How to write a bootloader from scratch (2019)(interrupt.memfault.com)
interrupt.memfault.com
How to write a bootloader from scratch (2019)
https://interrupt.memfault.com/blog/how-to-write-a-bootloader-from-scratch
19 comments
I'm a bit jealous of the fact they have such good blog posts. I did a deep dive on a couple of different architectures and always wanted to write something so nice and useful for other people with the stuff I know.
Related:
From zero to main(): How to write a bootloader from scratch - https://news.ycombinator.com/item?id=24635383 - Sept 2020 (44 comments)
From zero to main(): How to write a bootloader from scratch - https://news.ycombinator.com/item?id=24635383 - Sept 2020 (44 comments)
"Zero to main" is the best resource there is about writing bootloaders. Wrote my first boatloader with the help of this series.
The CMSIS-DAP adapter referenced in the tutorial is discontinued. Can one of you hardware pros point me to a suitable replacement?
Also asked the author here: https://mobile.twitter.com/kaycebasques/status/1585708384531...
Also asked the author here: https://mobile.twitter.com/kaycebasques/status/1585708384531...
Author here. It's surprising how hard CMSIS-DAP adapters are to find. The two options shared by codys look reasonable, but I'm working on putting together a better option.
Other CMSIS-DAP compatible probes:
- https://makerdiary.com/products/pitaya-link
- https://www.l-tek.com/web-shop/cmsis-dap-debug-probe/
- https://makerdiary.com/products/pitaya-link
- https://www.l-tek.com/web-shop/cmsis-dap-debug-probe/
We use these in production because the IO signals are buffered: https://www.artekit.eu/products/debug/ak-cmsis-dap/
I'm using https://github.com/probe-rs/hs-probe most of the time, sometimes I get out a jlink
(For a Cortex-M based microcontroller)
Yes, an important qualifier! In practice the vast majority of embedded projects out there are on cortex M, with a bit of xtensa (ESP32) and a bit of RISC-V.
and 32-Bit microprocessors are still only about 1/3 of total microprocessors by volume (source https://www.ednasia.com/8-bit-mcu-still-alive-and-kicking/)
(n00b q) are there big differences for Cortex-A and Cortex-M bootloaders? Any helpful tutorials for Cortex-A if so?
Yes. DRAM initiazization, MMU, exception levels, trusted-firmware, initialization of specialized coprocessors for system power states management, more complicated storage access options (SD card/eMMC), etc. etc.
Most of those aren't Cortex-A specific, you've just listed mostly peripherals that are typically initialized in the application space (except for a secure boot).
E.g. you aren't going to go into power savings mode during boot, nor are you going to initialize peripherals (because their contexts will be lost once you jump to the application).
Initializing assymetric processors can be done in application space with a variety of tools (AMP) as well. I haven't seen a need to initialize them during bootloader1 phase, but there are lot of use cases.
E.g. you aren't going to go into power savings mode during boot, nor are you going to initialize peripherals (because their contexts will be lost once you jump to the application).
Initializing assymetric processors can be done in application space with a variety of tools (AMP) as well. I haven't seen a need to initialize them during bootloader1 phase, but there are lot of use cases.
Majority of Cortex-A cores are in complex SoCs, where you have to do all of the above and more if you want to boot something like Linux.
What are you talking about? The MCU is the first thing to boot. Wait, do you not know what a bootloader is? It's not GRUB.
Yes, I wrote one bootloader for a SoC, and read through several other bootloaders to be able to make modifications to them. I've also reverse engineered several binary only bootloaders for undocumented MCUs to be able to write sane Linux based flashing tools for them.
Kinda know what bootloader is. And, while I don't care about the naming, I know several people who would vehemently disagree with you that grub is not a bootloader. They'd say that only grub is a bootloader, and anything else running prior to that is platform firmware.
Cortex-A is not typically used in MCUs, so I don't see what you mean by "The MCU is the first thing to boot".
Kinda know what bootloader is. And, while I don't care about the naming, I know several people who would vehemently disagree with you that grub is not a bootloader. They'd say that only grub is a bootloader, and anything else running prior to that is platform firmware.
Cortex-A is not typically used in MCUs, so I don't see what you mean by "The MCU is the first thing to boot".
Cortex-A appears to be way more vendor-implementation dependent. For example, I remember reading that the Raspberry Pi SoC, which uses a Cortex-A72 core, actually boots off its GPU.
Even if you regularly write embedded code for Cortex-M MCUs, scroll through their list of posts and you'll find several that will teach you something new and useful.