Basic IL

What is interesting about this simple example is that it illustrates all of the major characteristics of the assembler.

Assembler directives begin with a dot and the first directive:

.assembly extern mscorlib {}

informs the assembler that we are going to be using objects and methods within the  mscorlib assembly, i.e. the console class and its WriteLine method.

Notice that already we have objects and the .NET Framework involved in our assembler.

The next two directives simply give the program that we are creating an assembly and module name. Without these the assembler and the runtime don’t really know what to do with our program – declaring it to be an assembly means that it can be run.

.assembly Hello {}
.module Hello.exe

The next line declares a class and states that it inherits from Object

.class Hello.Program
extends [mscorlib]System.Object
{

We then define a CIL managed static method:

.method static void Main(string[] args)
cil managed
{

All more evidence of object orientation and use of the Framework.

The entrypoint directive marks where the program should be started from and every runnable program has to have one:

.entrypoint

Finally we get to some IL instructions, and it’s all over very quickly!

 ldstr "Hello World"
 call void [mscorlib]System.Console::WriteLine(string)
 ret
 }
}

The ldstr, i.e. LoadString, instruction loads the string “Hello World” onto the stack. The call instruction calls the WriteLine method of the static Console class.

The method picks up its parameters from the stack and what looks like a parameter definition, i.e. (string), is a type definition that says that the top of stack item is to be a string. The void return type means that the method doesn’t leave a return value on the stack. The ret, or Return, completes the method and our program.

As you can see even this simple example demonstrates how stack- and object-oriented the language is and how it uses both typing and the Framework. 

The stack

Of all of the features of IL, the one that high level language programmers tend to find strange is the central role the stack plays.

If you need a refresher on what a stack is, how it works and what it is used for then see - Stacks and Stack and Heap Memory Allocation..

In this case the stack is a little more sophisticated than a simple block of memory with a pointer. You need to think of it in terms of a strongly typed stack made up of “slots” that hold a complete data type.

When you push data onto and pop data off the stack it always works in terms of a complete data type. Nearly all IL instructions work by popping input data from the stack and pushing their results on the stack.

As an example, let’s add two numbers together.

The first instruction is ldc or LoaD Constant

ldc.i4 0x01

This pushes the 4-byte integer constant, i.e. an Int32, onto the stack. The ldc part of the instruction tells you what is to happen and the code after the dot tells you the data type - i4 is a four byte integer in this case. There are ldc instructions for all the standard data types - for example ldc.r8 pushes a float32 on the stack.

To perform an add we need two values on the stack so we need to push another int32 and then give the add command:

ldc.i4 0x02
add

The add command takes the top two items on the stack adds them together and pushes the result back on the top of the stack.

Now we have the result of adding 1 and 2, i.e. 3, on the top of the stack and we can use WriteLine again to display the result and issue a return to complete the job:

call void [mscorlib]System.Console::WriteLine(int32)
ret

Notice that everything is still strongly typed and the add instruction can discover the type of the two items on the top of the stack and push an appropriate type back on the stack – you can try the same with floating point numbers:

 ldc.r4 0.1
 ldc.r4 0.2
 add
 call void [mscorlib]System.Console::WriteLine(float32)
 ret

The range of primitive data types available to you is similar to those in C# or VB with some changes to the names used.