After the 05 material (05-qemu, 05-versatile and 05-jiffies) I ask you
to think about the following items:

1- In order to properly allocate a stack size and check for possible
overflows, we should devise a way to check stack usage.
Suggest a way to track stack usage and/or detect overflow.

2- boot.S is "ugly", as we would like to have everything in C.
Is it possible to replace it with boot.c?  Try to write boot.c
and evaluate why it does or doesn't work.