17.4.5. Setting top of memory and stack heap values

This example demonstrates how you can set permanent top of memory and stack heap values for a given target using your board file. It shows how to do this by updating the information in the Advanced_Information block. After you have defined the settings, they are used whenever you connect to the target with RealView Debugger.

This example uses the MCUeval board with a core module, but the procedure for amending the settings is the same for any target.

The top_of_memory variable is used to enable the semihosting mechanism to return the top of stack. You can create your own settings to specify the bottom of the stack address, the size of the stack, the bottom of the heap address, and the size of the heap. If you do not set these values manually, RealView Debugger uses the target-dependent defaults. If your application is scatterloaded, you must define the stack and heap limits.


The value of top_of_memory must be higher than the sum of the program base address, program code size, and program data size. If set incorrectly, the program might crash because of stack corruption or because the program overwrites its own code.

There is no requirement that the top of memory address is at the true top of memory. A C or assembler program can use memory at higher addresses.

The default value of top_of_memory for ARM processors is 0x20000. You can set the value of top_of_memory in a BOARD description of the target or in the CONNECTION you use to connect to the target.

To set the top of memory and stack heap values in the CONNECTION:

  1. Ensure that RealView Debugger is not connected to a target.

  2. Expand the following entries of the selected board group:


    2. Advanced_Information

    3. Default

    4. ARM_config

  3. Set the value of Top_memory, in the right pane, as required. For example, set it to 0x40000 if your target has 256KB of RAM starting at location 0.


    Be sure to specify a value that is supported by your debug target.

    When you load a program, the debugger sanity-checks top_of_memory by checking that the words below top_of_memory are writable. It issues a warning if they are not. However, your program might require much more RAM than the debugger checks for.

  4. Double-click on the Stack_Heap group, in the right pane, to display the contents (Figure 17.25).

    Figure 17.25. Settings in the Stack_Heap group

    Settings in the Stack_Heap group

    This shows the currently selected size and location of the stack and heap. A blank or zero Heap_base value is modified by the ARM C library runtime code, setting it to the address of the end of program data space (Figure 17.26).

    Figure 17.26. Relating top_of_memory to single section program layout

  5. If you require more control over the stack and heap location for a semihosted program, set the values as required.

    If your application requires control over the stack and heap location, or if the application is scatterloaded, the application must include a user-defined function, __user_initial_stackheap, that defines the stack and heap limits.

  6. Select File → Save and Close to close the Connection Properties window.

Whenever you load a program compiled with the standard ARM C library to this target, the top of memory, stack, and heap values you have set are used. For details on how to connect, see Connecting to a target.

Copyright © 2003, 2004 ARM Limited. All rights reserved.ARM DUI 0234B