This article discusses x86-64 CPUs (AMD64 and Intel's equivalent EM64T implementation). IA-64 (Itanium) is really a different beast and not addressed here.| wiki.osdev.org
Real Mode is a simplistic 16-bit mode that is present on all x86 processors. Real Mode was the first x86 mode design and was used by many early operating systems before the birth of Protected Mode. For compatibility purposes, all x86 processors begin execution in Real Mode.| wiki.osdev.org