copyright 2013 – noah mendelsohn process memory noah mendelsohn tufts university email:...

Copyright 2013 – Noah Mendelsohn

Process Memory

Noah MendelsohnTufts University Email: [email protected]: http://www.cs.tufts.edu/~noah

COMP 40: Machine Structure and

Assembly Language Programming (Fall 2014)

mailto:[email protected]

http://www.cs.tufts.edu/~noah

© 2010 Noah Mendelsohn3

Sharing the Computer

© 2010 Noah Mendelsohn

Operating systems do two main things for us:

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• They make the computer easier to use

• The facilitate sharing of the computer by multiple programs and users


Sharing the Computer


MAIN MEMORY

CPU

Sharing Memory

Angry Birds Play Video Browser

Multiple ProgramsRunning at once

All programs share memory

OPERATING SYSTEM


MAIN MEMORY

CPU

Sharing the CPU

Angry Birds Play Video Browser

Multiple ProgramsRunning at once

OPERATING SYSTEM

CPU is shared…can only do one thing at a time*

*Actually, modern CPUs can do a few things at a time, but for nowassume a simple, traditional computer


ProcessesThe Fundamental Unit of Work


Processes

CPUMEMORY

Angry Birds Angry Birds Browser

Disk PrinterKeyboard,mouse,display

There is one OS “process” for each running copy of a program

The operating switches rapidly between them…giving the illusion they are all

running at once


Processes

CPUMEMORY

Angry Birds Angry Birds Browser

Disk PrinterKeyboard,mouse,display

The operating system uses special virtual memory

hardware to give each process the illusion of it’s own private memory


Process Memory


The process memory illusion

Process thinks it's running in a private space

Separated into segments, from address 0

Stack: memory for executing subroutines

Heap: memory for malloc/new

Global static variables

Text segment: where program lives

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Now we’ll learn how your program uses these!









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Stack

Text(code)

Static initialized

Static uninitialized

Heap(malloc’d)

argv, environ

Loaded with your program

0









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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0

Our systems use 48 bit addressing, but it turns out the “top” half of memory is for the

operating system.

Therefore the highest address you’ll see in your program is (in binary):

11111111111111111111111111111111111111111111111

Yes, that’s 47 ‘1’ bits…however









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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0


operating system.

Therefore the highest address you’ll see in your program is (in binary):

11111111111111111111111111111111111111111111111

Yes, that’s 47 ‘1’ bits…however

This is a good time for a first tutorial on “hex” numbering…


Brief interlude……writing numbers in hex (base 16)


Hexadecimal

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You already know base 10 and base 2, hex is just base 16

Base Digits

2 0,1

10 0,1,2,3,4,5,6,7,8,9

16 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,

A few simple examples:

16(dec) = 10(hex)19(dec) = 13(hex)32(dec) = 20(hex)256(dec) = 100(hex)

As a professional programmer you must be expert using hex!!

Hex conversion and arithmetic will be significant on the

midterm!


Conversions between hex and binary are trivial

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Consider the decimal number 538

Which happens to be 1000011010 (binary) and 21A (hex)

2 1 A

You get hex from binary by grouping the bits

…and that 48 bit address looks a lot better this way:

011111111111111111111111111111111111111111111111 (bin)7fffffffffff (hex)


Process Memory(continued)









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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0


operating system.

Therefore the highest address you’ll see in your program is (in hex):

7fffffffffff

7fffffffffff



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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0

char notInitialized[10000];char initialized[] = “I love COMP 40”;

int main(int argc, char *argvp[]*) { float f; int i;

// yes, we should check return codes char *cp = malloc(10000);}

7fffffffffff



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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0




7fffffffffff



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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0




Program file tells how much is needed

7fffffffffff



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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0


int main(int argc, char *argv[]) { float f; int i;


7fffffffffff









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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0

malloc/free are library functions that run in user mode to allocate space within the heap….

…when they run out of space to play with, they call the sbrk system call to ask the OS to grow

the heap.

7fffffffffff



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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0




7fffffffffff



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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0




7fffffffffff



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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0


int main(int argc, char *argv[]) { float f; int i;


7fffffffffff


int factorial(int n){ if (n == 0) return 1; else return n * factorial(n - 1);}

Of course, the stack enables recursion

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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0

n=4n=3n=2n=1

7fffffffffff


int factorial(int n){ if (n == 0) return 1; else return n * factorial(n - 1);}

Of course, the stack enables recursion

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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0

n=4n=3n=2

7fffffffffff









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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ


0

In Linux, there is a system call to grow the heap, but not the stack. When the system faults on an access to the next page below

the end of the stack, the OS maps more memory automatically.

(up to configurable limit).

7fffffffffff



Process thinks it's running in a private space.

Separated into segments, from address 0:

Text segment: where program lives.

Global variables.

Heap: memory for malloc/new.

Stack: memory for executing subroutines.

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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ

Nothing here

Memory between the stack and the heap is not mapped by the OS.

As far as the process is concerned, it doesn’t exist. Access causes a segfault.

7fffffffffff



Process thinks it's running in a private space.

Separated into segments, from address 0:

Text segment: where program lives.

Global variables.

Heap: memory for malloc/new.

Stack: memory for executing subroutines.

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Stack

Text(code)

Static initialized


Heap(malloc’d)

argv, environ

Kernel

Surprisingly: the kernel actual lives in every process space at the “top”.

The maps are set so ordinary user code segfaults on access, but…

…when in the kernel executing a system call, the sytem can access its own kernel

segments as well as the user’s.

7fffffffffff

copyright 2013 – noah mendelsohn process memory noah mendelsohn tufts university email:...

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