that immediately translates into one of the 6502's branching instructions.
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This allows you to write a conditional jump or block execution directly acting on the current values of the CPU's status register bits.
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The eight branching instructions of the CPU each have an if-equivalent:
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``if_cs``, ``if_cc``, ``if_eq``, ``if_ne``, ``if_pl``, ``if_mi``, ``if_vs`` and ``if_vc``.
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So ``if_cc goto target`` will directly translate into the single CPU instruction ``BCC target``.
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.. note::
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For now, the symbols used or declared in the statement block(s) are shared with
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the same scope the if statement itself is in.
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Maybe in the future this will be a separate nested scope, but for now, that is
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only possible when defining a subroutine.
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Assignments
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-----------
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Assignment statements assign a single value to a target variable or memory location.
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Augmented assignments (such as ``A += X``) are also available, but these are just shorthands
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for normal assignments (``A = A + X``).
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Expressions
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-----------
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In most places where a number or other value is expected, you can use just the number, or a constant expression.
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The expression is parsed and evaluated by the compiler itself at compile time, and the (constant) resulting value is used in its place.
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Expressions can contain procedure and function calls.
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There are various built-in functions such as sin(), cos(), min(), max() that can be used in expressions (see :ref:`builtinfunctions`).
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You can also reference idendifiers defined elsewhere in your code.
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The compiler will evaluate the expression if it is a constant, and just use the resulting value from then on.
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Expressions that cannot be compile-time evaluated will result in code that calculates them at runtime.
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Arithmetic and Logical expressions
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Arithmetic expressions are expressions that calculate a numeric result (integer or floating point).
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Many common arithmetic operators can be used and follow the regular precedence rules.
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Logical expressions are expressions that calculate a boolean result, true or false
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(which in Prog8 will effectively be a 1 or 0 integer value).
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You can use parentheses to group parts of an expresion to change the precedence.
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Usually the normal precedence rules apply (``*`` goes before ``+`` etc.) but subexpressions
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within parentheses will be evaluated first. So ``(4 + 8) * 2`` is 24 and not 20,
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and ``(true or false) and false`` is false instead of true.
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Subroutines
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-----------
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Defining a subroutine
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^^^^^^^^^^^^^^^^^^^^^
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Subroutines are parts of the code that can be repeatedly invoked using a subroutine call from elsewhere.
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Their definition, using the sub statement, includes the specification of the required input- and output parameters.
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For now, only register based parameters are supported (A, X, Y and paired registers AX, AY and XY,
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and various flags of the status register P: Pc (carry), Pz (zero), Pn (negative), Pv (overflow).
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For subroutine return values, it is the same (registers, status flags).
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Subroutines can be defined in a Block, but also nested inside another subroutine. Everything is scoped accordingly.
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Calling a subroutine
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^^^^^^^^^^^^^^^^^^^^
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The output variables must occur in the correct sequence of return registers as specified
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in the subroutine's definiton. It is possible to not specify any of them but the compiler
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will issue a warning then if the result values of a subroutine call are discarded.
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If you don't have a variable to store the output register in, it's then required
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to list the register itself instead as output variable.
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Arguments should match the subroutine definition. You are allowed to omit the parameter names.
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If no definition is available (because you're directly calling memory or a label or something else),
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you can freely add arguments (but in this case they all have to be named).
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To jump to a subroutine (without returning), prefix the subroutine call with the word 'goto'.
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Unlike gotos in other languages, here it take arguments as well, because it
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essentially is the same as calling a subroutine and only doing something different when it's finished.
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**Register preserving calls:** use the ``!`` followed by a combination of A, X and Y (or followed
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by nothing, which is the same as AXY) to tell the compiler you want to preserve the origial
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value of the given registers after the subroutine call. Otherwise, the subroutine may just
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as well clobber all three registers. Preserving the original values does result in some
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stack manipulation code to be inserted for every call like this, which can be quite slow.
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.. caution::
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Note that *recursive* subroutine calls are not supported at this time.
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If you do need a recursive algorithm, you'll have to hand code it in embedded assembly for now,
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or rewrite it into an iterative algorithm.
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.. _builtinfunctions:
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Built-in Functions
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------------------
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There's a set of predefined functions in the language. These are fixed and can't be redefined in user code.
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You can use them in expressions and the compiler will evaluate them at compile-time if possible.
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sin(x)
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Sine.
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cos(x)
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Cosine.
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abs(x)
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Absolute value.
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acos(x)
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Arccosine.
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asin(x)
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Arcsine.
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tan(x)
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Tangent.
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atan(x)
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Arctangent.
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log(x)
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Natural logarithm.
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log10(x)
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Base-10 logarithm.
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sqrt(x)
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Square root.
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round(x)
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Rounds the floating point to the closest integer.
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floor (x)
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Rounds the floating point down to an integer towards minus infinity.
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ceil(x)
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Rounds the floating point up to an integer towards positive infinity.
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rad(x)
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Degrees to radians.
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deg(x)
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Radians to degrees.
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max(x)
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Maximum of the values in the non-scalar (array or matrix) value x
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min(x)
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Minimum of the values in the non-scalar (array or matrix) value x
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avg(x)
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Average of the values in the non-scalar (array or matrix) value x
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sum(x)
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Sum of the values in the non-scalar (array or matrix) value x
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len(x)
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Number of values in the array or matrix value x, or the number of characters in a string (excluding the size or 0-byte).
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Note: this can be different from the number of *bytes* in memory if the datatype isn't a byte.
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lsb(x)
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Get the least significant byte of the word x.
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msb(x)
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Get the most significant byte of the word x.
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any(x)
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1 ('true') if any of the values in the non-scalar (array or matrix) value x is 'true' (not zero), else 0 ('false')
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all(x)
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1 ('true') if all of the values in the non-scalar (array or matrix) value x are 'true' (not zero), else 0 ('false')
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rnd()
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returns a pseudo-random byte from 0..255
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rndw()
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returns a pseudo-random word from 0..65535
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rndf()
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returns a pseudo-random float between 0.0 and 1.0
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lsl(x)
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Shift the bits in x (byte or word) one position to the left.
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Bit 0 is set to 0 (and the highest bit is shifted into the status register's Carry flag)
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Modifies in-place but also returns the new value.
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lsr(x)
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Shift the bits in x (byte or word) one position to the right.
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The highest bit is set to 0 (and bit 0 is shifted into the status register's Carry flag)
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Modifies in-place but also returns the new value.
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rol(x)
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Rotate the bits in x (byte or word) one position to the left.
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This uses the CPU's rotate semantics: bit 0 will be set to the current value of the Carry flag,
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while the highest bit will become the new Carry flag value.
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(essentially, it is a 9-bit or 17-bit rotation)
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Modifies in-place, doesn't return a value (so can't be used in an expression).
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rol2(x)
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Like _rol but now as 8-bit or 16-bit rotation.
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It uses some extra logic to not consider the carry flag as extra rotation bit.
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Modifies in-place, doesn't return a value (so can't be used in an expression).
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ror(x)
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Rotate the bits in x (byte or word) one position to the right.
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This uses the CPU's rotate semantics: the highest bit will be set to the current value of the Carry flag,
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while bit 0 will become the new Carry flag value.
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(essentially, it is a 9-bit or 17-bit rotation)
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Modifies in-place, doesn't return a value (so can't be used in an expression).
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ror2(x)
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Like _ror but now as 8-bit or 16-bit rotation.
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It uses some extra logic to not consider the carry flag as extra rotation bit.
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Modifies in-place, doesn't return a value (so can't be used in an expression).
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P_carry(bit)
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Set (or clear) the CPU status register Carry flag. No result value.
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(translated into ``SEC`` or ``CLC`` cpu instruction)
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P_irqd(bit)
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Set (or clear) the CPU status register Interrupt Disable flag. No result value.
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(translated into ``SEI`` or ``CLI`` cpu instruction)
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