EFLAGSRFLAGS

EFLAGS is a 64-bit general-purpose register in the x86 architecture, though often referred to as EFLAGS due to the original 32-bit nature of the architecture. It holds a collection of status flags and control flags that affect the operation of the processor. The most significant byte, RFLAGS, is dedicated to these flags. The lower 32 bits, EFLAGS, contains the original set of flags.

The status flags indicate the outcome of arithmetic and logical operations. These include the Carry Flag (CF), which indicates if a carry-out or borrow occurred; the Parity Flag (PF), which indicates if the number of set bits in the least significant byte is even; the Auxiliary Carry Flag (AF), used in BCD arithmetic; the Zero Flag (ZF), set if the result of an operation is zero; the Sign Flag (SF), which mirrors the most significant bit of the result, indicating a negative number in two's complement; and the Overflow Flag (OF), indicating if a signed arithmetic operation resulted in a value too large to be represented.

Control flags modify processor behavior. The Trap Flag (TF) enables single-step mode for debugging. The Interrupt Enable Flag (IF) controls the response to maskable external interrupts. The Direction Flag (DF) determines the order of string operations. Other flags include the AC (Alignment Check) flag and the VM (Virtual-8086 Mode) flag.

The RFLAGS register is crucial for program execution, conditional branching, and debugging. Programs can inspect these flags to make decisions, and operating systems use them for managing interrupts and system calls.