X86-64 - AMD64 - Operating Modes

Operating Modes

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 Typical GPR width
Long mode 64-bit mode 64-bit operating system or bootloader 64-bit code 64 bits 32 bits 8, 16, 32, or 64 bits 16 registers per file 64 bits
Compatibility mode 64-bit operating system or bootloader 32-bit protected mode code 32 bits 32 bits 8, 16, or 32 bits 8 registers per file 32 bits
64-bit operating system or bootloader 16-bit protected mode code 16 bits 16 bits 8, 16, or 32 bits 8 registers per file 32 bits
Legacy mode Protected mode 32-bit operating system or bootloader, or 64-bit bootloader 32-bit protected mode code 32 bits 32 bits 8, 16, or 32 bits 8 registers per file 32 bits
16-bit or 32-bit operating system or bootloader, or 64-bit bootloader 16-bit protected mode code 16 bits 16 bits 8, 16, or 32 bits 8 registers per file 16 or 32 bits
Virtual 8086 mode 16-bit or 32-bit operating system 16-bit real mode code 16 bits 16 bits 8, 16, or 32 bits 8 registers per file 16 or 32 bits
Real mode 16-bit or 32-bit operating system or bootloader, or 64-bit bootloader 16-bit real mode code 16 bits 16 bits 8, 16, or 32 bits 8 registers per file 16 or 32 bits

Please note that the traditional x87 FPU register stack is not included in the register file size extension in 64-bit mode. It has been deprecated in favor of the XMM registers used by SSE2, which did get extended. The x87 register stack is not a register file because it does not provide true random access to its contents to the majority of instructions, and remains sized at 8 registers. Also note that 16-bit code written for the 80286 and below cannot use the 32 bit GPRs. 16-bit code written for the 80386 and above can use 32-bit GPRs, but defaults to using 16-bit operands.

The architecture has two primary modes of operation:

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