x64 Assembly Language Step-by-Step, 4th Edition

CHAPTER 9Bits, Flags, Branches, and Tables: Easing Into Mainstream Assembly Coding

As you've seen by now, my general method for explaining things starts with the “view from a height” and then moves down toward the details. That's how I do things because that's how people learn: by plugging individual facts into a larger framework that makes it clear how those facts relate to one another. It's possible (barely) to move from details to the big picture, but across 60-odd years of banging my head against various subjects in the pursuit of knowledge, it's become very clear that having the overall framework in place first makes it a lot easier to establish all those connections between facts. It's like carefully placing stones into a neat pile before shoveling them into a box. If the goal is to get the stones into a box, it's much better to have the box in place before starting to pick up the stones.

And so it is here. The big picture is mostly in place. From now on in this book, we'll be looking at the details of assembly code and seeing how they fit into that larger view.

Bits Is Bits (and Bytes Is Bits)

Assembly language is big on bits. Bits, after all, are what bytes are made of, and one essential assembly language skill is building bytes and taking them apart again. A technique called bit mapping is widely used in assembly language. Bit mapping assigns special meanings to individual bits within a byte to save space and squeeze the last little bit of utility out of a given ...