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Re: [Gnash-dev] open source AVM2 VMs

From: Benjamin Wolsey
Subject: Re: [Gnash-dev] open source AVM2 VMs
Date: Thu, 02 Sep 2010 18:13:49 +0200

Am Donnerstag, den 02.09.2010, 09:12 +0200 schrieb strk:
> On Wed, Sep 01, 2010 at 08:29:15PM -0600, Rob Savoye wrote:
> >   So I've spent a little time looking at AVM2 VMs that are also open
> > source. It seems that what we want to do is pass off any swf bytecodes
> > Gnash sees to another VM, and get back something like a display list. As
> > others have looked into this in much more depth than I have, I can't
> > barely wait to be corrected. :-)
> I belive the task of a VM is just to interpret bytecode and invoke
> exterior stuff for any method call or variable get/set.
> Should not have any concept of a "display list" at all.

This is correct: the VM simply interprets ECMAScript/ActionScript 3
bytecode. Tamarin itself implements only the covers the core
ActionScript classes, which include things like Dates, Booleans, XML

It's important to distinguish this core language, which is documented
openly, from the Flashplayer API, which adds a layer on top of
ActionScript and has no open technical documentation. That includes most
AS3 classes, such as BitmapData, NetStream etc, but also the entire
DisplayObject hierarchy.

Those classes are compatible with the VM, in that their methods and
associated objects interact with AS3 bytecode, but their most important
aspect - that they can be displayed - is down to native player functions
and completely separate from the VM. This separation seems much stricter
than in AVM1.

The VM supports static inheritance and runtime type information using
traits and a static prototype chain: this works with arbitrary types, so
the VM doesn't have to know about them.

AVM2 has a different display list from AVM1. It may be possible to
implement them together, but it's almost certainly not worth starting
that way.

Gnash already has a DisplayObject hierarchy that fairly closely
resembles the AVM2 hierarchy. One important question is whether to use
these DisplayObjects in AVM2, or whether to write a new (but effectively
very similar) set of DisplayObjects for AS3.

It's important to note that DisplayObjects are closely linked to
ActionScript: they interact with objects when certain events occur, and
they can be controlled by ActionScript. Because this happens in a
different way in the two VMs, it's not trivial for one class to interact
with both AVM1 and AVM2 objects.

Whatever VM is used, the interface between native objects
(DisplayObjects, BitmapData - see the AVM1 Relay system) and the AS
objects is important, and that's the bit that would make it tricky to
switch between different VMs.


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