How do OSes Handle context switching?

freertos.org. The entire OS is open source, and right there for you to see. And there are dozens of different ports to compare and contrast. For the context switch code, you will want to look in the ports directory, in any one of many files called port.c, port.asm, etc. And yes, in the case of freertos all context switches are performed in interrupts (a tick timer ISR, or any other SysCall interrupt).

A context switch is very-much processor specific, as the list of registers to save and the assembly code to save them varies between processor families, and sometimes within a given family. As a result each port has a separate file for this code.

The scheduling (selection of next task to run), on the other hand, is done in a file called tasks.c, which is common to all ports and references the port-specific code.

It is not the case than an RTOS simply context switches periodically - that is how most GPOS work. In an RTOS the scheduler runs on any scheduling event. These include system-tick, but also message post, event trigger, semaphore give, or mutex unlock for example.

On ARM Cortex-M the CMSIS 3.x includes an RTOS API (intended primarily for RTOS developers rather than a complete RTOS itself), the source for this will include a context switching mechanism.

If you want a detailed description for a simple RTOS you might consider reading µC/OS-II: The Real-Time Kernel or the slightly more sophisticated µC/OS-III: The Real-Time Kernel .

FreeRTOS is increasingly popular, though perhaps a little unconventional architecturally. A more complete (in that it is not just a scheduling kernel but a more complete OS) and very powerful option is eCos.