Flags registers - Can we read or write them directly?

Question

From what I've read, seems like there are 9 different flags. Is it possible to read/change them directly? I know I can know for example if the zero flag is set after doing a cmp/jmp instruction, but I'm asking if it's possible to do something like

mov eax, flags

or something.

Also, for writting, is it possible to set them by hand?

Thanks

Answer 1

You can use the pushf and popf instructions which will push the flags onto the stack, you can modify them, and then pop them back off.

Answer 2

If you need only the lower byte of the flags register (which contains SF,ZF,AF,PF,CF), then there is the odd but convenient instruction LAHF (ha ha), which loads the bottom 8 bits of the flags register into AH, and its counterpart SAHF to store AH into flags.

For the carry flag specifically, x86 offers CLC, STC and CMC, to clear, set, and complement the carry flag, respectively.

Answer 3

Some flags can be set or cleared directly with specific instructions:

• CLC, STC, and CMC: clear, set, and complement the carry flag
• CLI and STI: clear and set the interrupt flag (which should be done atomically)
• CLD and STD: clear and set the direction flag

For reading and writing the sign, zero, auxiliary carry, parity, and carry flags, you can use LAHF to load the lower 8 bits (those 5 flags plus 3 indeterminate bits) into the AH register, and you can use SAHF to store those values from AH back into the flags register.

You can also use the PUSHF instruction to push the flags onto the stack, read and modify them on the stack, and then use the POPF instruction to store them back into the flags register.

Note that you cannot set the VM and RF flags with POPF -- they retain their previous values. Similarly, you can only change the I/O privilege level when executing at privilege level 0, and the interrupt flag can only be changed when executing at a privilege level at least as privileged as the I/O privilege level.