CPU usually has some status flags that stored in a “flags register.” On x86-64, the status is stored in a 64 bit rflags register. It has a 32 bit counterpart, eflags.
While we can’t directly write to rflags, there are specific instructions that allow manipulation of the flags register. The CPU updates rflags automatically after executing most instructions, with different instructions affecting different flags
Below are some of the most commonly used flags:
| Flag | Name | Bit # | Meaning |
|---|---|---|---|
| CF | Carry flag | 0 | Set to 1 if the last operation resulted in a carry out of the most significant bit, and 0 otherwise |
| PF | Parity flag | 2 | Set to 1 if the least significant byte of the result contains an even number of 1 bits, and 0 otherwise |
| ZF | Zero flag | 6 | Set to 1 if the result of the last instruction was 0 and 0 otherwise |
| SF | Sign flag | 7 | Set to 1 if the most significant bit of the last result was 1 (two’s complement negative number), and 0 otherwise |
| OF | Overflow flag | 11 | Set to 1 if the last instruction resulted in signed integer overflow, and 0 otherwise |
It’s worth noting that the rflags register contains additional flags beyond these status flags. |