Understanding Instruction Cycle? A basic instruction cycle consists of these 5 stages.


  Instruction Cycle
  
  
  IF - Instruction Fetch
  RD - Instruction Decode and Register Read
  EX - Execute
  MA - Memory Access
  WB - Write Back
  


I understood the function of all the stages accept the fourth (Memory access). What's the significance of this stage?

Over Internet what I've found is this


  MA(Memory Access) does following operation
  
  
  If load or store instruction, then access memory
  If branch instruction, replace PC with destination address
  


The second point is clear, but I'm confused by the first. 

Suppose we have load and store instruction like this 
$$(I) R_1 \leftarrow m[A_1]$$ $$(II) m[A_2] \rightarrow R_2$$

what will the MA stage do for the above instructions?
You have to operate on the memory to do anything useful. Reading next instruction, getting variable to operate on or saving calculated value - all that requires accessing memory.