1

I've been experimenting with writing very basic operating system-like programs on the Arduino, and so far I have been running user-defined programs on a small virtual machine I wrote. This has been an interesting experience, but the VM runs on an assembly-like language and not C.

I want to be able to do something more. I would like to be able to write a core OS, and then install new programs, written in C. The core OS would get user input for which program to run, and then jump to that program.

Now, I know that I can do this trivially by writing the OS along with all of the programs into a single sketch. The programs are then just functions, and the OS just calls those functions. However, I want to do this with functions I've written after I've burned the main OS.

From what I can tell, this is just a bootloader with additional user functionality. However, I don't think that's quite right for two reasons:

First, I keep hearing that you can't write to progmem at runtime. But isn't that what a bootloader does? Which is correct?

Second, once a user program ends, I should return to the OS. As far as I understand it, on the ATMega, once the main function terminates it jumps into a no-operation function and that's that. I'd need it to jump back to the bootloader and continue the bootloader's operation. Is that even possible?

Improve this question
6
  • 1
    If this is what you want to do you have chosen the wrong MCU. Find something that can run from RAM, even if a bit inefficiently - competing ARM Cortex MCUs typically can. Also understand that this "C" code you speak of gets compiled to machine instructions before it runs - you can compile C to instructions for a virtual machine, too. – Chris Stratton Dec 8 '16 at 15:40
  • Yes, you're right, and I've got an M0 that I can play around with. However, I am using the AVR for a few reasons. First, I'm familiar with it. Second, because it doesn't just automatically have facilities to run like this I have to create them all myself. My goal with these projects is to learn HOW those facilities are developed, not to get anything useful out of them. So I chose a limited chip and made my own workarounds so that I understand how the more advanced chips do it all natively. – Michael Stachowsky Dec 8 '16 at 17:22
  • Your workaround might be an interesting puzzle, but it's not really how more capable chips would do it at all, but rather a workaround for the anachronistic limitations of the avr. – Chris Stratton Dec 8 '16 at 18:37
  • Seems you can fool the protection – Gerben Dec 8 '16 at 18:43
  • @ChrisStratton: yes, you're right. But doing it on my own means I get to make my own file system, loader style thing etc. However, I'm actually probably going to just avoid it all and use a Cortex now that I know they CAN execute from RAM. I guess it pays to do your homework – Michael Stachowsky Dec 8 '16 at 20:39
3

First, I keep hearing that you can't write to progmem at runtime. But isn't that what a bootloader does?

Indeed. My understanding is that non-bootloader code cannot write to the Flash, except on the smallest AVRs. You may need to check the datasheet of your target MCU.

Second, once a user program ends, I should return to the OS.

You could require for the user programs to be linked against a library you provide. Just implement void exit(int status) in that library and it will automatically override the version from avr-libc.

Improve this answer
1
  • Yes, a quick datasheet dive reveals the following: There are three sections in flash. Boot is a special section where code running can actively modify progmem. No-Read-While-Write is a section that does not allow for code to run while it is being updated (the CPU is halted). Read-While-Write does allow for code to run while it is being updated, BUT the code that runs can only run from the NRWW section. So it looks like I can write the OS to have an "installer" in BOOT, kernelspace in NRWW, and userspace in RWW. – Michael Stachowsky Dec 8 '16 at 14:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.