call JavaScript function on text changed if gridview textbo

Question

I have form with 1 textbox named "txtOutOfMarks" and gridview with student details. Grdview has template field with textbox named "txtObtainedMarks".
I have to call JScript while entering marks in gridviews textbox to check whether obtained marks are greater than outofmarks or not if yes  ontinue else alert with msg & clear current field.