How can chainloader jump to the new location? about rust-raspberrypi-os-tutorials HOT 2 CLOSED

The key part is that the GPU will load the binary from the SD card to 80_000 no matter what. And it will start executing code from there.

The chain loader binary, therefore, will run its first instructions from 0x80_000, despite having been linked for 0x2000000.

That is not an issue though, because we compiled the chain loader binary as position independent code (PIC). One of the first things the chain loader does is to then copy itself from the 0x80_000 address to the 32 MiB address, and then continue execution from there. This way we can safely put the binary that comes in over UART at the standard 0x80_000 because we do not execute from there anymore and as such do not overwrite the currently executing chain loader binary.

I’m summary, the load address in the linker script is in this case first and foremost only used to retrieve where to relocate the chain loader during runtime.

Hope that helps.

from rust-raspberrypi-os-tutorials.

Thanks for the super fast answer, and sorry for my super late reply! Indeed it was very useful, in my noobness I didn't know about the PIC option.

from rust-raspberrypi-os-tutorials.