Clock Interrupts and Preemption

Exercise 13. Modify kern/trapentry.S and kern/trap.c to initialize the appropriate entries in the IDT and provide handlers for IRQs 0 through 15. Then modify the code in env_alloc() in kern/env.c to ensure that user environments are always run with interrupts enabled.

在trap.c中:

for (i = 0; i < 16; ++i)
    SETGATE(idt[IRQ_OFFSET+i], 0, GD_KT, funs[IRQ_OFFSET+i], 0);

并在user env中启用EL_IF:

e->env_tf.tf_eflags |= FL_IF;

Exercise 14. Modify the kernel's trap_dispatch() function so that it calls sched_yield() to find and run a different environment whenever a clock interrupt takes place.

    if (tf->tf_trapno == IRQ_OFFSET + IRQ_TIMER) {
        lapic_eoi();
        sched_yield();
        return;
    }