Memory organization - storing variables efficiently in the RAM memory

Previously discussed: storing program instructions in RAM memory
- Instructions (encoded using binary numbers) of a computer program are stored in the RAM memory - (See: click here )
  Alternate (more comprehensive) view of the content of the RAM memory:
  (I am using a 1000000 byte RAM memory in the example - that's why the last address is 999999)
- Instructions in a Java program are static while the program is running:

Creating variables: how to reserve RAM memory

Variables are also placed in RAM memory
Example: create one variable
We have used the memory cell with address 1000 to store the value of the variable

$64,000 question:

If we want to create another variable, how can the computer know which memory cell to use ???
I.e.: how can the computer tell that the memory cell 1000 is now used (while previously, it was unused)

Answer:

The computer program reserves some RAM memory for special system variables in advance:
One of these system variables will remember the following information:
Example:

What happens when you create a variable:

Look up the start of the unused memory:
We find: 2000
Check in the list of used memory location if it is still unused:
Reserve memory location 2000 for the variable by marking the location 2000 as used:

The next time a variable is created, the system knows that the memory location 2000 is used !!!

Destroying variables

Here is the situation after creating 3 variables:
The memory locations 2000, 2001 and 2002 have been marked as used.
If variable 1 is destroyed, the memory call 2000 is no longer used.
Furthermore, the memory call can be re-used by some other variable !
How to destroy a variable:

Note:

If the program creates a new variable while in this situation:
It can detect that memory call 2000 is not used and can re-assign this memory call to another variable !!!

More efficient way to represent used memory cells
- Our current solution:
  Problem with this approach:
- A more efficient solution: use a range:
The Swiss cheese effect
- If the process of variable creation and variable destruction has gone on for some time, there will be regions of used and unused memory cells all over the place in memory:
- I call this the Swiss cheese effect
- Note:

A special sequence of variable creation and destruction

The is one special sequence of variable creation and destruction that will not create holes in the RAM memory:

Example:

Create variable 1:
Create variable 2:
Create variable 3:
When you destroy the variables in the reverse order, you preserve the region...
Destroy variable 3:
Destroy variable 2:

Terminology: stack and heap

Stack:
Heap:

Note:

The name "stack" is derived from the similarity with a "stack of book":
The last book you put on a stack sits at the top (stack top)
The first book that you can remove without causing the stack to collapse is the book at the top of the stack

Memory organization: where different things are stored in memory

In order to use the computer RAM memory as efficiently as possible, the computer RAM memory is organized into 3 parts:

Explanation:

Area 1 contains information that are used throughout the execution of the entire program (or what I call "persistent information")

Information stored in this area consists of 2 types of items:

Computer instructions (you need all instructions in the computer program throughout its execution).
Class variables (this kind of variables store information that is used throughout the program execution).

The amount of memory space in area 1 remains unchanged throughout the execution of the program !!!

Area 2 contains instance variables that used by multiple methods (long term information)
The space of Area 2 can grow or shrink:

Area 3 contains local and parameter variables that used by one single method (short term information)

Local and parameter variables are created when a method is invoked (starts executing)
Local and parameter variables are destroyed when a method is returns (exits)

The space in Area 3 can also grow or shrink:

Area 3 will grow when a method is invoked
Area 3 will shrink when a method is returns (exits)

Direction of growth of the areas 2 and 3:

Initial memory organization
- Consider the following computer program:
- When a program first starts running, the computer RAM memory will only contain the persistent information
  I.e., only the program instructions and class variables are stored in RAM:
  (Since we do not learned about class variables, our example only has program instructions)
- The area 2 and area 3 will grow and shrink depending on whether some variables are created and destroyed.

Out-of-memory error

It is possible that the area 2 or area 3 grows so large that the RAM memory becomes exhausted:

Out-of-memory error:

When you try to create a variable and the computer has run out of unused RAM memory, it will cause a out-of-memory error
The program will terminate immediately

Some situations that can cause out-of-memory errors:

The program has an infinite loop and inside the loop, the program creates a new variable
When we discuss recursion later in the course, I will warn you about the "infinite recursion" scenario that can cause out-of-memory errors.

Terminology: System stack and System heap
- Later in the course, you will learn that the sequence of variable creation and destruction in area 3 is LIFO
  Therefore:
- Area 2 however, is a heap
  This area is called the System heap.
- Summary: