Orange Juice Liberation Front

I’ve recently been looking into assembler coding a little. I learned assembler theory back in High School in Mr. Trapp’s computer programming elective, and later learned a bit of 68000 assembler as well, but never got round to actually getting into it when the PPC arrived on the scene. So, when I recently heard at work how one can get a whole bunch of Intel reference books for free, I thought this might be a good opportunity to learn x86 assembler. After all, I’m a parser and compiler geek, it’s kind of a gap in my skill set if I can’t do the backend.

Now, trouble is, while there are many tutorials for Linux and Windows, I couldn’t find a single one for Mac OS X. So, I started googling, assembling C code and bothering some developers I know and others on mailing lists with my questions, and I thought I’d share my first findings:

• I got a link to Apple’s Mac OS X ABI docs. This is really good, as it documents an important part on OS X in detail: How to align the stack (on 16 bytes, no matter what Intel’s docs tell you), and how to call your own functions.
• Aforementioned 16-byte stack alignment is not always necessary, but when you call a function, you must give it a properly aligned stack. When you are called, however, the stack will have the return address on it, which is 4 bytes. So, after you push the base pointer on the stack (4 more bytes), you have to move the stack pointer by another 8 bytes at least to make it aligned on a 16-byte boundary again.
• A nice way to learn assembler is by writing very simple C programs and using gcc -S my_simple_c_program.c to get it translated into assembler code. Note that by simple, I recommend you start out with stuff that doesn’t use any system functions, because those are dynamically linked and make for rather complex assembler.
• To compile such a program, simply pass it to GCC again, as you would with a C source file. E.g. gcc my_simple_c_program.s -o my_simple_c_program

This might be a good point to mention my Memory Management chapter in the Masters of the Void C tutorial again, which illustrates how memory works. As I learn more, I may post supplements to that that slowly teach you assembler. Well, I’m not promising anything, but I’d love to do that.