When malware is stored on a disk, it is typically in binary form at the machine code level. As discussed, machine code is the form of code that the computer can run quickly and efficiently. When we disassemble malware (as shown in Figure 4-1), we take the malware binary as input and generate assembly language code as output, usually with a disassembler. (Chapter 5 discusses the most popular disassembler, IDA Pro.)

Assembly language is actually a class of languages. Each assembly dialect is typically used to program a single family of microprocessors, such as x86, x64, SPARC, PowerPC, MIPS, and ARM. x86 is by far the most popular architecture for PCs.

Most 32-bit personal computers are x86, also known as Intel IA-32, and all modern 32-bit versions of Microsoft Windows are designed to run on the x86 architecture. Additionally, most AMD64 or Intel 64 architectures running Windows support x86 32-bit binaries. For this reason, most malware is compiled for x86, which will be our focus throughout this book. (Chapter 21 covers malware compiled for the Intel 64 architecture.) Here, we’ll focus on the x86 architecture aspects that come up most often during malware analysis.