I'm studying for an exam called Computer Fundamental, and started taking some previous exam but the given documents i had is way inadequate to solve all the question mentioned, like this one : Which of the following is the correct combination of variousaddressing  modes?   Here,  X1 is an address which is stored in a program counter.  X2is an address part of an  instruction which is addressed by X1.X3is  an address in which an operand needed to  execute an instruction is  stored.   X4is a value in an index register.  (X2) means the  contents  of location X2.I tried to google Addressing mode and came up with some resources that i can't understand. Can someone recommend me some link where i can get to know more about addressing mode so that i can answer this question?Thanks alot! I'm sorry for the tag, can't find some thing like address or addressing mode 
Addressing mode
memory hardware
I recommend you start by understanding what direct addressing, indirect addressing, PC-relative addressing, and indexed addressing mean.  With any luck, your textbook will explain those.  If not, you should be able to find many explanations on the Internet.  At last resort, check out the manual for your favorite processor.