Programming Embedded Systems in C and C++

114 Chapter 8: Operating Systems

The contextSwitch routine is actually invoked by the scheduler, which is in turn

called from one of the operating system calls that disables interrupts. So it is not

necessary to disable interrupts here. In addition, because the operating system call

that invoked the scheduler is written in a high-level language, most of the running

task’s registers have already been saved onto its local stack. That reduces the

amount of work that needs to be done by the routines saveContext and

restoreContext. They need only worry about saving the instruction pointer, stack

pointer, and flags.

The actual behavior of contextSwitch at runtime is difficult to see simply by look-

ing at the previous code. Most software developers think serially, assuming that

each line of code will be executed immediately following the previous one. How-

ever, this code is actually executed two times, in pseudoparallel. When one task

(the new task) changes to the running state, another (the old task) must simulta-

neously go back to the ready state. Imagine what the new task sees when it is

restored inside the restoreContext code. No matter what the new task was doing

before, it always wakes up inside the saveContext code—because that’s where its

instruction pointer was saved.

How does the new task know whether it is coming out of saveContext for the first

time (i.e., in the process of going to sleep) ...