mirror of
https://github.com/isometimes/rpi4-osdev
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44 lines
2.6 KiB
Markdown
44 lines
2.6 KiB
Markdown
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Writing a "bare metal" operating system for Raspberry Pi 4
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==========================================================
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Building on the RPi4 itself
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---------------------------
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It's possible (but not super-simple) to follow this tutorial on the Raspberry Pi without need for an additional build device.
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It will require you firstly to re-image your Pi to use the 64-bit Raspberry Pi OS (Beta):
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* Download a zipped _.img_ image file from the [64-bit image list](https://downloads.raspberrypi.org/raspios_arm64/images/), picking the newest update
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* Unzip it and use the [Raspberry Pi Imager](https://www.raspberrypi.org/software/) to write it to your SD card, selecting "Use custom" from the options and pointing it at your downloaded _.img_ file
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* Boot the Pi and follow the setup wizard to ensure you have a working Internet connection
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* Just for luck, run `sudo apt update`
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You'll then need to download a cross-compiler from the Arm website.
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What you're looking for is the current [AArch64 ELF bare-metal target (aarch64-none-elf)](https://developer.arm.com/-/media/Files/downloads/gnu-a/10.2-2020.11/binrel/gcc-arm-10.2-2020.11-aarch64-aarch64-none-elf.tar.xz). If this link is somehow broken, you can use Google to search for "Arm GNU-A linux hosted cross compilers".
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Then unpack the archive using `tar -xf _downloaded-filename_`. You'll end up with a _gcc_ directory (albeit with a slightly longer name), which itself contains a _bin_ subdirectory, wherein you'll find the _gcc_ executable (again - with a longer name!). Remember this path.
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Now let's build something:
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* Use `git` to clone this repo: `git clone https://github.com/isometimes/rpi4-osdev.git`
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* Decide which part you want to build - I like testing with _part5-framebuffer_ (it's visual, so you'll know when it works!)
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* Copy the _Makefile.gcc_ to _Makefile_
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* Edit the _Makefile_ and ensure the `GCCPATH` variable points to the _bin_ subdirectory that you just unpacked
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* Type `make` at the command line and it should build without errors
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If you want to then boot with this, you'll need to copy the _kernel8.img_ file to a prepped SD card as the tutorial discusses. For the purposes of testing this process, I did the following (NOTE: it will trash your OS install unless you backup the old files so you can move them back later):
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* `sudo cp kernel8.img /boot`
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* Then edit _/boot/config.txt_ to include only these lines (for _part5-framebuffer_ anyway, read the tutorial in full for any necessary config changes for other parts):
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```c
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hdmi_group=1
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hdmi_mode=16
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core_freq_min=500
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```
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Reboot and you should see the part5-framebuffer demo firing up!
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[Go to part1-bootstrapping >](./part1-bootstrapping/)
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