RFLAGS
This is a 64-bit status and control register that reflects the outcome of arithmetic and logical operations, controls CPU behavior, and provides information about system state. Each bit in the register has a specific purpose, such as the Carry Flag (CF) for unsigned arithmetic carries, the Zero Flag (ZF) for results equal to zero, the Sign Flag (SF) for negative results, the Overflow Flag (OF) for signed arithmetic overflow, and the Parity Flag (PF) for even parity in the least significant byte. Control bits like the Interrupt Flag (IF) enable or disable maskable interrupts, the Direction Flag (DF) governs the direction of string operations, and the Trap Flag (TF) allows single-step debugging. Other flags include privilege-level indicators (IOPL), nested task control (NT), and virtualization-related flags (VM, VIF, VIP). Many bits are reserved or used in special modes, but collectively, the RFLAGS register provides the processor with the necessary state information to make conditional decisions, manage system control, and support debugging and virtualization.
Label
CF
1
PF
0
AF
0
ZF
SF
TF
IF
DF
OF
IOPL
NT
0
RF
VM
AC
VIF
VIP
ID
0
Description
Carry Flag
Reserved
Parity Flag
Reserved
Auxiliary Carry Flag
Reserved
Zero Flag
Sign Flag
Trap Flag
Interrupt Flag
Direction Flag
Overflow Flag
I/O Privilege Level
Nested Task
Reserved
Resume Flag
Virtual-8086 Mode
Alignment Check / Access Control
Virtual Interrupt Flag
Virtual Interrupt Pending
Identification Flag
Reserved
Flags in the RFLAGS register can be thought of as boolean values, where a bit set to 1 represents “true” and a bit set to **0” represents “false.” This makes it easier to understand their role in conditional logic: for example, CF = 1 indicates that a carry or borrow occurred, ZF = 0 shows that the result of an operation was not zero, and SF = 1 signals that the result is negative. Interpreting the flags this way is especially useful when reasoning about conditional jumps, because these instructions test whether a specific flag is true or false to determine if the CPU should branch to another instruction.
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