docs/intro.pod - The Parrot Primer
This document provides a gentle introduction to the Parrot virtual machine for anyone considering writing code for Parrot by hand, writing a compiler that targets Parrot, getting involved with Parrot development or simply wondering what on earth Parrot is.
WHAT IS PARROT?
Parrot is a virtual machine. To understand what a virtual machine is, consider what happens when you write a program in a language such as Perl, then run it with the applicable interpreter (in the case of Perl, the perl executable). First, the program you have written in a high level language is turned into simple instructions, for example fetch the value of the variable named x, add 2 to this value, store this value in the variable named y, etc. A single line of code in a high level language may be converted into tens of these simple instructions. This stage is called compilation.
The second stage involves executing these simple instructions. Some languages (for example, C) are often compiled to instructions that are understood by the CPU and as such can be executed by the hardware. Other languages, such as Perl, Python and Java, are usually compiled to CPU-independent instructions. A virtual machine (sometimes known as an interpreter) is required to execute those instructions.
While the central role of a virtual machine is to efficiently execute instructions, it also performs a number of other functions. One of these is to abstract away the details of the hardware and operating system that a program is running on. Once a program has been compiled to run on a virtual machine, it will run on any platform that the VM has been implemented on. VMs may also provide security by allowing more fine-grained limitations to be placed on a program, memory management functionality and support for high level language features (such as objects, data structures, types, subroutines, etc).
Parrot is designed with the needs of dynamically typed languages (such as Perl and Python) in mind, and should be able to run programs written in these languages more efficiently than VMs developed with static languages in mind (JVM, .NET). Parrot is also designed to provide interoperability between languages that compile to it. In theory, you will be able to write a class in Perl, subclass it in Python and then instantiate and use that subclass in a Tcl program.
Historically, Parrot started out as the runtime for Perl 6. Unlike Perl 5, the Perl 6 compiler and runtime (VM) are to be much more clearly separated. The name Parrot was chosen after the 2001 April Fool's Joke which had Perl and Python collaborating on the next version of their languages. The name reflects the intention to build a VM to run not just Perl 6, but also many other languages.
Parrot concepts and jargon
Parrot can currently accept instructions to execute in four forms. PIR (Parrot Intermediate Representation) is designed to be written by people and generated by compilers. It hides away some low-level details, such as the way parameters are passed to functions. PASM (Parrot Assembly) is a level below PIR - it is still human readable/writable and can be generated by a compiler, but the author has to take care of details such as calling conventions and register allocation. PAST (Parrot Abstract Syntax Tree) enables Parrot to accept an abstract syntax tree style input - useful for those writing compilers.
All of the above forms of input are automatically converted inside Parrot to PBC (Parrot Bytecode). This is much like machine code, but understood by the Parrot interpreter. It is not intended to be human-readable or human-writable, but unlike the other forms execution can start immediately, without the need for an assembly phase. Parrot bytecode is platform independent.
The instruction set
The Parrot instruction set includes arithmetic and logical operators, compare and branch/jump (for implementing loops, if...then constructs, etc), finding and storing global and lexical variables, working with classes and objects, calling subroutines and methods along with their parameters, I/O, threads and more.
Registers and fundamental data types
The Parrot VM is register based. This means that, like a hardware CPU, it has a number of fast-access units of storage called registers. There are 4 types of register in Parrot: integers (I), numbers (N), strings (S) and PMCs (P). There are N of each of these, named I0,I1,..N0.., etc. Integer registers are the same size as a word on the machine Parrot is running on and number registers also map to a native floating point type. The amount of registers needed is determined per subroutine at compile-time.
PMC stands for Polymorphic Container. PMCs represent any complex data structure or type, including aggregate data types (arrays, hash tables, etc). A PMC can implement its own behavior for arithmetic, logical and string operations performed on it, allowing for language-specific behavior to be introduced. PMCs can be built in to the Parrot executable or dynamically loaded when they are needed.
Parrot provides garbage collection, meaning that Parrot programs do not need to free memory explicitly; it will be freed when it is no longer in use (that is, no longer referenced) whenever the garbage collector runs.
Obtaining, building and testing Parrot
Where to get Parrot
See http://www.parrot.org/download for several ways to get a recent version of parrot.
The first step to building Parrot is to run the Configure.pl program, which looks at your platform and decides how Parrot should be built. This is done by typing:
Once this is complete, run the
Configure.pl prompts you with. When this completes, you will have a working
Please report any problems that you encounter while building Parrot so the developers can fix them. You can do this by creating a login and opening a new ticket at https://github.com/parrot/parrot/issues/new. Please include the myconfig file that was generated as part of the build process and any errors that you observed.
The Parrot test suite
Parrot has an extensive regression test suite. This can be run by typing:
Substituting make for the name of the make program on your platform. The output will look something like this:
C:\Perl\bin\perl.exe t\harness --gc-debug t\library\*.t t\op\*.t t\pmc\*.t t\run\*.t t\native_pbc\*.t imcc\t\*\*.t t\dynpmc\*.t t\p6rules\*.t t\src\*.t t\perl\*.t t\library\dumper...............ok t\library\getopt_long..........ok ... All tests successful, 4 test and 71 subtests skipped. Files=163, Tests=2719, 192 wallclock secs ( 0.00 cusr + 0.00 csys = 0.00 CPU)
It is possible that a number of tests may fail. If this is a small number, then it is probably little to worry about, especially if you have the latest Parrot sources from the Git repository. However, please do not let this discourage you from reporting test failures, using the same method as described for reporting build problems.
SOME SIMPLE PARROT PROGRAMS
Create a file called hello.pir that contains the following code.
Then run it by typing:
As expected, this will display the text
Hello world! on the console, followed by a new line.
Let's take the program apart.
.sub main states that the instructions that follow make up a subroutine named
main, until a
.end is encountered. The second line contains the
We can modify hello.pir to first store the string
Hello world! in a register and then use that register with the print instruction.
PIR does not allow us to set a register directly. We need to prefix the register name with
$ when referring to a register. The compiler will map $S0 to one of the available string registers, for example S0, and set the value. This example also uses the syntactic sugar provided by the
= is simply a more readable way of using the
To make PIR even more readable, named registers can be used. These are later mapped to real numbered registers.
.local directive indicates that the named register is only needed inside the current subroutine (that is, between
.local is a type. This can be
int (for I registers),
float (for N registers),
string (for S registers),
pmc (for P registers) or the name of a PMC type.
PIR vs. PASM
PASM does not handle register allocation or provide support for named registers. It also does not have the
.end directives, instead replacing them with a label at the start of the instructions.
This example introduces some more instructions and PIR syntax. Lines starting with a
# are comments.
PIR provides a bit of syntactic sugar that makes it look more high level than assembly. For example:
Is just another way of writing the more assembly-ish:
Is the same as:
Is the same as:
As a rule, whenever a Parrot instruction modifies the contents of a register, that will be the first register when writing the instruction in assembly form.
As is usual in assembly languages, loops and selection are implemented in terms of conditional branch statements and labels, as shown above. Assembly programming is one place where using goto is not bad form!
Recursively computing factorial
In this example we define a factorial function and recursively call it to compute factorial.
The first line,
.param int n, specifies that this subroutine takes one integer parameter and that we'd like to refer to the register it was passed in by the name
n for the rest of the sub.
Much of what follows has been seen in previous examples, apart from the line reading:
The last line of PIR actually represents a few lines of PASM. The assembler builds a PMC that describes the signature, including which register the arguments are held in. A similar process happens for providing the registers that the return values should be placed in. Finally, the
factorial sub is invoked.
Right before the
.end of the
factorial sub, a
.return directive is used to specify that the value held in the register named
result is to be copied to the register that the caller is expecting the return value in.
The call to
factorial in main works in just the same was as the recursive call to
factorial within the sub
factorial itself. The only remaining bit of new syntax is the
:main, written after
.sub main. By default, PIR assumes that execution begins with the first sub in the file. This behavior can be changed by marking the sub to start in with
Compiling to PBC
To compile PIR to bytecode, use the
-o flag and specify an output file with the extension .pbc.
parrot -o factorial.pbc factorial.pir
What documentation you read next depends upon what you are looking to do with Parrot. The opcodes reference and built-in PMCs reference are useful to dip into for pretty much everyone. If you intend to write or compile to PIR then there are a number of documents about PIR that are worth a read. For compiler writers, the Compiler FAQ is essential reading. If you want to get involved with Parrot development, the PDDs (Parrot Design Documents) contain some details of the internals of Parrot; a few other documents fill in the gaps. One way of helping Parrot development is to write tests, and there is a document entitled Testing Parrot that will help with this.
The Parrot Mailing List
Much Parrot development and discussion takes place on the parrot-dev mailing list. You can subscribe by filling out the form at http://lists.parrot.org/mailman/listinfo/parrot-dev or read the NNTP archive at http://groups.google.com/group/parrot-dev/.
The Parrot IRC channel is hosted on irc.parrot.org and is named
#parrot. Alternative IRC servers are at irc.pobox.com and irc.rhizomatic.net.