How do OSes Handle context switching?

Question

As I can understand, every OS need to have some mechanism to periodically check if it should run some tasks and suspend others.

One way would be some kind of timer on whose expiry the OS will check if it should run/suspend some task.

Generally, say on a ARM system that would probably be some kind of ISR.

My real question, is that I've been ABLE to only visualize this and not see it somewhere. Could some one point to some free/open RTOS code where I can actually see the code that handles the preemption/scheduling?

Answer 1

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.