The status flags (bits 0, 2, 4, 6, 7, and 11) of the EFLAGS register indicate the results of arithmetic instructions.
The status flags allow a single arithmetic instructions to produce results for three different data types:
If the result of an arithmetic operation is treated as:
where:
When performing multiple-precision arithmetic on integers, the CF flag is used in conjunction with the add with carry (ADC) and subtract with borrow (SBB) instructions to propagate a carry or borrow from one computation to the next.
The condition instructions use one or more of the status flags as condition codes and test them for:
The status flag functions are:
Code | Bit | Name | Description |
---|---|---|---|
CF | 0 | Carry flag | Set if an arithmetic operation generates: * a carry * or a borrow out of the most significant bit of the result; cleared otherwise. This flag indicates an overflow condition for unsigned-integer arithmetic. It is also used in multiple-precision arithmetic. |
PF | 2 | Parity flag | Set if the least-significant byte of the result contains an even number of 1 bits; cleared otherwise |
AF | 4 | Auxiliary Carry flag | Set if an arithmetic operation generates: * a carry * or a borrow out of bit 3 of the result; cleared otherwise. This flag is used in binary-coded decimal (BCD) arithmetic. |
ZF | 6 | Zero flag | Set if the result is zero; cleared otherwise. |
SF | 7 | Sign flag | Set equal to the most-significant bit of the result, which is the sign bit of a signed integer. (0 indicates a positive value and 1 indicates a negative value.) |
OF | 11 | overflow flag | Set if the integer result is: * too large a positive number * or too small a negative number (excluding the sign-bit) to fit in the destination operand; cleared otherwise. This flag indicates an overflow condition for signed-integer (two’s complement) arithmetic. |