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: http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/01/intro-computer2.html)

Previously discussed: storing program instructions in RAM memory (cont.)

• 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)

Previously discussed: storing program instructions in RAM memory (cont.)

• Instructions in a Java program are static while the program is running:

• The instructions of a Java program will not change for the entire duration that the Java program is executing

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

Creating variables: how to reserve RAM memory

• $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)

Creating variables: how to reserve RAM memory (cont.)

• Answer:

• The computer program reserves some RAM memory for special system variables in advance:

Creating variables: how to reserve RAM memory (cont.)

• One of these system variables will remember the following information:

• The start of the unused portion of memory

• A list of memory locations that has been used.

Creating variables: how to reserve RAM memory (cont.)

Example:

Creating variables: how to reserve RAM memory (cont.)

• What happens when you create a variable:

• Look up the start of the unused memory:

We find: 2000

Creating variables: how to reserve RAM memory (cont.)

• Check in the list of used memory location if it is still unused:

Creating variables: how to reserve RAM memory (cont.)

• Reserve memory location 2000 for the variable by marking the location 2000 as used:

Creating variables: how to reserve RAM memory (cont.)

• 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.

Destroying variables (cont.)

• 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 !

Destroying variables (cont.)

• How to destroy a variable:

• We simply remove the memory location from the used memory location list:

Destroying variables (cont.)

• 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:

• We enter an individual memory address in the used memory location list to indicate that the memory location has been used:

More efficient way to represent used memory cells (cont.)

• Problem with this approach:

• Need to use a large number of entries

More efficient way to represent used memory cells (cont.)

• 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:

The Swiss cheese effect (cont.)

• I call this the Swiss cheese effect

• Note:

• We need to use 1 pair of addresses to record a used region of memory locations

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:

• the Last In, First Out sequence:

• A variable that is created later is always destroyed first.

A special sequence of variable creation and destruction (cont.)

• Example:

• Create variable 1:

A special sequence of variable creation and destruction (cont.)

• Create variable 2:

A special sequence of variable creation and destruction (cont.)

• Create variable 3:

A special sequence of variable creation and destruction (cont.)

• When you destroy the variables in the reverse order, you preserve the region...

Destroy variable 3:

A special sequence of variable creation and destruction (cont.)

Destroy variable 2:

Terminology: stack and heap

• Stack:

• Heap:

• Stack = an area of RAM memory used for variables that are created and destroyed in a Last In, First Out (LIFO) manner

• Heap = an area of RAM memory used for variables that are created and destroyed in a non-LIFO manner

Terminology: stack and heap (cont.)

• Note:

• The name "stack" is derived from the similarity with a "stack of book":

Terminology: stack and heap (cont.)

• 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

• The sequence of book inserting and book deletion is Last In, First Out.

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:

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:

Memory organization: where different things are stored in memory (cont.)

• 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).

Memory organization: where different things are stored in memory (cont.)

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

Memory organization: where different things are stored in memory (cont.)

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

• Area 2 will grow when the program creates new instance variables

• Area 2 will shrink when some c instance variables are destroyed

Memory organization: where different things are stored in memory (cont.)

• 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)

Memory organization: where different things are stored in memory (cont.)

• 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)

Memory organization: where different things are stored in memory (cont.)

• Direction of growth of the areas 2 and 3:

• In order to use the available RAM memory as efficient as possible, the direction of growth of areas 2 and 3 is towards each other

Graphically:

Initial memory organization

• Consider the following computer program:

Initial memory organization (cont.)

• 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:

Initial memory organization (cont.)

Initial memory organization (cont.)

• (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 (cont.)

• 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

Out-of-memory error (cont.)

• 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 3 is a stack !!!

In fact, this region of memory is called the System stack

(There are no "holes" of unused regions in this area)

Terminology: System stack and System heap (cont.)

• Area 2 however, is a heap

This area is called the System heap.

Terminology: System stack and System heap (cont.)

• Summary: