JTAG_UART interrupt (Lab 4 )

Here is my assembly code

--- Quote Start ---

# include "nios2.h"

.text

.equ JTAG_UART_base_address , 0x00011020

.org 0x20

ISR:

rdctl et,ctl4

beq et,r0,Skip_ea_dec

subi ea,ea,4

Skip_ea_dec:

movia r4,JTAG_UART_base_address

ldwio r5,0(r4)

andi r5,r5,0x00ff

stwio r5,0(r4)

End_isr:

eret

.global main

main:

movia r8,JTAG_UART_base_address

movia r7,1

stwio r7,0(r8)

movia r7,1

wrctl ctl3,r7

movia r7,1

wrctl ctl0,r7

End:

br End

.end

--- Quote End ---

Please , explain how the interrupt exception active ?