Real mode (16-bit)
Long mode (64-bit)
Virtual 8086 mode (16-bit)
System Management Mode (16-bit)
Since the 64-bit registers allow access for many sizes and locations, we define a byte as 8 bits, a word as 16 bits, a double word as 32 bits, a quadword as 64 bits, and a double quadword as 128 bits. Intel stores bytes "little endian," meaning lower significant bytes are stored in lower memory addresses.