About
The status flags (bits 0, 2, 4, 6, 7, and 11) of the EFLAGS register indicate the results of arithmetic instructions.
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Usage
One instruction, multiple Data Type Result
The status flags allow a single arithmetic instructions to produce results for three different data types:
- and BCD integers.
If the result of an arithmetic operation is treated as:
- an unsigned integer, the CF flag indicates an out-of-range condition (carry or a borrow);
- a signed integer (two’s complement number), the OF flag indicates a carry or borrow;
- a BCD digit, the AF flag indicates a carry or borrow.
where:
- The SF flag indicates the sign of a signed integers.
- The ZF flag indicates either a signed- or an unsigned integer zero.
Multiple-precision arithmetic on integers
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.
Conditional value
The condition instructions use one or more of the status flags as condition codes and test them for:
- setbyte,
- or end-loop conditions
Management
List
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. |
Modification
- Of these status flags, only the CF flag can be modified directly, using the STC, CLC, and CMC instructions.
- The bit instructions (BT, BTS, BTR, and BTC) copy a specified bit into the CF flag.
Documentation / Reference
- 3.4.3.1 - Status Flags