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Learning ILAsm, the backbone of .NET

Who cares about ILAsm or x86 assembly language anyway? I'm sure a lot of you are wondering why anyone would care about learning ILAsm.  It's not like you ever see it unless you disassemble an application.  ILasm or MSIL is the human readable translation of Microsoft .NET intermediate language.  ILAsm is a lot like classic assembly language.  It is a low level programming language that allows you to write programs one instruction at a time with a very minimal syntax.  I've explained the benefits of learning assembly language in my previous post, Why Learn Assembly Language.  In a nutshell, if you learn .NET at the low level of IL, you will have an understanding of what makes any .NET language tick.  You will have the knowledge to disassemble any .NET binary and debug your software at the instruction level.   This post is my first tutorial on writing code in ILAsm.  I hope you'll join me and become proficient in this language of kings.  You will have an edge over the competition and it will change the way you look at high level coding.  Enough talking, let us code! How to compile ILasm If you are a .NET developer, you most likely have an ILasm compiler on your computer which by the way is appropriately named ILasm.exe.  Simply launch the Visual Studio Command Prompt, navigate to the folder that you create your IL files and issue the following command. ilasm.exe is fine if you like to write code in notepad and drop out to a command prompt to write your code.  I myself prefer an IDE.  Visual Studio is an amazing IDE but for some reason the people over at Microsoft didn't see it fit to add support for ILasm into the product.  Never fear!  There is a free, open source alternative that is nearly identical to Visual Studio and it allows you to create and compile IL projects with syntax highlighting that works on Linux, Windows, and Mac OSX!  What is this application you ask? MonoDevelop   "Hello... World?" I know, I know, it's a tired, worn out cliche but far be it from me to interrupt the order of the programming gods and illustrate a programming language without starting with the infamous "Hello World!" example. //import the mscorlib assembly to give us access to Console and other basic classes .assembly extern mscorlib{} //define our assembly .assembly HelloAssembly { //define the version of this assembly .ver 1:0:0:0 } //define the executable module .module helloworld.exe //defin our main method .method static void main() cil managed { //set up the stack. In ILAsm, all values are placed on the stack and then manipulated. //here we will allocate memory for one value to be on the stack at a given time .maxstack 1 //define the main entry point to the application .entrypoint //load the emphamis phrase on the stack ldstr "Hello ILAsm!" //print the string from the stack to the console call void [mscorlib]System.Console::WriteLine(string) //return to end the program ret } If you have never seen asm or ILasm, I can imagine how strange this code snippet may look.  As I've stated before, ILasm is a very cryptic, low level language.  Let's breakdown the application. We start by importing the mscorlib library which contains much of the base .NET classes.  As the comments state, this library gives us access to the System.Console object.   Next, we define the assembly for our program.  All .NET executables are called assemblies.  Here we name our assembly as well as set a version number.  After we define our assembly, we define the executable module.  This is required in any ILasm application.   Now it's time to define our main method that performs the loading and printing of the string.  We start by defining the maxstack, that is, the maximum number of values that can be held in memory at a given time.  In ILAsm, you push values on the stack, perform operations on the values or use them as parameters to methods.  Since we have a maxstack of one, that means we can have only one piece of data to work with at any given time.  We use ldstr to load a string onto the stack.  If we were to load another string on the stack directly after the first ldstr call, the application will simply push the first value out of memory and the new string will be available to access. Finally, we call the WriteLine method on the System.Console object and we tell it to use the current string on the stack as it's input source.   So now you can load a string onto the stack and display it.  It's pretty interesting, although very limited as well.  How about we work with more than one value?  Let's try adding two numbers! Sum it up //reference to mscorlib .assembly extern mscorlib {} //define our assembly .assembly MiniCalculator { //the assembly version number .ver 1:0:0:0 } //create the required module .module MiniCalculator.exe //define our main method .method static void main() cil managed { //we plan to work with two integers this time .maxstack 2 //the main entry point to our application .entrypoint //load a string of instructions on the stack ldstr "OK. Class is in session. Who can tell the class what is the sum of 2 + 2? That's right, the answer is " //display the instructions to the user call void [mscorlib]System.Console::Write (string) //put the number 2 on the stack. Currently the previously loaded string, and the //number 2 are both on the stack. ldc.i4 2 //when we move another integer to the stack, this pushes the string off //now we have two instances of the number 2 on the stack ldc.i4 2 //add will add the two numbers on the stack and store the result add //lets tell the computer to look for an int on the stack and print it to the console call void [mscorlib]System.Console::Write (int32) //return to exit the application ret } We start the application as before by importing mscorlib, defining our assembly and module, and creating the method to perform our work.  We then load a string on the stack and use Console's WriteLine method to display the string.  We then load two integers onto the stack, pushing the string off of the stack.  We call add whichs adds the two integers and stores the result on the stack.  We use Console's WriteLine once again to display the answer. This concludes part one of my ILasm tutorial series. Please check back soon for my next installment in which we will tackle data types, loops, and classes! Until next time.. ~/Buddy James kick it on DotNetKicks.com  


Why Learn Assembly Language?

Here is my post from http://www.codeproject.com/Articles/89460/Why-Learn-Assembly-Language "Assembly language? Isn't that the hard to read instructions on how to assemble your brand new computer desk?"... No.. What is Assembly Language? x86 Assembly is a programming language for the x86 class of processors (specifically the 32bit x86 processors IA-32 - http://en.wikipedia.org/wiki/IA-32). The instruction set defined by the IA-32 architecture is targeted towards the family of microprocessors installed in the vast majority of personal computers on the planet. Assemblylanguage is machine specific and considered a "low level" language. This means that the code and syntax is much closer to the computer's processor, memory, and I/O system. A high level language is designed with keywords, libraries, and a syntax that introduces a high level of abstraction between the language and the hardware. Background I thought assembly was a dead language, why waste the time? Though it's true, you probably won't find yourself writing your next customer's app in assembly, there is still much to gain from learning assembly. Today, assembly language is used primarily for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level embedded systems, and real-time systems (EDIT:Thanks Trollslayer). The fact of the matter is, the more complex high level languages become, and the more ADT (abstract data types) that are written, the more overhead is incurred to support these options. In the instances of .NET, perhaps bloated MSIL. Imagine if you knew MSIL. This is where assembly language shines. (EDIT)Assembly language is as close to the processor as you can get as a programmer so a well designed algorithm is blazing -- assembly is great for speed optimization. It's all about performance and efficiency. Assembly language gives you complete control over the system's resources. Much like an assembly line, you write code to push single values into registers, deal with memory addresses directly to retrieve values or pointers. To write in assembly is to understand exactly how the processor and memory work together to "make things happen". Be warned, assembly language is cryptic, and the applications source code size is much much larger than that of a high-level language. But make no mistake about it, if you are willing to put in the time and the effort to master assembly, you will get better, and you will become a stand out in the field. So why should you care?     Points of Interest Wirth's Law I remember dialling into a BBS on my 486 with my brand new 2400bps modem. Fast-forward 14 years and now we are only limited by our imagination. With all of these amazing technological breakthroughs, there is a glaring anomaly; a paradox. This is referred to as Wirth's law. Wirth's law states that software is getting slower more rapidly than hardware becomes faster. There's no one reason why this is the case, but I'd like to think that the further we as developers drift away from the lower level details of software development, we write less than stellar (inefficient code). Hold the phone! I'm not calling anyone stupid. It's just that these new languages and supercharged processors have abstracted us so far from the machine, that we no longer have to be concerned with things like garbage collection, variable initialization, memory address pointers, etc. All of these features and more are now standard in today's languages/runtimes/IDEs. The result is a new breed of developers that rely on superior hardware power for performance rather than striving to write concise, cohesive, efficient code. My Eyes are Open! I realize now that learning assembly language will teach me about the inner workings of the computer. I'll learnhow the CPU/CPU registers work with memory addresses to achieve the end result one instruction at a time. This doesn't mean that I'm going to begin coding everything in assembly, however, I will learn which data types to use and when. I'll learn how to write smaller, faster, more efficient routines. I will understand software development at a level that most of my peers don't. I'm even opening up to the possibility of looking into writing my own compiler. So if you are serious about getting a leg up on the competition in your field, I'd recommend trying to learnassembly language. Resources on Learning Assembly How To Use Debug - http://www.codeproject.com/KB/miscctrl/Debug.aspx Introduction to x86 Assembly Language http://www.c-jump.com/CIS77/ASM/Assembly/lecture.html kick it on DotNetKicks.com  


About the author

My name is Buddy James.  I'm a Microsoft Certified Solutions Developer from the Nashville, TN area.  I'm a Software Engineer, an author, a blogger (http://www.refactorthis.net), a mentor, a thought leader, a technologist, a data scientist, and a husband.  I enjoy working with design patterns, data mining, c#, WPF, Silverlight, WinRT, XAML, ASP.NET, python, CouchDB, RavenDB, Hadoop, Android(MonoDroid), iOS (MonoTouch), and Machine Learning. I love technology and I love to develop software, collect data, analyze the data, and learn from the data.  When I'm not coding,  I'm determined to make a difference in the world by using data and machine learning techniques. (follow me at @budbjames).  

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