Mastering Reverse Engineering

Registers

In programming, processing data requires variables. You can simply think of registers as variables in assembly language. However, not all registers are treated as plain variables, but rather, each register has a designated purpose. The registers are categorized as being one of the following:

• General purpose registers
• Segment registers
• Flag registers
• Instruction pointers

In x86 architecture, each general purpose register has its designated purpose and is stored at WORD size, or 16 bits, as follows:

• Accumulator (AX)
• Counter (CX)
• Data (DX)
• Base (BX)
• Stack pointer (SP)
• Base pointer (BP)
• Source index (SI)
• Destination index (DI)

For registers AX, BX, CX, and DX, the least and most significant bytes can be accessed by smaller registers. ...