Zero to main(): How to Write a Bootloader from Scratch | Interrupt

[francois]

Hi Chandan,

Welcome to interrupt!

@chandan_bhatia

In session “Relocating our app from flash to RAM”, I believe this is not mandatory step. We can execute code from approm something like below. Correct ?

You are correct. I wanted to cover this technique, but it is not required or even appropriate for every project. Reasons you might want to relocate to RAM are:

1. It might be faster than ROM on some chips
2. Not every MCU supports executing from flash

@chandan_bhatia

Regarding “shared.h” text code, I believe we will have 2 copies of same code( .text) in bootrom and approm. Was thinking that Is there any way to make only 1 copy. Is this possible?

You are correct, you end up with two copies of that code. It is technically possible to use a single copy of the code shared between the app and the bootloader, but it is non trivial. In the past I have done this to avoid having two copies of libc (in bootloader & app). The trick is to link your app against your bootloader. Perhaps we will cover this in a future post.

@chandan_bhatia

During “start_app” definition you have added keyword " attribute ((naked)) ".Can you please tell reason of adding? If we don’t add, can it create issue?

You can find documentation for the naked attribute here. This attribute is used to tell the compiler it should not add a function prologue / epilogue. It is recommended when a function contains only inline assembly. In this case, omitting it likely would not cause problems.

@chandan_bhatia

Also facing one strange issue, I am getting linker script variables value completely wrong. For example for variable _shared_data_start, I am getting its value as “0x4b04d002” but this is not correct address. Any suggestion. I am using IDE STM32 workbench

I am not familiar with the STM32 workbench, but remember that you should be taking a reference to the variable. I would expect &_shared_data_start to be 0x20000300, but if you remove the & you won’t get the expected value.

Hope that helps!