Hello
Basically the interrupts are enabled again when the OS changes the context to the first process, which needs to be activated.
When you create a task, the OS is initializing the Task stack with the start address of the task main function and a SR value where interrupt are disabled.
When the fist task is activated, the return PC and SR value is read from the stack and the interrupts are enabled.,
I do not think you need to understand the internal mechanism. Point is when the first task gets started the interrupts are enabled.
CrasyCat