问题
i want to know about NEG instruction, does it affects the overflow flag too!!!..i got to know that is just negate the value of a variable but couldn't know that is it affects the Overflow flag
回答1:
If you want to know what instructions do, consult the reference manuals.
The essential reference, namely the Intel instruction set manual says this about the NEG instruction:
Flags Affected
The CF flag set to 0 if the source operand is 0; otherwise it is set to 1.
The OF, SF, ZF, AF, and PF flags are set according to the result.
So it is clear that the NEG instruction sets the O flag; therefore it affects the O flag, which is the OP's original question. And it does so every time it is executed. (People should not confuse "didn't change" from "not set").
That particular reference manual doesn't provide a specific algorithm to indicate when O is set to zero or one. However, Intel CPUs are 2's complement machines. The Subtract instruction has the exact same verbiage. NEG X is equivalent to (0 SUBTRACT X). So NEG should set the O bit according to "overflow" for (0 SUBTRACT X); this will set O when X is 0x8000000.
Inspecting the Intel Basic Archiecture Manual, we find this description of the OF bit:
OF (bit 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
confirming our understanding.
来源：https://stackoverflow.com/questions/26440463/does-neg-instruction-in-assembly-language-sets-the-overflow-flag