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<H2><A NAME="s10">10. Useful Tips</A></H2>
<H2><A NAME="ss10.1">10.1 Stack and Heap</A>
</H2>
<H3>Overview</H3>
<P>Here we view how "stack" and "heap" are allocated in memory
<P>
<H3>Memory allocation</H3>
<P>
<PRE>
FF.. | | <-- bottom of the stack
/|\ | | |
higher | | | | stack
values | | | \|/ growing
| |
XX.. | | <-- top of the stack [Stack Pointer]
| |
| |
| |
00.. |_________________| <-- end of stack [Stack Segment]
Stack
</PRE>
<P>Memory address values start from 00.. (which is also where Stack
Segment begins) and they grow going toward FF.. value.
<P>
<P>XX.. is the actual value of the Stack Pointer.
<P>
<P>Stack is used by functions for:
<P>
<P>
<OL>
<LI>global variables </LI>
<LI>local variables</LI>
<LI>return address
</LI>
</OL>
<P>For example, for a classical function:
<P>
<P>
<PRE>
|int foo_function (parameter_1, parameter_2, ..., parameter_n) {
|variable_1 declaration;
|variable_2 declaration;
..
|variable_n declaration;
|// Body function
|dynamic variable_1 declaration;
|dynamic variable_2 declaration;
..
|dynamic variable_n declaration;
|// Code is inside Code Segment, not Data/Stack segment!
|return (ret-type) value; // often it is inside some register, for i386 eax register is used.
|}
we have
| |
| 1. parameter_1 pushed | \
S | 2. parameter_2 pushed | | Before
T | ................... | | the calling
A | n. parameter_n pushed | /
C | ** Return address ** | -- Calling
K | 1. local variable_1 | \
| 2. local variable_2 | | After
| ................. | | the calling
| n. local variable_n | /
| |
... ... Free
... ... stack
| |
H | n. dynamic variable_n | \
E | ................... | | Allocated by
A | 2. dynamic variable_2 | | malloc & kmalloc
P | 1. dynamic variable_1 | /
|_______________________|
Typical stack usage
Note: variables order can be different depending on hardware architecture.
</PRE>
<H2><A NAME="ss10.2">10.2 Application vs Process</A>
</H2>
<H3>Base definition</H3>
<P>We have to distinguish 2 concepts:
<P>
<P>
<UL>
<LI>Application: that is the useful code we want to execute</LI>
<LI>Process: that is the IMAGE on memory of the application (it depends
on memory strategy used, segmentation and/or Pagination).
</LI>
</UL>
<P>Often Process is also called Task or Thread.
<P>
<H2><A NAME="ss10.3">10.3 Locks</A>
</H2>
<H3>Overview</H3>
<P>2 kind of locks:
<P>
<P>
<OL>
<LI>intraCPU</LI>
<LI>interCPU
</LI>
</OL>
<H2><A NAME="ss10.4">10.4 Copy_on_write</A>
</H2>
<P>Copy_on_write is a mechanism used to reduce memory usage. It
postpones memory allocation until the memory is really needed.
<P>
<P>For example, when a task executes the "fork()" system call (to
create another task), we still use the same memory pages as the
parent, in read only mode. When a task WRITES into the page, it causes
an exception and the page is copied and marked "rw" (read, write).
<P>
<P>
<PRE>
1-) Page X is shared between Task Parent and Task Child
Task Parent
| | RO Access ______
| |---------->|Page X|
|_________| |______|
/|\
|
Task Child |
| | RO Access |
| |----------------
|_________|
2-) Write request
Task Parent
| | RO Access ______
| |---------->|Page X| Trying to write
|_________| |______|
/|\
|
Task Child |
| | RO Access |
| |----------------
|_________|
3-) Final Configuration: Either Task Parent and Task Child have an independent copy of the Page, X and Y
Task Parent
| | RW Access ______
| |---------->|Page X|
|_________| |______|
Task Child
| | RW Access ______
| |---------->|Page Y|
|_________| |______|
</PRE>
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