i want to know about the flag under flow(in micro processor). please can anybody help me out. thanks in advance

Underflow is the opposite of overflow; when you increase/decrease a value beyond the interval that number is supposed to represent, over/underflow happens. For example, if you represent an integer with a two's-complement 8 bit register you can store from -128 to 127. If you have -85, substract 100 and store it in the same register, underflow happens, because you go below the lower limit of -128. Depending on the way the operation is implemented or configured it will generally store either -128 (the output is said to be "saturated") or more likely 71 (it truncates to the least significant bits), and it raises an "underflow" flag to let you know the register value does not represent the value it should.