Assembly code question
When I had my Windows laptop, I downloaded FASM and out of curiosity, opened one of the DLLs that came along with it. I noticed it had a long list of what looked like system variables or constants and noticed that if I put the value of one of these in once of the sample programs, I could get the machine to do different things. Eg, one of them popped open the disc drive, another played a Windows sound, another one would change the value of the keys on the keyboard etc.
Now I'm on Linux, I use NASM, but I would like to know how I can do the same things as mentioned above and what this activity is called. I thought it would be called system calls, but that seems to be something else.