Understanding Condition Codes
Stephanie Domas, Christopher Domas · 2024
Assembly instructions commonly include destination registers where the result of an operation will be stored. However, some instructions can have effects beyond those recorded in this destination register. x86 uses condition codes to track these effects. This chapter explores these condition codes and describes the main ones readers need to understand to effectively reverse engineer x86 applications. Most architectures, including x86, need a means of tracking the basic properties of previous operations. The computer has a special-purpose register called flags. Status flags can be affected by various operations. Four examples include add, sub, cmp, and test. Tracking the condition codes is essential to understanding the current state of an application when reverse engineering it.