My best introduction to any asm language is just my C compiler putting out asm (gcc -S, I think). I can create small programs to do what I want, and see what the compiler puts out.

https://gcc.godbolt.org is also helpful, it colorizes the assembly output corresponding to the C/C++ source. Furthermore, you can simply change between Intel and AT&T synatx.

The -S flag also works in clang, although the output may differ. For example, last time I checked a comparison check of i < 10 in a for loop would become:

  cmpl    $9, -4(%rbp)
  jle .L3

under GCC ('if i <= 9, loop') and:

  cmpl    $10, -8(%rbp)
  jge .LBB0_4

under clang ('if i >= 10, end the loop'). Neither GCC or clang does an exact conversion, and they both produce different assembly instructions (optimisations disabled in both cases).

If you build a cross-compiler, you can also output assembly for architectures other than your local machine, though this can be quite fiddly (see crosstool-ng for a project which has done most of the work for you).

I wonder if there is a nice and easy to use x86 simulator like the MARS simulator for MIPS [1]? Some place where I can run (step through) little x86 assembly programs study their effect of the each register.

[1] http://courses.missouristate.edu/KenVollmar/MARS/

GDB?

By GDB, you mean the GNU debugger? I have not used it for over a decade. Is it a convenient interface for toying and experimenting with x86 assembler?

See my comment: https://news.ycombinator.com/item?id=11700030