X86-64
Operating modes
The architecture has two primary modes of operation, long mode and legacy mode. Note that 16-bit code written for the 80286 and below does not use 32-bit operand instructions.

X86-64

Section Title: Operating modes
Table Section Text: The architecture has two primary modes of operation, long mode and legacy mode. Note that 16-bit code written for the 80286 and below does not use 32-bit operand instructions.

Operating mode	Operating sub-mode	Operating system required	Type of code being run	Default address size	Default operand size	Supported typical operand sizes	Register file size

Long mode	64-bit mode	64-bit	64-bit code	64 bits	32 bits	8, 16, 32, or 64 bits	16 registers per file

Compatibility mode	32-bit code	32 bits	32 bits	8, 16, or 32 bits	8 registers per file

16-bit code	16 bits	16 bits	8, 16, or 32 bits	8 registers per file

Legacy mode	Protected mode	32-bit	32-bit code	32 bits	32 bits	8, 16, or 32 bits	8 registers per file

16-bit protected mode	16-bit code	16 bits	16 bits	8, 16, or 32 bits	8 registers per file

Virtual 8086 mode	16-bit protected mode or 32-bit	some of real mode code	16 bits	16 bits	8, 16, or 32 bits	8 registers per file

Real mode	16-bit real mode	real mode code	16 bits	16 bits	8, 16, or 32 bits	8 registers per file

Sentence(s)

Under a 64-bit operating system, 64-bit programs run under 64-bit mode, and 32-bit and 16-bit run under compatibility mode.

final_sentence: Under a 64-bit operating system, 64-bit programs run under 64-bit mode, and 32-bit and 16-bit run under compatibility mode.