I am trying to understand how real time operating systems work. I have looked at the source codes of some RTOSes. I want to learn by creating my simple RTOS, something like FLIRT.
I am using PIC16 series and XC8 C compiler with MPLABX. I also want to implement this very simple RTOS to PIC12 series.
So, I decided that I should start by learning how to manipulate stack (like supercat did in this answer) and I started searching and came across AN818 which is titled "Manipulating the Stack of the PIC18 Microcontroller". Cited from the application note:
Traditionally, the microcontroller stack has only been
used as a storage space for return addresses of subroutines
or interrupt routines, where all ‘push’ and ‘pop’
operations were hidden.
For the most part, users had
no direct access to the information on the stack. The
PIC18 microcontroller diverges from this tradition
slightly. With the new PIC18 core, users now have
access to the stack and can modify the stack pointer
and stack data directly.
I am confused. How come there are RTOSes made for PIC microcontrollers that work with PIC16 cores? For example, OSA RTOS is available for PIC12/16 with mikroC compiler.
Can you direct me to some resources, or if possible give examples, so that I can learn about stack switching?