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Introduction To Dynamic Recompilation
Author: zenogais Difficulty Level: Advanced


    Dynamic recompilation, while certainly not a new concept as evidenced by the number of emulators which successfully employ it, is relatively difficult for many beginners to grasp and then later successfully implemented. This tutorial is an attempt by myself to lay down the basics of dynamic recompilation so that any who look at this tutorial will be able to understand somewhat how it works.
    Dynamic recompilation is the name for an emulation technique in which programs written for a completely different CPU architecture (from hereout called the "target architecture") are decoded and made to run on the current CPU architecture (from hereout called the "client architecture"). This is done by decoding individual CPU opcodes for the target architecture CPU and then translating them into their equivalent opcode or set of opcodes for the client architecture CPU, of course its a bit more complex than this, but explaining the specifics of why is beyond the scope of this tutorial. In order to get the most out of this technique it is quite necessary for one to have a very decent understanding of assembly language so that they can choose the most efficient representation for the client architecture.
    Firstly, there are three basic components of a recompiler, they are as follows: 

  • The code generation buffer - this buffer contains all the binary code generated by the emitters.
  • The opcode emitter functions - these emit various client architecture opcodes to the code generation buffer.
  • The target architecture opcode translation functions - these functions translate CPU opcodes into client architecture opcodes by passing data to the opcode emitter functions.
     As you can see these functions form a somewhat layered architecture, each one relying on the layer before itself, this relationship can be illustrated by the following image:


    This creates a sort of "trickle up" architecture. The layer at the top being the simplest, but also the most vital to the success of the system. What this should mean to you is that if you cannot allocate the buffer but your emulator is running in dynamic recompilation mode then you should throw a fatal error and take the whole system down, because as with any critical component if it doesn't work then the system can't function. Now that I've explained the basic workings of a dynamic recompiler to you, its time that you check out some source code containing the basic implementation details of a dynamic recompiler class. Firstly though I'd like to explain that this tutorial will only be covering the code generation buffer and opcode emitter function components of a dynamic recompiler, the last part is emulator specific and thus does not belong in a document introducing the topic. Now here is the source code:

enum ASSEMBLER_HELP_VALUES
{
	SIB    = 4,  ///< SIB Value Passed To SibSB
	DISP32 = 5   ///< DISP32 Value Passed To ModRM
};
	
typedef enum
{
	EAX = 0,
	EBX = 3,
	ECX = 1,
	EDX = 2,
	ESI = 6,
	EDI = 7,
	EBP = 5,
	ESP = 4 
} X86RegisterType;

///////////////////////////////////////////////////////////////////////////////////////////////////

class X86Emitter
{
public:
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Constructors
	
	/** The default constructor.
	  */
	X86Emitter(unsigned long bufferSize = 256);
	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Destructors

	/** The default destructor.
	  */
	~X86Emitter();
	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Block Execution
	
	/** This function will execute the block in memory.
	  */
	void ExecuteBlock();
	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Move Opcode Emitters
	
	/** Move 16 bits of data from memory into an x86 CPU register.
	  */
	void Mov16RtoM(unsigned int to, X86RegisterType from);
	/** Move 16 bits of data from an x86 CPU register to memory.
	  */
	void Mov16MtoR(X86RegisterType to, unsigned int from);
	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Add Opcode Emitters
	
	/** Adds an immediate value to an x86 CPU register.
	  */
	void Add32ItoR(X86RegisterType to, unsigned int from);
	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Return Opcode Emitter
	
	/** This opcode must end every recompiled block.
	  */
	void Ret();
protected:	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Emitter Functions
	
	/** Emits 8 bits of data to our code generation buffer.
	  */
	void EmitByte(unsigned char byte);
	/** Emits 16 bits of data to our code generation buffer.
	  */
	void EmitWord(unsigned short word);
	/** Emits 32 bits of data to our code generation buffer.
	  */
	void EmitDword(unsigned long dword);
	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Emitter Modifier Functions
	
	/** standard opcode emitter helper.
	  */
	void ModRM(unsigned char mod, unsigned char rm, X86RegisterType register);
	/** standard opcode emitter helper.
	  */
	void SibSB(unsigned char sib, unsigned char rm, unsigned char index);
	
	////////////////////////////////////////////////////////////////////////////////////////////////
	// Variables
	
	unsigned char* mBuffer;        ///< The pointer to the allocated code generation buffer.
	unsigned long  mBufferSize;    ///< The size of the buffer in bytes.
	unsigned long  mBufferPointer; ///< The current index into the allocated buffer.
};

    As you can no doubt tell writing a dynamic recompiler, even a simple one like the one we're creating takes alot of work. This is a class that is basic enough to allow us to write values to a CPU register and add to them then read them back, but even this requires a hefty amount of code. In the constructor of our class we will be allocating our code generation buffer, the default size for this buffer will be 256 bytes, this should be enough to encapsulate everything that we will need for our simple demo. Our simple demo should result in output similar to the following:

 

Moving value 14 to register EAX...
Adding value 14 to register EAX...
Expected Result :: 28
Result Found :: 28

    This is about as simple as dynamic recompilation programs will ever get. This program is merely designed to test our dynamic recompiler implementation and to ensure that we don't have any nasty errors hanging about in the basics of our dynamic recompiler. All the names of the functions for our class should be familiar to you if you've met the prerequisite of being familiar with assembly language, as they are very standard assembly language functions. For those who don't know though, the MOV opcode on x86 CPUs moves a value into one of the CPU registers (i.e. EAX, EBX, ECX, etc..) and the ADD opcode on x86 CPUs adds an immediate value to one of the CPU registers, an immediate value being one not read from memory or another register. All the implementation details of these opcodes, including the emitted hexcode values I found in two documents (The Nasm Manual and Intel P4 Processor Manuals). Basically implementing opcodes requires research and accuracy, and it can often be a slow and somewhat boring process. Since I'm writing this tutorial though I've done the hard work for you, so here's the implementation for of this simple emitter class:

					
X86Emitter::X86Emitter(unsigned long bufferSize)
: mBuffer( NULL ),
  mBufferSize( bufferSize ),
  mBufferPointer( 0 ),
{
	mBuffer = new unsigned char[bufferSize];
}

///////////////////////////////////////////////////////////////////////////////////////////////////

X86Emitter::~X86Emitter()
{
	if(mBuffer != NULL)
	{
		delete [] mBuffer;
		mBuffer = NULL;
	}
}

///////////////////////////////////////////////////////////////////////////////////////////////////

void X86Emitter::ExecuteBlock()
{
	if(mBlockPointer > 0)
	{
		void (*BlockFunction)() = (void(*)())&mBlock;
		BlockFunction();
	}
}

///////////////////////////////////////////////////////////////////////////////////////////////////

void X86Emitter::Mov16RtoM(unsigned int to, X86RegisterType from)
{
	EmitByte( 0x66 );
	EmitByte( 0x89 );
	ModRM(0, from, DISP32);
	EmitDword( from );
}

void X86Emitter::Mov16MtoR(X86RegisterType to, unsigned int from)
{
	EmitByte( 0x66 );
	EmitByte( 0x8B );
	ModRM(0, to, DISP32);
	EmitDword( from );
}

///////////////////////////////////////////////////////////////////////////////////////////////////

void X86Emitter::Add32ItoR(X86RegisterType to, unsigned int from)
{
	if(to == EAX)
	{
		EmitByte( 0x05 );
	} else {
		EmitByte( 0x81 );
		ModRM(3, 0, to);
	}
	
	EmitDword( from );
}

///////////////////////////////////////////////////////////////////////////////////////////////////

void X86Emitter::Ret()
{
	EmitByte( 0xC3 );
}

///////////////////////////////////////////////////////////////////////////////////////////////////

void X86Emitter::EmitByte(unsigned char byte)
{
	mBuffer[ mBufferPointer++ ] = byte;
}

void X86Emitter::EmitWord(unsigned short word)
{
	*((unsigned short*)(mBuffer + mBufferPointer)) = word;
	mBufferPointer += 2;
}

void X86Emitter::EmitDword(unsigned long dword)
{
	*((unsigned long*)(mBuffer + mBufferPointer)) = dword;
	mBufferPointer += 4;
}

///////////////////////////////////////////////////////////////////////////////////////////////////

void X86Emitter::ModRM(unsigned char mod, unsigned char rm, X86RegisterType register)
{
	EmitByte((mod << 6) | (rm << 4 ) | (register));
}

void X86Emitter::SibSB(unsigned char sib, unsigned char rm, unsigned char index)
{
	EmitByte(sib << 6) | (rm << 4) | (index));
}

    Now, that wasn't too hard. Hopefully that doesn't look too scary to you, and if it does maybe you should go look it over one more time and make sure you understand what's going on. Basically as you can see the heart of the system revolves around the emitters which simply write different sized chunks of data to our buffer and increment the internal buffer pointer. But now that you've got that big nasty class down, why not write a simple little program to show you what its all about, we'll do the example program whose output I showed above, and here it is:

					
#include "X86Emitter.hpp"
#include <iostream>
using namespace std;

int main(int argc, char** argv)
{
	X86Emitter CodeEmitter;
	
	// This is the code to do the moving of the value and the adding.
	cout<<"Moving Value 14 To Register EAX..."<<endl;
	CodeEmitter.MovMtoR(EAX, (unsigned int)&value);
	cout<<"Adding Value 14 To Register EAX..."<<endl;
	CodeEmitter.AddItoR(EAX, value);
	cout<<"Expected Result :: 28"<<endl;
	
	// Now we move the value back into our variable.
	CodeEmitter.Mov16RtoM((unsigned int)&value, EAX);
	CodeEmitter.Ret();
	
	// Now We Run Our Program And Check The Output.
	CodeEmitter.ExecuteBlock();
	
	cout<<"Result Found :: "<< value <<endl;
	return EXIT_SUCCESS;
}


    If everthing went well then we should get the output shown above. If something went wrong then its probably time to go back and debug. Anyway, I hope that I have made the topic of dynamic recompilation at least a little less scary through this tutorial, and hopefully I have show you that, while its not simple to make one, they aren't necessarily that difficult either.

Credits: I give major credits for this document to GoldFinger the author of GoldRec, some of the source code in here is borrowed from his dynamic recompiler which I think is an excellent one, and certainly something to look out for.