When I first got my mac, I had no idea how to continue working on my toy operating system. After some research, I found a couple of effective ways to do assembly programming on the mac.
I recommend this method for learning basic assembly. In combination with debug.com, dosbox provides a good safe environment for learning the basics. A guy named Khoraski(on YouTube) has a really good series on how to do assembly programming in DEBUG.
Install dosbox. Just use the .dmg file to install it to a default directory.
Next create a working directory to store all you dosbox programs and source files. Open up a terminal window, and type something along the lines of ‘mkdir ~/DOSPrograms’ to make a folder.
Now that you have a working directory, just download the debug.com file (try this site) and put it into the directory you created earlier(e.g. DOSPrograms).
The rest of this method is just booting dosbox and using its built in commands.
This is another very safe way to practice Assembly, but provides a much broader toolset.
Download and install VirtualBox, and the latest Ubuntu .iso.
Install nasm via command line.
you should be good to go
Probably the most obvious option, you just install the tools directly into OSX and just program without the protection of a virtual machine. Download Xcode, enable the command-line tools, and you should be ready to go. Nasm, GCC, DD, VIM, and LD should be automatically installed and enabled.
I hope some of you found this helpful. Options 2 and 3 are probably the two I would most recommend for serious Assembly development, seeing as Option 1 is severely limited in scope and utility.