[DRAFT] PDD 5: Opcodes
Abstract
This PDD specifies how the opcode functions should behave and how they are called by the Parrot interpreter.
{{ NOTE: this PDD is only loosely connected to the current state of Parrot. }}
{{ NOTE: standardize on underscores or no underscores? }}
Version
$Revision$
Description
The opcode functions are the workhorse of the Parrot engine. They control program flow and do most of the work in a program. (The rest being done by the variable vtable functions)
Opcode functions have very few limitations or restrictions on them. In particular, opcode functions:
- Can assume a working interpreter
- Must leave all interpreter registers the way they were found, unless the opcode signature indicates otherwise
Each opcode has two separate functions. The first function takes two parameters, the current interpreter pointer and current interpreter PC, and returns the address of the next opcode to execute. The second function takes zero or more parameters as addresses, register numbers, integers, or floating point numbers and optionally returns either the address of the next opcode or the register number holding the address of the next opcode. These are referred to as the wrapping function and the inner function, respectively.
The wrapping function is required, as this is the code that the interpreter will call. Normally this is automatically generated.
The inner function is the code that gets directly executed when parrot gets TIL-ified. If there is no inner function for some reason, then your opcode will likely run slower (as the interpreter would need to set up the registers and other stuff that would normally get stripped away for speed)
The wrapping function
This is the function that the interpreter actually executes. It has all the intimate knowledge of its parameters embedded in it, and is responsible for figuring out what register data it needs and from where.
This function is generally created automatically by opcode_process.pl,
so the programmer doesn't have to create it.
If,
for some reason,
you do need or want to write it (for example if you have no inner function) that's fine.
The inner function
The inner function is the code that actually does the work. This is generally a chunk of C code, though the interpreter will be able to call perl code soon.
Implementation
Prototype declaration of inner function
RETURN function(INPUT[, INPUT[, INPUT...]])
The RETURN type may be one of:
- void Indicates the function returns nothing. The wrapping function will automagically figure out what address to return based on the size of the current opcode.
- void * Indicates the function returns the address of the next opcode to execute.
- I Indicates the function returns the number of the
PMC register that holds address of the next opcode to be execute.The ITEM may be one of:
- IV Indicates the item is an integer
- IV * Indicates the item is a pointer to an integer
- NV Indicates the item is a float
- NV * Indicates the item is a pointer to a float
- STRING Indicates the item is a parrot string pointer
- PMC Indicates the item is a pointer to a PMC
- INT Indicates the item is a pointer to an bigint structure
- NUM Indicates the item is a pointer to a bignum structure
- Ix Indicates the item is an integer register number.
- Nx Indicates the item is a float register number.
- Sx Indicates the item is a string register number.
- Px Indicates the item is a PMC register number.
The function starts with the first open brace, which should generally be on the first non-empty line.
For example:
void addI(Ix out, Ix in1, Ix in2)
{
INTREG(out) = INTREG(in1) + INTREG(in2);
}
is a simple opcode function that corresponds to the addI opcode.
TODO
References
Oploop PDD, PDD 4 (Internal types).