Stacks and HeapsCS-502 Fall 2007 1
A Short DigressionStacks and Heaps
CS-502, Operating SystemsFall 2007
(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne and from Modern Operating Systems, 2nd ed., by Tanenbaum)
Stacks and HeapsCS-502 Fall 2007 2
Digression: the “Stack”
• Imagine the following program:–int factorial(int n){if (n <= 1)
return (1);else
int y = factorial(n-1);return (y * n);
}
• Imagine also the caller:–int x = factorial(100);
• What does compiled code look like?
Stacks and HeapsCS-502 Fall 2007 3
Compiled code: the caller
int x = factorial(100);
• Put the value “100” somewhere that factorial can find
• Put the current program counter somewhere so that factorial can return to the right place in caller
• Provide a place to put the result, so that caller can find it
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Compiled code: factorial function
• Save the caller’s registers somewhere• Get the argument n from the agreed-upon place• Set aside some memory for local variables and
intermediate results – i.e., y, n - 1
• Do whatever it was programmed to do
• Put the result where the caller can find it• Restore the caller’s registers• Transfer back to the program counter saved by the
caller
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Question: Where is “somewhere”?
• So that caller can provide as many arguments as needed (within reason)?
• So that called routine can decide at run-time how much temporary space is needed?
• So that called routine can call any other routine, potentially recursively?
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Answer: a “Stack”
• Stack – a linear data structure in which items are added and removed in last-in, first-out order.
• Calling program• Push arguments & return address onto stack• After return, pop result off stack
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“Stack” (continued)
• Called routine• Push registers and return address onto stack• Push temporary storage space onto stack
• Do work of the routine
• Pop registers and temporary storage off stack• Leave result on stack• Return to address left by calling routine
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Stack (continued)
• Definition: context – the region of the stack that provides the execution environment of a particular call to a function
• Implementation• Usually, a linear piece of memory and a stack
pointer contained in a (fixed) register• Occasionally, a linked list
• Recursion• Stack discipline allows multiple contexts for the
same function in the stack at the same time
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Stacks in Modern Systems
• All modern programming languages require a stack
• Fortran and Cobol did not (non-recursive)
• All modern processors provide a designated stack pointer register
• All modern process address spaces provide room for a stack
• Able to grow to a large size• May grow upward or downward
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Process Address Space (Typical)
0x00000000
0xFFFFFFFF
Virtual
address space
program code(text)
static data
heap(dynamically allocated)
stack(dynamically allocated)
PC
SP
See also Silbershatz, figure 3.1
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Stacks in Multi-threaded Environments
• Every thread requires its own stack• Separate from all other stacks• Each stack may grow separately• Address space must be big enough to accommodate
stacks for all threads
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Stacks in Multi-threaded Address Space
0x00000000
0xFFFFFFFF
Virtual
address space
code(text)
static data
heap
thread 1 stack
PC (T2)
SP (T2)thread 2 stack
thread 3 stack
SP (T1)
SP (T3)
PC (T1)PC (T3)
SP
PC
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Stacks in Multi-threaded Environments
• Every thread requires its own stack• Separate from all other stacks• Each stack may grow separately• Address space must be big enough to accommodate
stacks for all threads
• Some small or RT operating systems are equivalent to multi-threaded environments
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Heap
• A place for allocating memory that is not part of last-in, first-out discipline
• I.e., dynamically allocated data structures that survive function calls
• E.g., strings in C• new objects in C++, Java, etc.
Stacks and HeapsCS-502 Fall 2007 15
Process Address Space (Typical)
0x00000000
0xFFFFFFFF
Virtual
address space
program code(text)
static data
heap(dynamically allocated)
stack(dynamically allocated)
PC
SP
See also Silbershatz, figure 3.1
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Dynamically Allocating from Heap
• malloc() – POSIX standard function• Allocates a chunk of memory of desired size• Remembers size• Returns pointer
• free () – POSIX standard function• Returns previously allocated chunk to heap for
reallocation• Assumes that pointer is correct!
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Dynamically Allocating from Heap
• malloc() – POSIX standard function• Allocates a chunk of memory of desired size• Remembers size• Returns pointer
• free () – POSIX standard function• Returns previously allocated chunk to heap for
reallocation• Assumes that pointer is correct!
• Storage leak – failure to free something
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Heaps in Modern Systems
• Many modern programming languages require a heap
• C++, Java, etc.• NOT Fortran
• Typical process environment• Grows toward stack
• Multi-threaded environments• All threads share the same heap• Data structures may be passed from one thread to
another.
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Heap in Multi-threaded Address Space
0x00000000
0xFFFFFFFF
Virtual
address space
code(text)
static data
heap
thread 1 stack
PC (T2)
SP (T2)thread 2 stack
thread 3 stack
SP (T1)
SP (T3)
PC (T1)PC (T3)
SP
PC
Heap
Stacks and HeapsCS-502 Fall 2007 20
Stacks in Multi-threaded Address Space
0x00000000
0xFFFFFFFF
Virtual
address space
code(text)
static data
heap
thread 1 stack
PC (T2)
SP (T2)thread 2 stack
thread 3 stack
SP (T1)
SP (T3)
PC (T1)PC (T3)
SP
PC
What’s this?
Stacks and HeapsCS-502 Fall 2007 21
Questions?