Today, I get a chance to be THAT senior who everyone turned to for help at their college. The Lecture on Embedded lInux that I am supposed to deliver starts in 90 mins lads, and I am nervous as fuck when it comes to this. Because this is a chance for me to introduce my students to concepts and topics that their college keeps telling them are "way too advanced" for them to learn. After the general pleasantries and the introduction about linux and how it's actually NOT an OS and is just a kernel; and the customary differences between Desktop Linux and Embedded Linux, here is what I plan to talk about. I am walking then through the journey of booting Linux on a Single Board Computer (SBC). The board of choice - a BeagleBone Black. Based around a Texas Instrument's AM335x Processing unit, it follows a 3-stage booting process. Stage 1 has the BootROM, SPL, and the MLO. The BootROM is what runs when you power on the board. It brings the board to a point, where it effectively scans 3 different address points from where the next stage of the booting process shall arrive from. Those can be USB, SDCard, the on-board eMMC, or even network booting too! In secure boots, it also scans the incoming images and performs authentication on them BEFORE letting it run. Then, the SPL (Secondary Program Loader). This piece runs on the SRAM of the processor, and it's only job is to ensure that the much bigger DRAM (or DDR) is initialized, and ready to pick up the next batch of images called the MLO. This is necessary because the SRAM may not be big enough for the incoming images, and proper, flawless init of the DRAM is important for the boot to happen. Imagine your un-initialized / unverified DRAM bricking DURING your boot process - it can very well be rendered unrecoverable. The SPL is then discarded once it runs. Finally, we have the MLO or Memory LOader. This includes a basic U-boot, zImage, and the Device Tree Binary for the processor. These binary files are loaded onto the memory pre-emptively for the kernel. When the kernel init occurs, it uses these files to initialize what is essentially the configured hardware. Stage 2 has the U-boot proper and the kernel itself. The stage 1 ensured that all hardware and memory elements are initialized and ready. This stage, is where the bootloader (often U-Boot) and the kernel itself are loaded onto the memory of the board. First, we have the U-Boot. It loads the kernel onto the memory, the DTB onto the memory, and holds env variables and scripts. It enables the storage and network accesses for the device, and tells the kernel where to find the rootFS. The kernel itself, holds code that controls the CPU subsystems, the memory management unit's control ssoftware, the scheduler, init controller, device drivers, and then the specific drivers from the DTB - along with so much more. At this point, the SBC is all initialized, memory elements holding all pieces of software, CPU fully up and running, network interfaces ready to be accessed - all that is missing, is the link between the user and these interfaces to interact. What might that be? Stage 3 - the RootFS. This is the link that ties the user to the world of the SBC. It holds the Directories, the app data, package managers, dev tools, userspace tools, the Window manager and the Desktop environment, and quite literally anything else that we can think of, when we imagine a linux based system. If you are still reading this, I shall conclude today's lecture with implementations of the communication protocols that are commonly seen in Microcontrollers. A very short look at the UART devices - dev/tty, the SPI devices - dev/spidev, and the I2C devices - dev/i2c-0(or 1). This is supposed to be an hour long lecture, but I don't think they'll have an issue if I take a bit longer than that! Finally, I shall leave them with books, and links to additional resources to explore more - should their hearts wish for. All in all - this is about to be blast for me, and I shall do my best to spark a passion for this in my students too, so that they can learn on their own, and do some cooler things than what I could in my college days. If you have read so far - I can only thank you for listening to my ramblings. Now back to your regularly scheduled scrolling!