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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