What is memory model in computer organization?
um I'm not sure if it is what you are looking for or not. I assume by memory model you mean the part in which we place input and do some calculation (like paper which we write something to remember and calculate).

The memory model depends on the definition of the model itself. In CS we have different kinds of computing model with different power.
One of the simplest one is Deterministic Finite Automata and informally its memory just contains an input string and nothing more.
One of the most powerful one is Turing machine that its memory is a infinite tape.
There are other models between these two such as PushDown Automata and Linear Bounded Automata where loosely speaking their memories contains stack of symbols and finite tape.

Also there are other models with same power as Turing machine, like two stack PDA, cellular automata and etc. 

So by how to define your computing model and program, your memory model details will be different.