Why image address is 82000000 and entry point 80008000?

jose · December 15, 2023, 10:01pm

Hi,
I am new in BBB world. I cannot find any explanation explaining why the image is kept in address 82000000 and the entry point is 80008000
My guess is that AM3358 expects the image to be 82000000 and the entry point must have 8000 of offset but I cannot really find any doc with that info.
Does anyone know why these 2 addresses have to be used.

thanks

jose · December 15, 2023, 11:23pm

I guess am335x-boneblack-common.dtsi seems to say 0x80000000 is where DDR memory is mapped but that is as close as I have gotten so far.

RobertCNelson · December 15, 2023, 11:45pm

The omap2 family (which the am335x of the bbb is part of)… Memory starts at 0x80000000…

As for why certain address are used, take a look at this section of u-boot, where much though has been put in place for location of items…

github.com

u-boot/u-boot/blob/master/include/configs/ti_armv7_common.h#L19-L46


      
          /*
           * We setup defaults based on constraints from the Linux kernel, which should
           * also be safe elsewhere.  We have the default load at 32MB into DDR (for
           * the kernel), FDT above 128MB (the maximum location for the end of the
           * kernel), and the ramdisk 512KB above that (allowing for hopefully never
           * seen large trees).  We say all of this must be within the first 256MB
           * as that will normally be within the kernel lowmem and thus visible via
           * bootm_size and we only run on platforms with 256MB or more of memory.
           *
           * As a temporary storage for DTBO blobs (which should be applied into DTB
           * blob), we use the location 15.5 MB above the ramdisk. If someone wants to
           * use ramdisk bigger than 15.5 MB, then DTBO can be loaded and applied to DTB
           * blob before loading the ramdisk, as DTBO location is only used as a temporary
           * storage, and can be re-used after 'fdt apply' command is done.
           */
          #define DEFAULT_LINUX_BOOT_ENV \
          	"loadaddr=0x82000000\0" \
          	"kernel_addr_r=0x82000000\0" \
          	"fdtaddr=0x88000000\0" \
          	"dtboaddr=0x89000000\0" \

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Regards,

jose · December 17, 2023, 9:18am

Thanks Robert,
I also saw that in the dts.
memory@0x80000000 {
device_type = “memory”;
reg = <0x80000000 0x20000000>; /* 512 MB */
};

so memory physical address is 0x80000000 and the size 0x20000000 that is 512 MiB as it was confirmed by your header file.
I hope it starts to sink in my head )

thanks
José