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Parrot Calling Conventions




    Maintainer: Dan Sugalski
    Class: Internals
    PDD Number: 03
    Version: 1.3
    Status: Developing
    Last Modified: 2 May 2003
    PDD Format: 1
    Language: English


Version 1.3

2 May 2003

Version 1,2

11 March 2003

Version 1.1

16 September 2002

version 1

None. First version


Version 1.3

No longer use the stack, with overflow going into the array in P3.

Clarified some muddy language.

Version 1.2

Dropped the number of registers passed in and out of subs.

Version 1.1

We now call with a frame, rather than pushing on the stack, and we return frames, rather than returning a stack. We also pass in context information for the return.

Version 1.0

None. First version


This PDD describes Parrot's inter-routine calling conventions.


Please note that the following conventions are only necessary when exposing subs and methods via the generic parrot routine exposure mechanism. What does this mean?

It means the caller needs to follow these conventions only when calling into subs and methods that it has looked up symbolically via parrot's default lookup system. If a language has a lighter-weight calling mechanism that's safe to use in some circumstances, it's perfectly fine to use that.

This means that if you write a C compiler that targets Parrot, for example, you don't need to use parrot's caller-save, full-on calling conventions when one C function calls another if the compiler knows at compile (or possibly link) time what parameters are being passed into the function.

This can potentially save a significant amount of time when dealing with languages that are fully, or nearly fully, bound at compile time, and especially when dealing with languages, such as Forth, that would otherwise spend an inordinate amount of time calling small functions.

If a function isn't exposed at all, it doesn't need to have any way to call into it with the standard calling conventions. It's also perfectly acceptable for there to be two ways to call into a function--one with a language's private calling method, and another that follows the standard conventions.

When this document refers to an "array PMC", used for passing overflow parameters in and out, this means a PMC that can be accessed by integer index and which can return its length. It doesn't need to be of any particular class, it just needs to act like an array. It specifically does not need to be able to extend itself, and may be considered (and actually be) a constant array.

Responsibility for environment preservation

The caller is responsible for preserving any environment it is interested in keeping. This includes any and all registers, lexical scoping and scratchpads, opcode libraries, and so forth.

Use of the savetop opcode is recommended if the caller wishes to save everything, and the restoretop opcode to restore everything savetop saved. This saves off the top 16 of each register type, leaving the bottom 16 registers, which generally contain the return values, intact.

Calling conventions

The following registers are used in calling all subs and methods


Holds the object representing the subroutine.


Holds the return continuation for the caller.


Holds the object the sub was called on. (For method calls)


The overflow parameters. Everything that wouldn't fit in a register is in here. This PMC should act as an array, and belongs to the called sub/function/method. The caller should not assume anything about the state of the PMC passed in here after the call is made.


The fully qualified name of the method or sub being called


True if the sub is being called with prototyped parameters.


The number of items pushed onto the parameter list held in P3. Since the parameter list is all the parameters that won't fit into registers, the count in I1 should reflect only those items held in the list in P3. If there are 14 parameters and 11 of them are in PMC registers (as they should be) then there will be three parameters in the overflow list, and I1 will be set to 3.


The number of parameters in PMC registers.


The return type expected. This is the identifier number for the class. A return type of 0 is void context, -1 is unknown, and -2 and down are the number of expected return variables, negated, minus one. (So -2 means we expect 1 variable, -3 means we expect 2, -4 means we expect 3, and so forth)


Hash value of method or sub name. 0 means no hash value

The following registers, with the exception of P registers, are used only when calling a subroutine for which there is a compile-time prototype. The first 11 PMC parameters may be passed in registers P5 through P15.

I5 through I15

The first 11 integer parameters.

S5 through S15

The first 11 string parameters.

N5 through N15

The first 11 numeric parameters.

P5 through P15

The first 11 PMC parameters.

Overflow parameters go in the array PMC passed in P3. Overflow entries are in there in order, so element 0 is the first overflow parameter, element 1 the second, and so on.

The PMC for a hash, array, or list is passed if one of the entries in the parameter list is a hash, array, or list. The aggregate is not flattened. (Though when accessing the parameters it may be)

Parameters are passed in S, I, and N registers only if the sub's prototype specifically indicates it takes parameters of that type. I0 must be set to 1 if that is the case. If I0 is 0, then the S, I, and N registers can be assumed to be garbage.

Note that it doesn't matter what the order of the integer, string, numeric, and PMC parameters are in the parameter list up until overflow occurs. Once overflow occurs the parameters must be taken in the exact order that they appear in the signature.

Prototyped parameters

A sub or method can be called in two ways--either prototyped or non-prototyped.

A prototyped call means that the caller has an idea of what parameters the sub takes, and has placed its parameters in the appropriate S, I, N, and P registers. I0 will be true in this case.

An unprototyped call means the caller doesn't know what the sub takes, so has stuffed all its parameters into PMCs, and put those PMCs first in the PMC registers with any overflow in the overflow array. I0 will be false in this case.

The sub being called is responsible for runtime checking the parameter types to see if there is a parameter mismatch problem, if it cares. (Often it doesn't) This isn't a replacement for that sort of parameter type checking. What it is, instead, is a means of allowing shortcutting parameter placement checking for the called sub.

For example, assume we have a subroutine with a signature that looks like:

    sub foo(int a, int b, string c, PMC d, float e);

If we were calling without prototyping, all five parameters would be passed in as PMCs, in registers P5 through P9. If, on the other hand, we were calling with prototyping, a would be in I5, b in I6, c in S5, d in P5, and e in N5.

Return conventions

On return from a function, the following registers are set. Return values are placed in I, S, and N registers only if the function is prototyped to do so. It is unlikely that more than one of these registers is set, but you never know. (Perl 6 having fancier return types if nothing else). If a prototyped return is indicated then register I0 must be set to 1. If I0 is set to 0, then this is a non-prototyped return and only the PMC registers and overflow in P3 may be set.

Overflow return values are put in the array PMC in register P3.


True if prototyped return, false otherwise


Holds the number of return values in integer registers.


Holds the number of return values in string registers.


Holds the number of return values in PMC registers.


Holds the number of return values in numeric registers.


Holds the overflow return values in an array PMC


PMC return values


Integer return values


String return values


Numeric return values


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