.SWF File Format – ABC file

DoABC (82, SWF 9)

Flex 3 applications write flags 1 and names such as frame1, one ABC a frame; a Flex debug build writes one DoABC a class, named by its path, such as mx/core/IChildList; other tools write flags 0 and an empty name. The code runs only when FileAttributes sets the ActionScript3 flag. SymbolClass links characters to the classes, character 0 being the document class.

Tag 72 (DoABCDefine, not in the specification)

The ABC file with no flags and no name: the whole body is the ABC. The specification documents only tag 82. That tag 72 came from Flash Player 9’s pre-release tools is an inference from its absence there; JPEXS calls 72 DoABC and 82 DoABC2. No movie examined for this site has one.

Numbers in ABC

FF 81 FC 07 as an s32: 0x7F + 0x01 × 2⁷ + 0x7C × 2¹⁴ + 0x07 × 2²¹ = 16,711,935, which is 0xFF00FF.

abcFile

A major version of 47 with minors 12 to 14 is reported for a float extension that never shipped in released players; that is not confirmed.

Constant pool

Seven pools, in this order, each a u30 count and then the entries. Every count is the number of entries plus one: entry 0 is implied and not stored, and a count of 0 or 1 means the pool is empty.

method_info

An option’s kind, and a slot’s vkind, says which pool its value index is in:

metadata_info

The overview draws the items as key and value pairs; the compilers, Tamarin and RABCDAsm write all the keys and then all the values, and the explorer reads them that way. The size is the same either way. Only metadata the compiler is told to keep is written: Flex keeps Bindable, Event and a few others.

instance_info

class_info

Class i’s class_info goes with instance_info i.

script_info

A script’s initialiser runs when a name it defines is first needed; the last script in the file runs at once.

method_body_info

Native methods and interface methods have no body.

exception_info

traits_info

The attributes are Final 0x1, Override 0x2 and Metadata 0x4. A slot_id or disp_id of 0 lets the AVM2 choose one. A slot’s vkind is one of the constant kinds.

A worked example

An uncompressed SWF 14 movie, a Flex 4.6 debug build. Its first tags are FileAttributes (44 11 19 00 00 00: HasMetadata, ActionScript3 and UseNetwork), Metadata, EnableDebugger2, DebugID, ScriptLimits, SetBackgroundColor, ProductInfo and FrameLabel. The first DoABC follows at 0x265:

00000265  BF 14 E3 07 00 00 01 00 00 00 6D 78 2F 63 6F 72
00000275  65 2F 49 46 6C 65 78 4D 6F 64 75 6C 65 46 61 63
00000285  74 6F 72 79 00 10 00 2E 00 00 00 00 47 00 4A 45
00000295  3A 5C 64 65 76 5C …

What an ActionScript 3 class becomes

package com.example {
  import flash.display.Sprite;
  public class Main extends Sprite {
    public static var count:int = 0;
    private var b:int;
    public function Main() { super(); }
    public function go(x:int):void { b = x; }
  }
}

The Flex compiler and Flash CS3 and later both write:

• Multinames QName(package com.example, "Main") and QName(package flash.display, "Sprite"), and QNames for count (PackageNamespace “”), b (PrivateNs) and go.
• An instance_info: name Main, super_name Sprite, flags ClassSealed and ClassProtectedNs with a ProtectedNamespace “com.example:Main”, iinit the constructor, and the traits Slot b:int and Method go.
• A class_info at the same index: cinit, the static initialiser, and the trait Slot count:int, with its default as vindex and vkind, or set in cinit’s code instead.
• A method_info for the constructor, cinit, go (param_count 1, param_type int, return_type void) and the script initialiser, and a method_body_info for each. A constructor body usually starts getlocal_0; pushscope; getlocal_0; constructsuper 0.
• A script_info whose traits hold Class Main, and whose init method builds the class: it pushes the scope of each base class in turn (getscopeobject 0, getlex Object, pushscope, getlex flash.events.EventDispatcher, pushscope …), then getlex Sprite, newclass, the popscopes and initproperty Main.

A movie compiled by Flash CS3 or later with timeline code also has a class for each timeline, such as MainTimeline, with each frame’s code as a method frame1, frame2 … registered with addFrameScript in the constructor. See also ActionScript 3.0 compiled bytecode.