It's fundamental in the sense that assembly doesn't translate into anything else. It is the lowest level machine language to give instructions to a processor. All the major tasks are broken down into the tiniest instructions. And they cannot be broken down into smaller pieces. It's like atoms. Atoms are the most fundamental building blocks of the world. And if you want to mix chemicals and do stuff, you probably need a science lab! But you might say, if they are the most fundamental, then why do I need a sciance lab? Why can't I just mix something in my backyard or in my hand? You can, but most chemical engineers don't do it that way.

However, if you want to do it your way, in DOS, there's a program for that. It's called DEBUG.EXE. With this program, you can type your assembly code directly into the memory and run it. But I am not sure if Linux or OSX has a similar program. Most programmers, and I mean 99.99% of people type their assembly program in plain text and save it as a text file and then use an assembler to translate the code to machine language. Then they run the executable and watch it in action. Or they may load a debugger and use it to trace through the program to see how it works.