Memory management

ws# Memory Management

Memory Segmentation Schema

            +------------------+
            |                  |
    ⌈       |  Code Segment    |    Binary of source code
 Static     |                  |
 Memory     +------------------+
 Layout     |                  |
    ⌊       |  Data Segment    |    Static, Global variables & Constants
            |                  |
            +------------------+
            |                  |
    ⌈       |  Heap Segment    |    Dynamic variable size allocation
 Dynamic    |                  |
 Memory     +------------------+
 Layout     |                  |
    ⌊       |  Stack Segment   |    Local variables & functions
            |                  |
            +------------------+

Data Segment

• Initialized (data)
• Uninitialized (bss)
• Read-Only Section
• Read-Write Section

+--------------------+
|   Code Segment     |
|                    |
|   Instructions     |
|   of the Program   |
|                    |
+--------------------+
|   Data Segment     |
|                    |
| Initialized Data   |   <-- .data section / Modifiable Data (R.W.)
| (static, globals,  |
|          e.g.)     |
| Constants          |   <-- .rodata section (or within .data)
|                    |
+ -   -   -   -   -  +
|                    |
| Uninitialized      |   <-- .bss section / Zeroed Data
|   Data (zeroed)    |
| (globals without   |
|  explicit values)  |
|                    |
+--------------------+
|                    |
|   Heap Segment     |
|                    |
|  (Dynamic Memory)  |
|                    |
+--------------------+
|   Stack Segment    |
|                    |
|   Local Variables  |
|   and Functions    |
|                    |
+--------------------+

Share Memory Structure For a Thread

+--------------------+
|    ...             |
|    Code     ┑      |
|    Data  section   |
|    Heap     ┙      |
|    as same as      |
|    above           |
|    ...             |
+--------------------+
|   Stack (Thread 1) |      Share the stack segment
|                    |
|   Local Variables  |
|   and Functions    |
|                    |
+--------------------+
|   Stack (Thread 2) |
|                    |
|   Local Variables  |
|   and Functions    |
|                    |
+--------------------+
|   Stack (Thread 3) |
|                    |
|   Local Variables  |
|   and Functions    |
|                    |
+--------------------+

Memory Section by Threads

Memory Region	Shared	Private
Code Segment (Text)	Yes	No
Data Segment (Initialized global/static variables) BSS (Uninitialized global/static variables)	Yes	No
Heap	Yes	No
Mapped Memory	Yes	No
Thread-Local Storage (TLS)	No	Yes
Stack	No	Yes
Registers (CPU)	No	Yes

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External Resources

• Memory Management in Operating System
• Memory Layout of C Programs
• Memory Segmentation in 8086 Microprocessor
• C Memory Division Text (code segment), Data and BSS
• Memory Management on C programming
• Memory Layout in C

How Does C Code Become Assembly

The compiler does a lot of stuff to translate C code to assembly

• Choose assembly instructions to implement C oeprations
• Implement C conditionals and loops using jumps and branches
• Choose registers and memory locations to store data
• Move data among the registers and memory to satifisy dependecies
• Coordinate function calls
• Try to make the assembly fast.