RFLAGS

RFLAGS is the 64-bit processor status and control register in the x86-64 architecture. It is the 64-bit extension of the legacy EFLAGS register used in 32-bit mode. RFLAGS holds a mix of condition flags, system flags, and privilege-related controls that influence instruction behavior, interrupt handling, and task state.

The lower portion of RFLAGS contains the familiar condition flags: the carry flag (CF), parity flag (PF), auxiliary carry flag (AF), zero flag (ZF), and sign flag (SF). It also includes control and state flags such as the trap flag (TF) for single-stepping, the interrupt-enable flag (IF), the direction flag (DF), and the overflow flag (OF). The I/O privilege level (IOPL) field, typically a two-bit field, and the nested task flag (NT) are also part of this region. In addition, the resume flag (RF) and other system-related bits appear here.

In 64-bit mode, the lower 32 bits of RFLAGS preserve the traditional EFLAGS behavior, while the upper 32 bits are generally reserved and must be zero on current processors. In some contexts, additional bits related to virtualization and system state (such as VM, AC, VIF, VIP, and ID) are defined, though their exact availability and semantics can vary with processor and operating mode. The RF, VM, AC, VIF, VIP, and ID flags provide mechanisms for advanced features such as virtual-8086 support, alignment checks, and software/hardware interrupt controls.

RFLAGS is accessed and modified by instructions such as PUSHFQ/POPFQ (for 64-bit pushes and pops) and, in 32-bit mode, by PUSHF/POPF. It governs the behavior of conditional jumps, string operations, interrupt handling, and various privilege-level and debugging features.

See also EFLAGS and FLAGS register for related 32-bit behavior.