Why mips eret failed?

["wilbur.chan" <wilbur512@gmail.com>]

HI all!

I'm learning to write a timer interrupt handler by my own on
mips32(xls416 with 32bits cross compiled) , but to find that, eret
failed to quit.

detail:

I took the following steps:

1)  copy exception vector to the physical address 0x180, then set ebase with it.

     that is , memcpy these three  instructions to 0x80000180,with
size 0x80 bytes:

        lui    k1, HIGH(handle_int)
       addiu  k1, k1, LOW(handle_int)
       jr     k1


 2)     this is handle_int , which is  the entry of interrupts

    LEAF(handle_int)
         nop
         la     t9,do_IRQ
         nop
         jalr   t9
         nop
        eret
        nop
   END(handle_int)

 ps:

 'nop' is used to avoid delay slot,  and I did not add 'SAVE_ALL'  or
'RESTORE_ALL'  in handle_int, because it is just a demo, I want the
interrupt return

immediately.

3) this is do_IRQ

 void do_IRQ(void)
{
    ack_irq();    /* ack with compare register ,which is used to
generate timer interrupt*/
    print("do_irq enter\n");
}



4) there is a main loop like  this:


    void main_loop()
  {
     local_irq_enable(); /* enable timer interrupt*/
    while(1)
    {
         print("loop...\n");

    }
 }

I found that , the message in do_IRQ  prints  every 4s (I' ve set
timer of 4 seconds),  however, the message in main_loop did not appear


q1:  does that mean, the timer interrupt has never quit to main_loop ,
but a nested interrupt?



q2:  that is to say, eret in handle_int failed to quit to main_loop?

q3: why this happend?


Thank you !