...I want to learn assembler, in order to understand how the machine works, then indeed, to make a little C stuff ! it's the main goal...

Some well commented assembler source code will provide you with much of what you want to know.

Believe it or not, before I could afford a computer, I used to write Z80 assembler programs on paper. I made a little chart showing all of the registers, then I would manually execute the program using pencil and eraser.

When I took my assembler class, the first programs we wrote were in machine language. All input was via front panel switches, output was by front panel lights. We would load the program into memory one instruction at a time, and then step to the next memory location.

You only need to do a little of this to get the idea of how a cpu works.