Re: [distortos-development] Newbie getting started

[Kamil S. <dis...@di...>]

On Sat, 2018-02-03 at 15:09 -0500, Trampas Stern wrote:
> I was looking through some FreeRTOS documentation on how the Cortex M
> processors work with RTOSs.  I personally like the ground up approach
> to learning new things...   I was hoping that maybe you had a blog
> going through the ground up on DISTOREC?
> 
> I will go through and see if I can figure it out either way...

Generally all RTOSes work (more or less) the same. The "mechanics" is
mostly identical - PendSV is used for context switching and SysTick is
used as periodic timer. If anything wants to force a context switch, it
has to trigger PendSV interrupt. There may be some minor differences in
this particular aspect, but the general concept of context switching is
pretty much identical in any RTOS. It is not even very specific to ARM
Cortex-M - obviously the tiny details (assembly instructions,
interrupts used, registers that need to be saved, ...) will be
different, but the approach (push registers to stack, save stack
pointer of thread "A", load stack pointer of thread "B", pop registers
from stack) is still identical.

Unless you are interested in something other than context switching?
This definitely is one of a very few architecture-specific things in an
RTOS. Huge majority of code is architecture-agnostic - it can be seen
by how little code is in the architecture folder (at least I think
there's "little" (; ).

https://github.com/DISTORTEC/distortos/tree/master/source/architecture/ARM/ARMv6-M-ARMv7-M

If you ignore the stuff which is required for implementation of POSIX
signals (which would be SVC interrupt, supervisorCall and
requestFunctionExecution), all that is left is:
- code for context switching (PendSV),
- code to request context switching,
- code related to periodic tick interrupt (SysTick and
startScheduling),
- code to initialize thread's stack,
- various low-level initializers (including reset handler),
- code to enable, disable and restore interrupt masking (used for
critical sections),
- function which tells whether you are in interrupt context (used in
various integrity checks).

Please let me know if you have some specific questions, as the topic
you mention would nicely fill a small book, so the scope is a bit too
large for an e-mail (;

Regards,
KSz