What and where are the stack and heap?
Programming language books usually explain that value types are created on the stack, and reference types are created on the heap, without really explaining what these two things are.  With my only programming experience being in high level languages, I haven't read a clear explanation of this.  I mean I understand what a stack is, but where and what are they (relative to the physical memory of a real computer)?  
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Kayleigh Beer Created at: 2013-11-13 17:07:51 UTC By Kayleigh Beer
Darn near every tech phone screen I've had in the last year for ASP.Net MVC mobile-friendly web development with C# has asked about boxing/unboxing, value and reference types, and which one is on the stack and which one is on the heap. Sigh. - Garrison Hyatt
All the points the OP has gathered is through this question. And here I am struggling to reach 200. :P - Quentin Jerde
Is it snarky to point out that it is the success of modern "managed code" languages that awareness of stack and heap are no longer necessary (much) for C# and Java developers, because those of us who have to debug things that we did wrong in C and C++ needed to understand the model so we can fix our code when it breaks. In other words, "ignorance is power", because the abstraction works as designed, allowing me to work on a virtual machine that is easier to use than the real one.  Until you add threads, then it breaks again, in C#. - Ms. Bertha Hermiston
14 Answers
From WikiAnwser.


When a function or a method calls another function which in turns calls another function etc., the execution of all those functions remains suspended until the very last function returns its value.

This chain of suspended function calls is the stack, because elements in the stack (function calls) depend on each other.

The stack is important to consider in exception handling and thread executions.


The heap is simply the memory used by programs to store variables.
Element of the heap (variables) have no dependencies with each other and can always be accessed randomly at any time.

I like the accepted answer better since it's even more low level.
Others have directly answered your question, but when trying to understand the stack and the heap, I think it is helpful to consider the memory layout of a traditional UNIX process (without threads and mmap()-based allocators). The Memory Management Glossary web page has a diagram of this memory layout.

The stack and heap are traditionally located at opposite ends of the process's virtual address space. The stack grows automatically when accessed, up to a size set by the kernel (which can be adjusted with setrlimit(RLIMIT_STACK, ...)). The heap grows when the memory allocator invokes the brk() or sbrk() system call, mapping more pages of physical memory into the process's virtual address space. 

In systems without virtual memory, such as some embedded systems, the same basic layout often applies, except the stack and heap are fixed in size. However, in other embedded systems (such as those based on Microchip PIC microcontrollers), the program stack is a separate block of memory that is not addressable by data movement instructions, and can only be modified or read indirectly through program flow instructions (call, return, etc.). Other architectures, such as Intel Itanium processors, have multiple stacks. In this sense, the stack is an element of the CPU architecture.
The most important point is that heap and stack are generic terms for ways in which memory can be allocated.  They can be implemented in many different ways, and the terms apply to the basic concepts.

In a stack of items, items sit one on top of the other in the order they were placed there, and you can only remove the top one (without toppling the whole thing over).

In a heap, there is no particular order to the way items are placed.  You can reach in and remove items in any order because there is no clear 'top' item.

It does a fairly good job of describing the two ways of allocating and freeing memory in a stack and a heap.  Yum!

To what extent are they controlled by the OS or language runtime?

As mentioned, heap and stack are general terms, and can be implemented in many ways.  Computer programs typically have a stack called a call stack which stores information relevant to the current function such as a pointer to whichever function it was called from, and any local variables.  Because functions call other functions and then return, the stack grows and shrinks to hold information from the functions further down the call stack.  A program doesn't really have runtime control over it; it's determined by the programming language, OS and even the system architecture.

A heap is a general term used for any memory that is allocated dynamically and randomly; i.e. out of order.  The memory is typically allocated by the OS, with the application calling API functions to do this allocation.  There is a fair bit of overhead required in managing dynamically allocated memory, which is usually handled by the OS.
What is their scope?

The call stack is such a low level concept that it doesn't relate to 'scope' in the sense of programming.  If you disassemble some code you'll see relative pointer style references to portions of the stack, but as far as a higher level language is concerned, the language imposes its own rules of scope.  One important aspect of a stack, however, is that once a function returns, anything local to that function is immediately freed from the stack.  That works the way you'd expect it to work given how your programming languages work.  In a heap, it's also difficult to define.  The scope is whatever is exposed by the OS, but your programming language probably adds its rules about what a "scope" is in your application.  The processor architecture and the OS use virtual addressing, which the processor translates to physical addresses and there are page faults, etc.  They keep track of what pages belong to which applications.  You never really need to worry about this, though, because you just use whatever method your programming language uses to allocate and free memory, and check for errors (if the allocation/freeing fails for any reason).
What determines the size of each of them?

Again, it depends on the language, compiler, operating system and architecture.  A stack is usually pre-allocated, because by definition it must be contiguous memory (more on that in the last paragraph).  The language compiler or the OS determine its size.  You don't store huge chunks of data on the stack, so it'll be big enough that it should never be fully used, except in cases of unwanted endless recursion (hence, "stack overflow") or other unusual programming decisions.

A heap is a general term for anything that can be dynamically allocated.  Depending on which way you look at it, it is constantly changing size.  In modern processors and operating systems the exact way it works is very abstracted anyway, so you don't normally need to worry much about how it works deep down, except that (in languages where it lets you) you mustn't use memory that you haven't allocated yet or memory that you have freed.
What makes one faster?

The stack is faster because all free memory is always contiguous.  No list needs to be maintained of all the segments of free memory, just a single pointer to the current top of the stack.  Compilers usually store this pointer in a special, fast register for this purpose.  What's more, subsequent operations on a stack are usually concentrated within very nearby areas of memory, which at a very low level is good for optimization by the processor on-die caches.
(I have moved this answer from another question that was more or less a dupe of this one.)

The answer to your question is implementation specific and may vary across compilers and processor architectures. However, here is a simplified explanation.

Both the stack and the heap are memory areas allocated from the underlying operating system (often virtual memory that is mapped to physical memory on demand).
In a multi-threaded environment each thread will have its own completely independent stack but they will share the heap. Concurrent access has to be controlled on the heap and is not possible on the stack.
The heap

The heap contains a linked list of used and free blocks. New allocations on the heap (by new or malloc) are satisfied by creating a suitable block from one of the free blocks. This requires updating list of blocks on the heap. This meta information about the blocks on the heap is also stored on the heap often in a small area just in front of every block.
As the heap grows new blocks are often allocated from lower addresses towards higher addresses. Thus you can think of the heap as a heap of memory blocks that grows in size as memory is allocated. If the heap is too small for an allocation the size can often be increased by acquiring more memory from the underlying operating system.
Allocating and deallocating many small blocks may leave the heap in a state where there are a lot of small free blocks interspersed between the used blocks. A request to allocate a large block may fail because none of the free blocks are large enough to satisfy the allocation request even though the combined size of the free blocks may be large enough. This is called heap fragmentation.
When a used block that is adjacent to a free block is deallocated the new free block may be merged with the adjacent free block to create a larger free block effectively reducing the fragmentation of the heap.

The stack

The stack often works in close tandem with a special register on the CPU named the stack pointer. Initially the stack pointer points to the top of the stack (the highest address on the stack).
The CPU has special instructions for pushing values onto the stack and popping them back from the stack. Each push stores the value at the current location of the stack pointer and decreases the stack pointer.  A pop retrieves the value pointed to by the stack pointer and then increases the stack pointer (don't be confused by the fact that adding a value to the stack decreases the stack pointer and removing a value increases it. Remember that the stack grows to the bottom). The values stored and retrieved are the values of the CPU registers.
When a function is called the CPU uses special instructions that push the current instruction pointer, i.e. the address of the code executing on the stack. The CPU then jumps to the function by setting the 
instruction pointer to the address of the function called. Later, when the function returns, the old instruction pointer is popped from the stack and execution resumes at the code just after the call to the function.
When a function is entered, the stack pointer is decreased to allocate more space on the stack for local (automatic) variables. If the function has one local 32 bit variable four bytes are set aside on the stack. When the function returns, the stack pointer is moved back to free the allocated area.
If a function has parameters, these are pushed onto the stack before the call to the function. The code in the function is then able to navigate up the stack from the current stack pointer to locate these values.
Nesting function calls work like a charm. Each new call will allocate function parameters, the return address and space for local variables and these activation records can be stacked for nested calls and will unwind in the correct way when the functions return.
As the stack is a limited block of memory, you can cause a stack overflow by calling too many nested functions and/or allocating too much space for local variables. Often the memory area used for the stack is set up in such a way that writing below the bottom (the lowest address) of the stack will trigger a trap or exception in the CPU. This exceptional condition can then be caught by the runtime and converted into some kind of stack overflow exception.

  Can a function be allocated on the heap instead of a stack?

No, activation records for functions (i.e. local or automatic variables) are allocated on the stack that is used not only to store these variables, but also to keep track of nested function calls.

How the heap is managed is really up to the runtime environment. C uses malloc and C++ uses new, but many other languages have garbage collection.

However, the stack is a more low-level feature closely tied to the processor architecture. Growing the heap when there is not enough space isn't too hard since it can be implemented in the library call that handles the heap. However, growing the stack is often impossible as the stack overflow only is discovered when it is too late; and shutting down the thread of execution is the only viable option.
The Stack
When you call a function the arguments to that function plus some other overhead is put on the stack. Some info (such as where to go on return) is also stored there.
When you declare a variable inside your function, that variable is also allocated on the stack. 

Deallocating the stack is pretty simple because you always deallocate in the reverse order in which you allocate. Stack stuff is added as you enter functions, the corresponding data is removed as you exit them. This means that you tend to stay within a small region of the stack unless you call lots of functions that call lots of other functions (or create a recursive solution).

The Heap
The heap is a generic name for where you put the data that you create on the fly. If you don't know how many spaceships your program is going to create, you are likely to use the new (or malloc or equivalent) operator to create each spaceship. This allocation is going to stick around for a while, so it is likely we will free things in a different order than we created them. 

Thus, the heap is far more complex, because there end up being regions of memory that are unused interleaved with chunks that are - memory gets fragmented. Finding free memory of the size you need is a difficult problem. This is why the heap should be avoided (though it is still often used).

Implementation of both the stack and heap is usually down to the runtime / OS. Often games and other applications that are performance critical create their own memory solutions that grab a large chunk of memory from the heap and then dish it out internally to avoid relying on the OS for memory. 

This is only practical if your memory usage is quite different from the norm - i.e for games where you load a level in one huge operation and can chuck the whole lot away in another huge operation.

Physical location in memory
This is less relevant than you think because of a technology called Virtual Memory which makes your program think that you have access to a certain address where the physical data is somewhere else (even on the hard disc!). The addresses you get for the stack are in increasing order as your call tree gets deeper. The addresses for the heap are un-predictable (i.e implimentation specific) and frankly not important.
This helps:

Fixed size items (like an int and cls1 (the object pointer reference)) go on the stack 'cos we know their size.

[Note, I had used the phrase static items which was misleading 'cos some people where interpreting this to mean static variables. I have now changed the phrase to fixed size to be clearer.]

Objects (which vary in size as we update them) go on the heap 'cos we don't know their size.

To summarise, if you know the size of the variable (e.g. an int) it goes on the stack. If not, it goes on the heap. And objects always, always, always go on the heap.

More information can be found here:


and here: 


This article is the source of picture above: http://www.codeproject.com/Articles/76153/Six-important-NET-concepts-Stack-heap-value-types#Stack%20and%20Heap

but be aware it may contain some inaccuracies. 
Others have answered the broad strokes pretty well, so I'll throw in a few details.

Stack and heap need not be singular.  A common situation in which you have more than one stack is if you have more than one thread in a process.  In this case each thread has its own stack.  You can also have more than one heap, for example some DLL configurations can result in different DLLs allocating from different heaps, which is why it's generally a bad idea to release memory allocated by a different library.
In C you can get the benefit of variable length allocation through the use of alloca, which allocates on the stack, as opposed to alloc, which allocates on the heap.  This memory won't survive your return statement, but it's useful for a scratch buffer.
On windows making a huge temporary buffer that you don't use much of is not free. This is because the compiler will generate a stack probe loop that is called every time your function is entered to make sure the stack exists (because windows uses a single guard page at the end of your stack to detect when it needs to grow the stack, if you access memory more than one page off the end of the stack you will crash).  Example:
void myfunction()
   char big[10000000];
   // do something that only uses for first 1K of big 99% of the time.


Stored in computer RAM just like the heap.
Variables created on the stack will go out of scope and automatically deallocate.
Much faster to allocate in comparison to variables on the heap.
Implemented with an actual stack data structure.
Stores local data, return addresses, used for parameter passing
Can have a stack overflow when too much of the stack is used. (mostly from inifinite (or too much) recursion, very large allocations)
Data created on the stack can be used without pointers.
You would use the stack if you know exactly how much data you need to allocate before compile time and it is not too big.
Usually has a maximum size already determined when your program starts

Stored in computer RAM just like the stack.
Variables on the heap must be destroyed manually and never fall out of scope.  The data is freed with delete, delete[] or free
Slower to allocate in comparison to variables on the stack.
Used on demand to allocate a block of data for use by the program.
Can have fragmentation when there are a lot of allocations and deallocations
In C++ data created on the heap will be pointed to by pointers and allocated with new or malloc
Can have allocation failures if too big of a buffer is requested to be allocated.
You would use the heap if you don't know exactly how much data you will need at runtime or if you need to allocate a lot of data.
Responsible for memory leaks

int foo()
  char *pBuffer; //<--nothing allocated yet (excluding the pointer itself, which is allocated here on the stack).
  bool b = true; // Allocated on the stack.
    //Create 500 bytes on the stack
    char buffer[500];

    //Create 500 bytes on the heap
    pBuffer = new char[500];

   }//<-- buffer is deallocated here, pBuffer is not
}//<--- oops there's a memory leak, I should have called delete[] pBuffer;

The stack is a portion of memory that can be manipulated via several key assembly language instructions, such as 'pop' (remove and return a value from the stack) and 'push' (push a value to the stack), but also call (call a subroutine - this pushes the address to return to the stack) and return (return from a subroutine - this pops the address off of the stack and jumps to it).  It's the region of memory below the stack pointer register, which can be set as needed.  The stack is also used for passing arguments to subroutines, and also for preserving the values in registers before calling subroutines.

The heap is a portion of memory that is given to an application by the operating system, typically through a syscall like malloc.  On modern OSes this memory is a set of pages that only the calling process has access to.

The size of the stack is determined at runtime, and generally does not grow after the program launches.  In a C program, the stack needs to be large enough to hold every variable declared within each function.  The heap will grow dynamically as needed, but the OS is ultimately making the call (it will often grow the heap by more than the value requested by malloc, so that at least some future mallocs won't need to go back to the kernel to get more memory.  This behavior is often customizable)

Because you've allocated the stack before launching the program, you never need to malloc before you can use the stack, so that's a slight advantage there.  In practice, it's very hard to predict what will be fast and what will be slow in modern operating systems that have virtual memory subsystems, because how the pages are implemented and where they are stored is an implementation detail.  
I think many other people have given you mostly correct answers on this matter.

One detail that has been missed, however, is that the "heap" should in fact probably be called the "free store".  The reason for this distinction is that the original free store was implemented with a data structure known as a "binomial heap."  For that reason, allocating from early implementations of malloc()/free() was allocation from a heap.  However, in this modern day, most free stores are implemented with very elaborate data structures that are not binomial heaps.
To clarify, this answer has incorrect information (thomas fixed his answer after comments, cool :) ). Other answers just avoid explaining what static allocation means. So I will explain the three main forms of allocation and how they usually relate to the heap, stack, and data segment below. I also will show some examples in both C/CPP and Python to help people understand.

"Static" (aka statically allocated) variables are not allocated on the stack. Do not assume so - many people do only because "static" sounds a lot like "stack". They actually exist in neither the stack nor the heap. The are part of what's called the data segment.

However it is generally better to consider "scope" and "lifetime" rather than "stack" and "heap". 

Scope refers to what parts of the code can access a variable. Generally we think of local scope (can only be accessed by the current function) versus global scope (can be accessed anywhere) although scope can get much more complex.

Lifetime refers to when a variable is allocated and deallocated during program execution. Usually we think of static allocation (variable will persist through the entire duration of the program, making it useful for storing the same information across several function calls) versus automatic allocation (variable only persists during a single call to a function, making it useful for storing information that is only used during your function and can be discarded once you are done) versus dynamic allocation (variables whose duration is defined at runtime, instead of compile time like static or automatic).

Although most compilers and interpreters implement this behavior similarly in terms of using stacks, heaps, etc, a compiler may sometimes break these conventions if it wants as long as behavior is correct. For instance, due to optimization a local variable may only exist in a register or be removed entirely, even though most local variables exist in the stack. As has been pointed out in a few comments, you are free to implement a compiler that doesn't even use a stack or a heap, but instead some other storage mechanisms (rarely done, since stacks and heaps are great for this).

I will provide some simple annotated C code to illustrate all of this. The best way to learn is to run a program under a debugger and watch the behavior. If you prefer to read python, skip to the end of the answer :)

//statically allocated in the data segment when the program/DLL is first loaded 
//deallocated when the program/DLL exits
//scope - can be accessed from anywhere in the code
int someGlobalVariable;

//statically allocated in the data segment when the program is first loaded
//deallocated when the program/DLL exits
//scope - can be accessed from anywhere in this particular code file
static int someStaticVariable;

//"someArgument" is allocated on the stack each time MyFunction is called
//"someArgument" is deallocated when MyFunction returns
//scope - can be accessed only within MyFunction()
void MyFunction(int someArgument) {

    //statically allocated in the data segment when the program is first loaded
    //deallocated when the program/DLL exits
    //scope - can be accessed only within MyFunction()
    static int someLocalStaticVariable;

    //allocated on the stack each time MyFunction is called
    //deallocated when MyFunction returns
    //scope - can be accessed only within MyFunction()
    int someLocalVariable;

    //a *pointer* is allocated on the stack each time MyFunction is called
    //this pointer is deallocated when MyFunction returns
    //scope - the pointer can be accessed only within MyFunction()
    int* someDynamicVariable; 

    //this line causes space for an integer to be allocated in the heap
    //when this line is executed. note this is not at the begining of 
    //the call to MyFunction(), like the automatic variables
    //scope - only code within MyFunction() can access this space
    //*though this particular variable*
    //however, if you pass the address somewhere else, that code
    //can access it too
    someDynamicVariable = new int; 

    //this line deallocates the space for the integer in the heap
    //if we did not write it, the memory would be "leaked"
    //note a fundamental difference b/w the stack and heap
    //the heap must be managed. the stack is managed for us
    delete someDynamicVariable;

    //in other cases, instead of deallocating this heap space you 
    //might store the address somewhere more permanent to use later
    //some languages even take care of deallocation for you... but
    //always it needs to be taken care of at runtime by some mechanism

    //when the function returns, someArgument, someLocalVariable
    //and the pointer someDynamicVariable are deallocated.
    //the space pointed to by someDynamicVariable was already
    //deallocated prior to returning

//note that someGlobalVariable, someStaticVariable and 
//someLocalStaticVariable continue to exist, and are not 
//deallocated until the program exits

A particularly poignant example of why it's important to distinguish between lifetime and scope is that a variable can have local scope but static lifetime - for instance, "someLocalStaticVariable" in the code sample above. Such variables can make our common but informal naming habits very confusing. For instance when we say "local" we usually mean "locally scoped automatically allocated variable" and when we say global we usually mean "globally scoped statically allocated variable". Unfortunately when it comes to things like "file scoped statically allocated variables" many people just say... "huh???". 

Some of the syntax choices in c/cpp exacerbate this problem - for instance many people think global variables are not "static" because of the syntax shown below.

int var1; //has global scope and static allocation
static int var2; //has file scope and static allocation

int main() {return 0;}

Note that putting the keyword "static" in the declaration above prevents var2 from having global scope. Nevertheless, the global var1 has static allocation. This is not intuitive! For this reason, I try to never use the word "static" when describing scope, and instead say something like "file" or "file limited" scope. However many people use the phrase "static" or "static scope" to describe a variable that can only be accessed from one code file. In the context of lifetime, "static" always means the variable is allocated at program start and deallocated when program exits.

Some people think of these concepts as C/CPP specific. They are not. For instance, the Python sample below illustrates all three types of allocation (there are some subtle differences possible in interpreted languages that I won't get into here).

from datetime import datetime

class Animal:
    _FavoriteFood = 'Undefined' #_FavoriteFood is statically allocated

    def PetAnimal(self):
        curTime = datetime.time(datetime.now()) #curTime is automatically allocatedion
        print("Thank you for petting me. But it's " + str(curTime) + ", you should feed me. My favorite food is " + self._FavoriteFood)

class Cat(Animal):
    _FavoriteFood = 'tuna' #note since we override, Cat class has its own statically allocated _FavoriteFood variable, different from Animal's

class Dog(Animal):
    _FavoriteFood = 'steak' #likewise the Dog class gets its own static variable. Important to note - this one static variable is shared among all instances of Dog, hence it is not dynamic!

if __name__ == "__main__":
    whiskers = Cat() #dynamically allocated
    fido = Dog() #dynamically allocated
    rinTinTin = Dog() #dynamically allocated


    Dog._FavoriteFood = 'milkbones'

# output is
# Thank you for petting me. But it's 13:05:02.255000, you should feed me. My favorite food is tuna
# Thank you for petting me. But it's 13:05:02.255000, you should feed me. My favorite food is steak
# Thank you for petting me. But it's 13:05:02.255000, you should feed me. My favorite food is steak
# Thank you for petting me. But it's 13:05:02.255000, you should feed me. My favorite food is tuna
# Thank you for petting me. But it's 13:05:02.255000, you should feed me. My favorite food is milkbones
# Thank you for petting me. But it's 13:05:02.256000, you should feed me. My favorite food is milkbones

You can do some interesting things with the stack.  For instance, you have functions like alloca (assuming you can get past the copious warnings concerning its use), which is a form of malloc that specifically uses the stack, not the heap, for memory.

That said, stack-based memory errors are some of the worst I've experienced.  If you use heap memory, and you overstep the bounds of your allocated block, you have a decent chance of triggering a segment fault.  (Not 100%: your block may be incidentally contiguous with another that you have previously allocated.)  But since variables created on the stack are always contiguous with each other, writing out of bounds can change the value of another variable.  I have learned that whenever I feel that my program has stopped obeying the laws of logic, it is probably buffer overflow.
Simply, the stack is where local variables get created. Also, every time you call a subroutine the program counter (pointer to the next machine instruction) and any important registers, and sometimes the parameters get pushed on the stack. Then any local variables inside the subroutine are pushed onto the stack (and used from there). When the subroutine finishes, that stuff all gets popped back off the stack. The PC and register data gets and put back where it was as it is popped, so your program can go on its merry way.

The heap is the area of memory dynamic memory allocations are made out of (explicit "new" or "allocate" calls). It is a special data structure that can keep track of blocks of memory of varying sizes and their allocation status.

In "classic" systems RAM was laid out such that the stack pointer started out at the bottom of memory, the heap pointer started out at the top, and they grew towards each other. If they overlap, you are out of RAM. That doesn't work with modern multi-threaded OSes though. Every thread has to have its own stack, and those can get created dynamicly.
The stack is the memory set aside as scratch space for a thread of execution.  When a function is called, a block is reserved on the top of the stack for local variables and some bookkeeping data.  When that function returns, the block becomes unused and can be used the next time a function is called.  The stack is always reserved in a LIFO order; the most recently reserved block is always the next block to be freed.  This makes it really simple to keep track of the stack; freeing a block from the stack is nothing more than adjusting one pointer.

The heap is memory set aside for dynamic allocation.  Unlike the stack, there's no enforced pattern to the allocation and deallocation of blocks from the heap; you can allocate a block at any time and free it at any time.  This makes it much more complex to keep track of which parts of the heap are allocated or free at any given time; there are many custom heap allocators available to tune heap performance for different usage patterns.

Each thread gets a stack, while there's typically only one heap for the application (although it isn't uncommon to have multiple heaps for different types of allocation).

To answer your questions directly:  

  To what extent are they controlled by the OS or language runtime?

The OS allocates the stack for each system-level thread when the thread is created.  Typically the OS is called by the language runtime to allocate the heap for the application.

  What is their scope?

The stack is attached to a thread, so when the thread exits the stack is reclaimed.  The heap is typically allocated at application startup by the runtime, and is reclaimed when the application (technically process) exits.

  What determines the size of each of them?  

The size of the stack is set when a thread is created.  The size of the heap is set on application startup, but can grow as space is needed (the allocator requests more memory from the operating system).

  What makes one faster?

The stack is faster because the access pattern makes it trivial to allocate and deallocate memory from it (a pointer/integer is simply incremented or decremented), while the heap has much more complex bookkeeping involved in an allocation or free.  Also, each byte in the stack tends to be reused very frequently which means it tends to be mapped to the processor's cache, making it very fast.
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