UART minimal files and config on SD-Card, e.g. gpu_mem=??? about rpi4-osdev HOT 5 CLOSED

Have fun and best wishes for it! :) I actually started with an x86 minimal OS before I bought the Pi. So much more enjoyable on AArch64 though, I must say...

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Thanks for the feedback. I must confess that I never looked to minimise the set of files on the SD card (but I am intrigued now, so will look into it). I simply stuck with what the Raspberry Pi Imager put on there and just replaced kernel8.img with my own. I expect you'll need to at least add fixup4.dat to your list.

As for config.txt, mine looks like this and the UART runs just fine:

hdmi_group=1
hdmi_mode=16
core_freq_min=500

You shouldn't need a gpu_mem line at all. Are you running on a standard Raspberry Pi 4 Model B?

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Hi, thanks for your answer. It is a "normal" Pi4 B with 4GB ram. Maybe there are early Hardware versions and I use a different/newer EEPROM (recovery.img), start4.elf, fixup4.elf and DTB file.

The miniuart example really works without fixup4.elf on my pi and I really need exact this gpu_mem line. It is ugly, that these elf files coming from Broadcom are closed source. I want to understand the whole boot process and that is quiet hard.

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Yes - it's annoying that we can't strip right back... It stems from the fact that when the Raspberry Pi powers on, the ARM CPU is off and the GPU is on.

The GPU runs the first bootloader, which is held in ROM on the SoC (System on Chip). This is similar to the BIOS in conventional PCs.

This bootloader reads the SD card and loads the second bootloader from bootcode.bin.

The second bootloader then reads the GPU firmware, also from the SD card, named start.elf.

Finally, start.elf reads kernel.img and allows the ARM CPU to execute it.

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The hardware is open enough to explain some base OS techniques to students. That's why I try to migrate a minimalistic BIOS x86 32bit based OS (c++ and some asm) to aarch64 (primary the Pi4). I think the PL0/1 and paging instead of Ring1/0 and "sections and GDT" will be the next big trouble in my tasklist ;-D

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