Chapter 1. Assembly Language

This chapter provides a brief introduction to assembly language (ASM), in order to lay the groundwork for the reverse engineering chapters in Part I. This is not a comprehensive guide to learning ASM, but rather a brief refresher for those already familiar with the subject. Experienced ASM users should jump straight to Chapter 2.

From a cracker's point of view, you need to be able to understand ASM code, but not necessarily program in it (although this skill is highly desirable). ASM is one step higher than machine code, and it is the lowest -level language that is considered (by normal humans ) to be readable. ASM gives you a great deal of control over the CPU. Thus, it is a powerful tool to help you cut through the obfuscation of binary code. Expert crackers dream in assembly language.

In its natural form, a program exists as a series of ones and zeroes. While some operating systems display these numbers in a hex format (which is much easier to read than a series of binary data), humans need a bridge to make programming ”or understanding compiled code ”more efficient.

When a processor reads the program file, it converts the binary data into instructions. These instructions are used by the processor to perform mathematical calculations on data, to move data around in memory, and to pass information to and from inputs and outputs, such as the keyboard and screen. However, the number of instruction sets and how they work varies, depending on the processor type and how powerful it is. For example, an Intel processor, such as the Pentium 4, has an extensive set of instructions, whereas a RISC processor has a limited set. The difference can make one processor more desirable in certain environments. Issues such as space, power, and heat flux are considered before a processor is selected for a device. For example, in handheld devices, a RISC-based processor such as ARM is preferable. A Pentium 4 would not only eat the battery in a few minutes, but the user would have to wear oven mitts just to hold the device.