Programming Embedded Systems in C and C++

106 Chapter 8: Operating Systems

to make such a guarantee is to disable interrupts for the duration of the critical

section. So enterCS is called at the beginning of the critical section to save the

interrupt enable state and disable further interrupts. And exitCS is called at the end

to restore the previously saved interrupt state. We will see this same technique

used in each of the routines that follow.

There are several other routines that I’ve called from the constructor in the previ-

ous code, but I don’t have the space to list here. These are the routines contextInit

and os.readyList.insert. The contextInit routine establishes the initial context for a

task. This routine is necessarily processor-specific and, therefore, written in assem-

bly language.

contextInit has four parameters. The first is a pointer to the context data structure

that is to be initialized. The second is a pointer to the startup function. This is a

special ADEOS function, called run, that is used to start a task and clean up

behind it if the associated function later exits. The third parameter is a pointer to

the new Task object. This parameter is passed to run so the function associated

with the task can be started. The fourth and final parameter is a pointer to the new

task’s stack.

The other function call is to os.readyList.insert. This call adds the new task to the

operating system’s internal list of ready tasks. ...