Adobe AVM2概述：2007年的ActionScript虚拟机技术详解

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Adobe ActionScript Virtual Machine 2 (AVM2) 是Adobe Systems在2007年发布的一个关键的技术文档，它详细介绍了当时Adobe用于支持其ActionScript编程语言的虚拟机（VM）的架构和功能。AVM2是Adobe Flash Player的核心组成部分，为创建交互式网页应用、动画和游戏提供了强大的执行环境。 AVM2概述主要涵盖了以下几个关键知识点： 1. **虚拟机架构**：AVM2的设计旨在高效地解析、编译和执行ActionScript代码。它采用了解释器与即时编译器（JIT，Just-In-Time Compiler）相结合的模式，允许动态代码优化，提升了性能。 2. **执行效率**：AVM2通过引入指令集优化和垃圾回收机制，提高了代码执行速度，同时减少了内存管理中的开销。这使得开发者能够编写出更复杂的交互式应用程序，而无需过多担心性能瓶颈。 3. **安全性**：为了保障用户的安全，AVM2实施了严格的沙箱机制，限制了脚本访问系统资源的能力，仅允许在预定义的安全范围内运行。这有助于防止恶意代码的执行。 4. **兼容性与升级**：AVM2不仅支持ActionScript 2.0和ActionScript 3.0，还随着时间的推移进行了多次迭代和优化，以适应不断发展的编程语言特性。这使得开发者可以利用最新的ActionScript特性，同时保持向后兼容。 5. **版权与许可**：文档强调所有信息受Adobe Systems的版权保护，未经书面许可，任何复制或传播行为都是违法的。Adobe的商标，如Adobe、Adobe Logo和ActionScript，分别代表着公司的品牌和产品的专有权。 6. **法律免责声明**：Adobe Systems对文档内容不承担任何责任，不保证其准确性，并且明确排除所有形式的明示或暗示担保，包括但不限于产品性能和适用性。 AVM2 Overview是开发人员理解和开发基于Flash的应用程序的重要参考，它揭示了如何在Adobe的平台生态系统中实现高性能、安全的ActionScript应用程序。随着技术的发展，AVM2的后续版本可能已经发生了变化，但这份文档的历史价值在于它揭示了Adobe对早期虚拟机设计的思考和实践。

Adobe ActionScript Virtual Machine 2 (AVM2) Overview 16

3.3.4 Execution mechanics

Execution begins with the first instruction of the code in the method_body_info. The address of this first

instruction is 0. The first byte of each instruction is the opcode, which is followed by zero or more bytes of

operands. The instruction may modify the local data areas as well as objects on the heap, and it may create new

objects on the heap either directly or indirectly. (The AVM2 instruction set is defined in Section 2.5.)

Branch and jump instructions add a signed offset to the program counter to effectuate the branch. The base

value for the program counter is the instruction address following the instruction.

3.3.5 Calling and returning

When one of the call instructions is executed, a new local data area is created for the called function. The called

function has no access to the local data area of its caller. The actual parameter values are coerced to the types

expected by the called function as part of the call protocol.

The return instructions transfer a single value from the returning function’s stack, or the implied value

undefined, to the caller. The returned value is coerced to the returning function’s declared return type as part

of the return protocol. The coerced value replaces the operands of the call instruction in the calling method’s

stack, and the called method’s local data area is destroyed.

3.3.6 Exception handling

Exception handlers are defined by a table associated with each method. The table defines a range of bytecode

addresses across which a particular handler is active, the bytecode address of the handler, and a type which is

used to determine whether the handler will handle a particular exception.

When an exception is thrown, the call stack is unwound until a method is found that contains a handler which

covers the current program counter and whose type is a supertype of the type of the object thrown. The

method containing the handler is reactivated and the program counter is set to point to the first address of the

handler. The value and scope stacks in the handling method are cleared before the handler is entered.

finally clause is normally translated as an exception handler that accepts any type of value, which it catches

and rethrows after the body of the

finally block finishes executing.

3.4 Verification

As noted earlier, verification does not happen all at once. Verification is often put off until a datum is actually

needed or until some dependent object has been loaded, so that forward references will be possible.

The following list summarizes some of the verification errors signaled by the AVM2. (Not all verification errors

are listed.)

• There must be no nonzero bits above bit 30 in a u30 value.

• No control flow instruction must cause control to be transferred outside the code vector or into the

middle of another instruction.

• Multiple control flow instructions to the same instruction must have compatible scope stacks, operand

stacks, and register values.

• Named types (for example, in

coerce instructions or in a base class reference) must always be

uniquely resolvable.

• Names may not reference index zero of the name pool unless explicitly specified for that particular

name field.

Adobe ActionScript Virtual Machine 2 (AVM2) Overview 18

4 The ActionScript Byte Code (abc) format

Syntactically complete sections of ActionScript code are processed by a compiler into ActionScript Byte Code

segments. These segments are described by the

abcFile structure, which is defined below. The abcFile

structure is the unit of loading and execution used by the AVM2.

The

abcFile structure describes the interpretation of a block of 8-bit bytes. Despite the name, the contents of

abcFile does not need to be read from a file in the file system; it can be generated dynamically by a run-

time compiler or other tools. The use of the word “file” is historical.

The

abcFile structure comprises primitive data, structured data, and arrays of primitive and structured data.

The following sections describe all the data formats.

Primitive data include integers and floating-point numbers encoded in various ways.

Structured data, including the

abcFile itself, are presented here using a C-like structural notation, with

individual named fields. Fields within this structure are in reality just sequences of bytes that are interpreted

according to their type. The fields are stored sequentially without any padding or alignment.

4.1 Primitive data types

Multi-byte primitive data are stored in little-endian order (less significant bytes precede more significant bytes).

Negative integers are represented using two’s complement.

 The type u8 represents a one-byte unsigned integer value.

 The type u16 represents a two-byte unsigned integer value.

 The type s24 represents a three-byte signed integer value.

 The type u30 represents a variable-length encoded 30-bit unsigned integer value.

 The types u32 and s32 represent variable-length encoded 32-bit unsigned and signed integer values

respectively.

 The type d64 defines an 8-byte IEEE-754 floating point value. The high byte of the double value

contains the sign and upper bits of the exponent, and the low byte contains the least significant bits of

the significand.

The variable-length encoding for u30, u32, and s32 uses one to five bytes, depending on the magnitude of the

value encoded. Each byte contributes its low seven bits to the value. If the high (eighth) bit of a byte is set,

then the next byte of the

abcFile is also part of the value. In the case of s32, sign extension is applied: the

seventh bit of the last byte of the encoding is propagated to fill out the 32 bits of the decoded value.

Adobe ActionScript Virtual Machine 2 (AVM2) Overview 19

4.2 abcFile

abcFile

{

u16 minor_version

u16 major_version

cpool_info constant_pool

u30 method_count

method_info method[method_count]

u30 metadata_count

metadata_info metadata[metadata_count]

u30 class_count

instance_info instance[class_count]

class_info class[class_count]

u30 script_count

script_info script[script_count]

u30 method_body_count

method_body_info method_body[method_body_count]

}

The abcFile structure describes an executable code block with all its constant data, type descriptors, code, and

metadata. It comprises the following fields.

minor_version, major_version

The values of major_version and minor_version are the major and minor version numbers of the

abcFile format. A change in the minor version number signifies a change in the file format that is

backward compatible, in the sense that an implementation of the AVM2 can still make use of a file of an

older version. A change in the major version number denotes an incompatible adjustment to the file

format.

As of the publication of this overview, the major version is 46 and the minor version is 16.

constant_pool

The constant_pool is a variable length structure composed of integers, doubles, strings, namespaces,

namespace sets, and multinames. These constants are referenced from other parts of the

abcFile

structure.

method_count, method

The value of method_count is the number of entries in the method array. Each entry in the method array

is a variable length

method_info structure. The array holds information about every method defined in

this

abcFile. The code for method bodies is held separately in the method_body array (see below).

Some entries in

method may have no body—this is the case for native methods, for example.

metadata_count, metadata

The value of metadata_count is the number of entries in the metadata array. Each metadata entry is a

metadata_info structure that maps a name to a set of string values.

class_count, instance, class

The value of class_count is the number of entries in the instance and class arrays.

Each

instance entry is a variable length instance_info structure which specifies the characteristics of

object instances created by a particular class.

Each

class entry defines the characteristics of a class. It is used in conjunction with the instance field to

derive a full description of an AS Class.

script_count, script

The value of script_count is the number of entries in the script array. Each script entry is a

script_info structure that defines the characteristics of a single script in this file. As explained in the

previous chapter, the last entry in this array is the entry point for execution in the

abcFile.

Adobe ActionScript Virtual Machine 2 (AVM2) Overview 20

method_body_count, method_body

The value of method_body_count is the number of entries in the method_body array. Each method_body

entry consists of a variable length

method_body_info structure which contains the instructions for an

individual method or function.

4.3 Constant pool

The constant pool is a block of array-based entries that reflect the constants used by all methods. Each of the

count entries (for example,

int_count) must be one more than the number of entries in the corresponding

array, and the first entry in the array is element “1”. For all constant pools, the index “0” has a special meaning,

typically a sensible default value. For example, the “0” entry is used to represent the empty sting (

""), the any

namespace, or the any type (

*) depending on the context it is used in. When “0” has a special meaning it is

described in the text below.

cpool_info

{

u30 int_count

s32 integer[int_count]

u30 uint_count

u32 uinteger[uint_count]

u30 double_count

d64 double[double_count]

u30 string_count

string_info string[string_count]

u30 namespace_count

namespace_info namespace[namespace_count]

u30 ns_set_count

ns_set_info ns_set[ns_set_count]

u30 multiname_count

multiname_info multiname[multiname_count]

}

If there is more than one entry in one of these arrays for the same entity, such as a name, the AVM may or may

not consider those two entries to mean the same thing. The AVM currently guarantees that names flagged as

belonging to the “private” namespace are treated as unique.

int_count, integer

The value of int_count is the number of entries in the integer array, plus one. The integer array

holds integer constants referenced by the bytecode. The “0” entry of the

integer array is not present in

the

abcFile; it represents the zero value for the purposes of providing values for optional parameters and

field initialization.

uint_count, uinteger

The value of uint_count is the number of entries in the uinteger array, plus one. The uinteger array

holds unsigned integer constants referenced by the bytecode. The “0” entry of the

uinteger array is not

present in the

abcFile; it represents the zero value for the purposes of providing values for optional

parameters and field initialization.

double_count, double

The value of double_count is the number of entries in the double array, plus one. The double array

holds IEEE double-precision floating point constants referenced by the bytecode. The “0” entry of the

double array is not present in the abcFile; it represents the NaN (Not-a-Number) value for the purposes

of providing values for optional parameters and field initialization.

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