Opcodes and Runcores

The smallest executable component is not the compilation unit or even the subroutine, but is in fact the opcode. Opcodes in PASM, like opcodes in other assembly languages, are individual instructions that implement low-level operations in Parrot In the world of microprocessors, the word "opcode" typically refers to the numeric identifier for each instructions. The human-readable word used in the associated assembly language is called the "mnemonic". An assembler, among other tasks, is responsble for converting mnemonics into opcodes for execution. In Parrot, instead of referring to an instruction by different names depending on what form it's in, we just call them all "opcodes". Of course the list of things that qualify as "low-level" in Parrot can be pretty advanced compared to the functionality supplied by regular assembly language opcodes.

Before we talk about opcodes, we have to a little bit of talking about the various runcores that invoke them.

Runcores

During execution, the runcore is like the heart of Parrot. The runcore controls calling the various opcodes with the correct data, and making sure that program flow moves properly. Some runcores, such as the precomputed C goto runcore are optimized for speed and don't perform many tasks beyond finding and dispatching opcodes. Other runcores, such as the GC-Debug, debug and profiling runcores help with typical software maintenance and analysis tasks. Different runcores, because of the way they are structured, require the opcodes to be compiled into different forms. Because of this, understanding opcodes first requires an understanding of the Parrot runcores.

Types of Runcores

Parrot has multiple runcores. Some are useful for particular maintenance tasks, some are only available as optimizations in certain compilers, some are intended for general use, and some are just interesing flights of fancy with no practical benefits. One runcore that we've already seen is the debugging runcore which prompts the user for commands between executing each opcode. Another valuable mainteance runcore is the GC dubug core (which runs a full sweep of the garbage collector between each opcode).

=item* Slow Core

The slow core is a basic runcore design that treats each opcode as a separate function at the C level. Each function is called, and returns the address of the next opcode to be called by the core. The slow core performs bounds checking to ensure that the next opcode to be called is properly in bounds. Because of this modular approach where opcodes are treated as separate executable entities many other runcores, especially diagnostic and maintenance cores are based on this design.

=item* Fast Core

The fast core is a bare-bones core that doesn't do any of the bounds-checking or context updating that the slow core does.

=item* Computed Goto Core

Computed Goto is a feature of some C compilers where a label is treated as a piece of data that can be stored in an array. Each opcode is simply a lable in a very large function, and the labels are stored in an array. Calling an opcode is as easy as taking that opcode's number as the index of the label array, and calling the associated label. Sound complicated? It is a little, especially to C programmers who are not used to these kinds of features, and who have been taught that the goto keyword is to be avoided.

As was mentioned earlier, not all compilers support computed goto, which means that this core will not be built on platforms that don't support it.

=item* Precomputed Goto Core

Thought the Computed Goto core was hard enough to understand? Precomputed goto takes the concept a little further.

=item* Tracing Core

=item* Profiling Core

=item* GC Debug Core

=item* Debug Core

Opcodes

Opcodes are the smallest logical execution element in Parrot. An individual opcode corresponds, in an abstract kind of way, with a single machine code instruction for a particular hardware processor architecture. The difference is that Parrot's opcodes can perform some very complex tasks. Also, Parrot's opcodes can be dynamically loaded in from a special library file called a dynop library. We'll talk about dynops a little bit later

Opcode naming

Opcode Multiple Dispatch

Writing Opcodes

Writing Opcodes, like writing PMCs, is done in a C-like language which is later compiled into C code by the opcode compiler. The opcode script represents a thin overlay on top of ordinary C code: All valid C code is valid Opcode script. There are a few neat additions that make writing Opcodes easier.

Opcode Parameters

Opcode Control Flow

The Opcode Compiler

Dynops

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