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Why Doesn’t FVP Initialize Unused Memory to Zero (e.g., 0xcfdfdfdfdfdfdfcf)?

Qingyu Ma

I have a question about FVP (Fixed Virtual Platform) and its memory initialization behavior. Specifically, I noticed that unused memory in FVP is not set to 0, but instead contains non-zero values like 0xcfdfdfdfdfdfdfcf.

For example, when reading a non-used memory area in FVP, the values appear as non-zero by default. This behavior can lead to issues, such as:

  • If the OS running on FVP does not explicitly clear the .bss section, global variables in the OS could end up initialized with these non-zero default values.
  • This, in turn, may cause unexpected behavior or bugs during execution.

Given that FVP operates in a virtualized memory environment, I’m wondering why it doesn’t simply initialize all memory to zero. Is there a specific design rationale or technical limitation for this behavior?

  • +1 Arm Employee Badge

    I'm not sure which specific FVP you are using, but enter the command:

    <fvp_executable> --list-params > params.txt


    Then search for "fill" therein. You should find lines similar to:

    bp.dram.fill1=3755990991 # (int , init-time) default = '0xdfdfdfcf' : Fill pattern 1, initialise memory at start of simulation with alternating fill1, fill2 pattern
bp.dram.fill2=3487555551 # (int , init-time) default = '0xcfdfdfdf' : Fill pattern 2, initialise memory at start of simulation with alternating fill1, fill2 pattern


    You can then launch the FVP with command options (-C) to change these values, or specify them in an options file (-f), for example:
    -C bp.dram.fill1=0

    See also https://developer.arm.com/documentation/100966/1127/Getting-Started-with-Fixed-Virtual-Platforms/Configuring-the-model

  • 0 in reply to Ronan Synnott

    Thanks you very much, I successfully clear the memory of the OS's bss section.

  • 0 in reply to Ronan Synnott

    I have tested these configurations one by one and confirmed that the memory region where my kernel put is seems to be controlled by bp.s_dram. However, I’m still unsure about why this is the case.

    Could it be related to the security settings of the memory regions? For example, I understand that "P security memory regions block access from both Secure and Non-secure domains." And my kernel is put in `P` memory region, Could it be that regions blocking Non-secure access are controlled by bp.s_dram? Actually I didn't find any description of what there fill params mean.

    UPDATE:

    I set bp.secrue_memory=0, and the three DRAM regions is still controlled by bp.s_dram.fill1 & fill2, so that may not associate with security of the memory region.