eflags

I am unable to understand how does the direction flag work in x86 . The text in my lectures say that it increments or decrements the source or destination register but that does not make sense with its name. Can someone explain what it does? This flag is used in string operations, and specifies if strings begin at a low address and proceed to higher addresses or vice versa. For string instructions, ECX has the number of iterations, DS:ESI has the source address and ES:EDI has the destination (hence the s in ESI and the d in EDI ). After each iteration, ECX is decremented by one, and ESI and

I want to write a simple code (or algorithm) to set/clear overflow flag. For setting OF, I know that I can use signed values. But how can I clear that? user1354557 There are many possible solutions. For instance, test al, al will clear the OF flag without affecting register contents. Or, if you don't want to affect the other flags, you can just directly modify the *FLAGS register. For example, in 32-bit, this would look like: pushfd ; Push EFLAGS onto the stack and dword [esp], ~0x800 ; Clear bit 11 (OF) popfd ; Pop the modified result back into EFLAGS Edit: Changed or al, al to test al, al

I don't understand the JG/JNLE/JL/JNGE instructions, which come after CMP. for example, If I have: CMP al,dl jg label1 When al=101; dl =200 . On what we ask the jg ? Is it on al>dl ? or al-dl>0 ? Same prolbem on the next code: test al,dl jg label1 I don't understand what we compare, and on what we ask the " jg ". In other words, I don't understand when we would jump to label1, and when we wouldn't. When you do a cmp a,b , the flags are set as if you had calculated a - b . Then the jmp -type instructions check those flags to see if the jump should be made. In other words, the first block of