Memory

This is a basic overview of the memory model in Mbed OS.

+---------------------+   Last address of RAM
| Scheduler/ISR stack |
+---------------------+
|          ^          |
|          |          |
|                     |
|      Heap cont.     |
|---------------------|
| User thread n stack |
|---------------------|
| User thread 2 stack |
|---------------------|
| User thread 1 stack |
|---------------------|
|          ^          |
|          |          |
|                     |
|        Heap         |
+---------------------+
|                     |
| ZI: Global data     |
|                     |
+---------------------+
| ZI: Idle stack      |
+---------------------+
| ZI: Timer stack     |
+---------------------+
| ZI: Main stack      |
+---------------------+
|                     |
| ZI: Global data     |
|                     |
+---------------------+
|     Optional        |
|  Crash-Data-RAM     |
|      region         |
+---------------------+
| RW: Vector table    |
+=====================+   First address of RAM
|                     |   Last address of flash
|                     |
|     Application     |
|                     |
|                     |
+---------------------+
|                     |
| Optional bootloader |
|                     |
+---------------------+
| RO: Vector table    |
+---------------------+   First address of flash

There are at least two kinds of memory in the system: flash and RAM.

RAM

Inside RAM, you can distinguish two logical types: static and dynamic memory. Static memory is allocated at compile time and, consequently, does not change size during runtime. Dynamic memory is allocated at runtime. For example, program memory use grows and shrinks as you fork and join threads, and construct and destruct objects. The system uses each of them in different ways:

• Static:
  • Vector table (read and write).
  • Crash data RAM. (Please see the error handling documentation for more information on this region.)
  • Global data.
  • Static data.
  • Stacks for default threads (main, timer, idle and scheduler/ISR).
• Dynamic:
  • Heap (dynamic data).
  • Stacks for user threads. Mbed OS dynamically allocates memory on heap for user thread stacks.

Stack checking is turned on for all threads, and the kernel errors if it detects an overflow condition.

Flash

Flash is a read-only memory (ROM) that contains:

• Vector table (read only).
• Application code.
• Application data.
• Optional bootloader.