Memory Management

Part 1 Objectives

Define reference types
- Partially review, but hopefully this gives you a deeper understanding of why reference types behave the way they do.
Describe a pointer
Draw a memory diagram

Registers and Memory

Computers have two ways to store data. Registers and Memory.

variables, objects, and everything in a computer program is an abstraction around these mechanisms.

Registers

Registers are very small - each one can store 64 bits
Registers exist on the processor chip
Data must be loaded into registers before the processor can use it

Memory

All non-register storage can be considered memory
L1-L4 caches, RAM, and hard-disc space are ALL in memory

L1 cache

L2 cache

L3 cache

RAM

Hard Disk

The Processor and the Registers

Memory

"Memory"

Although there are some limitations to this mindset, for now we are going to treat *all* of memory as one big bucket

Memory & Programs

Throughout the life-cycle of a program, data moves between memory and registers.

"Memory"

Memory & Programs

Because registers have limited storage, data that exceeds the limit must live in memory.

In JavaScript each integer is 64 bits. This array is therefore (at least) 5 times to large for a register.

var x = [1, 2, 3, 4, 5];

Memory & Programs

So our register gets a pointer to a memory address

var x = [1, 2, 3, 4, 5];

x = 0xFF019A

0xFF019A:

0: 1

1: 2

2: 3

3: 4

4: 5

Memory & Programs

x = 0xFF019A

0xFF019A:

0: 1

1: 2

2: 3

3: 4

4: 5

Hold up: Whats this?

Memory & Programs

This pointer is why Objects are called reference types. Our variable really holds a 64-bit pointer to a memory address

var x = [1, 2, 3, 4, 5];

x = 0xFF019A

0xFF019A:

0: 1

1: 2

2: 3

3: 4

4: 5

Memory & Programs

If we make a new variable and set it equal to x, this variable references the same memory location.

var x = [1, 2, 3, 4, 5];
var y = x;

x = 0xFF019A

0xFF019A:

0: 1

1: 2

2: 3

3: 4

4: 5

y = 0xFF019A

Memory & Programs

If we change one of the properties at that location both variables reflect the change.

x[0] = 100;
console.log(y[0]); // Prints 100

x = 0xFF019A

0xFF019A:

0: 100

1: 2

2: 3

3: 4

4: 5

y = 0xFF019A

Memory & Programs

But if we change what one of the variables points to then the modified variable points somewhere new.

y = [7,8,9]
console.log(y[0]); // Prints 7
console.log(x[0]); // Still prints 100

x = 0xFF019A

0xFF019A:

0: 100

1: 2

2: 3

3: 4

4: 5

y = 0xFF0EEE

0xFF0EEE:

0: 7

1: 8

2: 9

Lets Examine

http://pythontutor.com

Memory & Programs

If all our registers are filled, even 64 bit primitive types may have to be stored into memory and fetched again when they are needed.

There is *a lot* of hardware and software that we rely on to handle this for us, and our mental model doesn't need to be perfect.

(and probably couldn't be without years of study in Computer Engineering, Computer Science, and Operating Systems)

Memory Diagrams

Lets go here https://github.com/gSchool/computer-science-curriculum/blob/master/Unit-0/02-memory-diagrams.md

And look at some "memory diagrams" before we create some ourselves.

Stop when you get to "Prototypes" -- we'll discuss those later.

Take 10 minutes right now to read this

Now, Practice

Grab a mini-whiteboard and a partner

https://github.com/gSchool/computer-science-curriculum/blob/master/Unit-0/03-memory-diagrams-exercises.md

Objectives Review

Define reference types
Describe a pointer
Draw a memory diagram

g35 Specific Slide

You all asked about Symbols, the new ES2016 (aka JavaScript 6) primitive type.

Now we have enough information to better understand Symbols.

var fooOne = new Symbol('foo');
var fooTwo = new Symbol('foo');

'foo' === 'foo'    // this evaluates to true
fooOne === fooTwo; // What do you think this evaluates to?

// Why?

Objectives Part 2

Describe "The Stack" and what data lives there
Describe "The Heap" and what data lives there
Describe "Garbage Collection" at a high level
Stretch goal - appreciate how many countless hours of frustration John McCarthy has saved us all by inventing Garbage Collection.

Memory Groups

Computer programs use two groupings for memory
The distinction between these is 100% software - both use the same memory hardware
They are called "The Stack" and "The Heap"

The Stack

The Stack is for localized program memory
Every time a function is called, space is allocated on the stack
This "Stack Frame" contains local variables and pointers to function calls as well as "return addresses"
Once a frame "pops off the stack" it is immediately freed as memory space.

The Stack

Learn by example

function fun1() {
    var a = 1;
    var b = 2;
    return fun2(a, b);
}

function fun2(x, y) {
    var c = x + y;
    return c;
}

fun1();

We have the code to the left. Two functions are defined, then we call fun1.

The Stack

At the "top level" we're on whats called the global frame. Before anything happens, it's empty.

// YOU ARE HERE
function fun1() {
    var a = 1;
    var b = 2;
    return fun2(a, b);
} 

function fun2(x, y) {
    var c = x + y;
    return c;
}

fun1();

Global Frame:
returnTo: null

Every stack frame knows where it was called from. The global stack frame is special in that it isn't called from anywhere.

The Stack

After we execute the two function definitions, our stack looks like this

function fun1() {
    var a = 1;
    var b = 2;
    return fun2(a, b);
} 

function fun2(x, y) {
    var c = x + y;
    return c;
}

// YOU ARE HERE
fun1();