How is Assembly used in the modern day (with C/C++ for example)?

Question

I understand how a computer works on the basic principles, such as, a program can be written in a \"high\" level language like C#, C and then it\'s broken down in to object code

Answer 1

You need to look upon it from the hardware's point of view, the assembly language is created with regard to what the CPU can do. Every time a new feature in a CPU is created an appropriate assembly instruction is created so that it can be used.

Assembly is thus very dependent on the CPU, the high level languages like C++ provides abstractions from this to allow us to not have to think about the details like CPU instructions as well as the compiler generates optimized assembly code.

EDIT:

How many assembly languages are there? How many work well with other languages?

as many as there are different types of CPU. The second question I didn't understand. Assembly per se is not interacting with any other language, the output, the machine code is.

How would someone go about writing a routine in assembly, and then compiling it in to object/binary code?`

The principle is similar to writing in any other compiled language, you create a text file with the assembly instructions, use an assembler to compile it to machine code. Then link it with eventual runtime libraries.

How would someone then reference the functions/routines within that assembly code from a language like C or C++?

C++ and C provide inline assembly so there is no need to link, but if you want to link you need to create the assembly object following the same/similar calling conventions as the host language. For instance some languages when calling a function push the arguments to the function on the stack in a certain order, so you would have to do the same.

How do we know the code we've written in assembly is the fastest it possibly can be?

Because it is closest to the actual hardware. When you are dealing with higher level languages you don't know what the compiler will do with your for loop. However more often than not they do a good and better job of optimizing the code than a human can do (of course in very special circumstances you can probably get a better result).