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I am in uboot and was wondering, how could I tell what RAM address range is being used by uboot.

uboot needs memory to run so it might be using some of the RAM. I want to avoid modifying those RAM addresses.

How do I know which region of memory is uboot loaded?

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2 Answers 2

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The DULG DebuggingUBoot page has this to say (the "relocation" it speaks of is copying itself from flash to RAM).

For debugging U-Boot after relocation we need to know the address to which U-Boot relocates itself to. When no exotic features like PRAM are used, this address usually is - CONFIG_SYS_MONITOR_LEN. In our example with 16MB RAM and CONFIG_SYS_MONITOR_LEN = 192KB this yields the address 0x1000000 - 0x30000 = 0xFD0000.

Further reading of the text seems to indicate it's dependent on the processor or board and that you may have to check the U-Boot source to know for sure.

Regarding the Guruplug:

On the RAM side, u-Boot has the first 8 megabytes reserved. The rest is free. Some developer’s will load kernels and filesystems at 0×800000 for programming to flash. Another common spot is at the 100 megabyte boundary (or offset 0×640000).

Depending on your version of U-Boot commands may be available to put a string somewhere in free RAM and then hunt the rest of RAM for that string, revealing the approximate location of U-Boot in memory.

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When I start the version of uboot that I am using, it automatically displays a "virtual kernel memory layout".

Memory: 859068k/859068k available, 25668k reserved, 0K highmem
Virtual kernel memory layout:
    vector  : 0xffff0000 - 0xffff1000   (   4 kB)
    fixmap  : 0xfff00000 - 0xfffe0000   ( 896 kB)
    DMA     : 0xf9e00000 - 0xffe00000   (  96 MB)
    vmalloc : 0xe0800000 - 0xf4000000   ( 312 MB)
    lowmem  : 0x80000000 - 0xe0000000   (1536 MB)
    pkmap   : 0x7fe00000 - 0x80000000   (   2 MB)
    modules : 0x7f000000 - 0x7fe00000   (  14 MB)
      .init : 0x80008000 - 0x8004d000   ( 276 kB)
      .text : 0x8004d000 - 0x808ad000   (8576 kB)
      .data : 0x808ce000 - 0x80937a40   ( 423 kB)

I found it was safe to use the lowmem area for various utility processes I wanted to run (like loading large files into memory). Ideally, I think you want to use high memory for that kind of thing, but I didn't have the option (as you can see in my output).

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  • 1
    Are you sure that's not Linux's memory map?
    – claymation
    Commented Apr 5, 2017 at 20:37
  • I'm not certain. That lack of highmem and the reference to "Virtual kernel" leads me to think otherwise. I'm a noob/hack when it comes to uboot. The important point here was that I was able to safely use that lowmem range without writing over anything that the uboot seemed to require.
    – BuvinJ
    Commented Apr 5, 2017 at 20:48
  • As I recall, this is displayed prior to the loading the OS btw.
    – BuvinJ
    Commented Apr 5, 2017 at 20:50

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