It was based on FreeRTOS, but FreeRTOS at the time was extremely bare bones and only provided a preemptive scheduler, task management, and synchronization primitives. Everything else (memory management, I/O, ...) had to come either from whatever libc implementation was in use, or be built from scratch.

Thanks, does that mean early FreeRTOS is a good beginner project for OS study?

In my opinion it's not always the most readable codebase, due to some idiosyncratic style choices, but it definitely has the advantages of being small and focused.

Thank you!

Thanks found it,

https://github.com/google/pebble/blob/main/src/fw/kernel/pbl...

Which calls heap malloc:

https://github.com/google/pebble/blob/main/src/libutil/heap....