The FLAGSregister is the status register in Intel x86 microprocessors that contains the current state of the processor. This register is 16 bits wide. Its successors, the EFLAGS and RFLAGS registers, are 32 bits and 64 bits wide, respectively. The wider registers retain compatibility with their smaller predecessors. The fixed bits at bit positions 1, 3 and 5, and carry, parity, adjust, zero and sign flags are inherited from an even earlier architecture, 8080 and 8085. The adjust flag used to be called auxiliary carry bit in 8080 and half-carry bit in the Zilog Z80 architecture.
FLAGS
Note: The mask column in the table is the AND bitmask to query the flag within FLAGS register value.
Usage
All FLAGS registers contain the condition codes, flag bits that let the results of one machine-language instruction affect another instruction. Arithmetic and logical instructions set some or all of the flags, and conditional jump instructions take variable action based on the value of certain flags. For example, jz, jc, and jo depend on specific flags. Other conditional jumps test combinations of several flags. FLAGS registers can be moved from or to the stack. This is part of the job of saving and restoring processor context, against a routine such as an interrupt service routine whose changes to registers should not be seen by the calling code. Here are the relevant instructions:
The PUSHF and POPF instructions transfer the 16-bit FLAGS register.
PUSHFD/POPFD transfer the 32-bit double register EFLAGS.
PUSHFQ/POPFQ transfer the 64-bit quadword register RFLAGS.
In 64-bit mode, PUSHF/POPF and PUSHFQ/POPFQ are available but PUSHFD/POPFD are not. The lower 8 bits of the FLAGS register is also open to direct load/store manipulation by SAHF and LAHF.
Example
The ability to push and pop FLAGS registers lets a program manipulate information in the FLAGS in ways for which machine-language instructions do not exist. For example, the cld and std instructions clear and set the direction flag, respectively; but there is no instruction to complement DF. This can be achieved with the following assembly code: pushf ; Use the stack to transfer the FLAGS pop ax ;...into the AX register push ax ; and copy them back onto the stack for storage xor ax, 400h ; Toggle DF only; other bits are unchanged push ax ; Use the stack again to move the modified value popf ;...into the FLAGS register ; Insert here the code that required the DF flag to be complemented popf ; Restore the original value of the FLAGS
By manipulating the FLAGS register, a program can determine the model of the installed processor. For example, the alignment flag can only be changed on the 486 and above. If the program tries to modify this flag and senses that the modification did not persist, the processor is earlier than the 486. Starting with the Intel Pentium, the CPUID instruction reports the processor model. However, the above method remains useful to distinguish between earlier models.
