Set option 'arm_64bit' in config.txt

[legendlc]

When I worked on lesson1, I installed Raspbian Buster(release date 2019-09-26). There are 3 kernel image under boot partition: kernel.img(32bit older RPI), kernel7.img(32bit RPI2&3) and kernel7l.img(32-bit RPI4).

After I deleted kernel7.img, copy kernel8.img, modified config.txt according to the tutorial and boot my Pi 3b, no UART message was printed. However, when I deleted all kernel*.img except kernel8.img, it worked. I searched the Internet for a while and couldn't figure out why. Maybe the latest firmware has changed the boot sequence?

Then I saw the arm_64bit option in the official document.

arm_64bit

If set to non-zero, forces the kernel loading system to assume a 64-bit kernel, starts the processors up in 64-bit mode, and sets kernel8.img to be the kernel image loaded, unless there is an explicit kernel option defined in which case that is used instead. Defaults to 0 on all platforms. NOTE: 64-bit kernels must be uncompressed image files.

Note that the 64-bit kernel will only work on the Pi4, Pi3, and Pi2B rev1.2 boards with latest firmware.

I tried it and it worked well without deleting any other kernel*.img. Maybe setting arm_64bit option is a better choice for current firmware?

[s-matyukevich]

Thanks, @legendlc for reporting this.

I made a fix in this commit.

I prefer to add instructions to delete all 'kernel*img' files rather than use the arm_64bit option because I am not sure how this option is going to work with different versions of Raspberry Pi boards as well as different firmware versions. Deleting all kernel*.img files looks like a more reliable way to me. Also, default config.txt is used in a lot of different places (source of all lessons, exercises, translations, etc) and updating all of them could be painful