my interrupt not back

HI.
I have a peroblem in usart interrupt . the interrupt when occure it not back and at the end of IRQ_handler function program jump to the undef_irq label in start up. it's means that the interrupt can't go back to main..whye?

in main I have a correct program. I think my problem is from the startup code.

; Setup Stack for each mode

        LDR     R0, = |Image$$ARM_LIB_STACK$$ZI$$Limit|

;  Enter IRQ Mode and set its Stack Pointer
        MSR     CPSR_c, #ARM_MODE_IRQ:OR:I_BIT:OR:F_BIT
        MOV     SP, R0
        SUB     R4, SP, #IRQ_Stack_Size

; Supervisor mode (interrupts enabled)
        MSR     CPSR_c, #ARM_MODE_SVC | F_BIT
        MOV     SP, R4

; Enter the C code

        IMPORT  __main
        LDR     R0, =__main
        BX      R0
loop4
        B       loop4

       END

IRQ_Handler




    ;   Save interrupt context on the stack to allow nesting */
        SUB     lr, lr, #4
        STMFD   sp!, {lr}
        MRS     lr, SPSR
        STMFD   sp!, {r0,r1,lr}




        ; Write in the IVR to support Protect Mode */
        LDR     lr, =AT91C_BASE_AIC
        LDR     r0, [r14, #AIC_IVR]
        STR     lr, [r14, #AIC_IVR]




        ; Branch to interrupt handler in Supervisor mode */
        MSR     CPSR_c, #ARM_MODE_SVC
        STMFD   sp!, {r1-r4, r12, lr}
        MOV     lr, pc
        BX      r0
        LDMIA   sp!, {r1-r4, r12, lr}
        MSR     CPSR_c, #ARM_MODE_IRQ | I_BIT




        ; Acknowledge interrupt */
        LDR     lr, =AT91C_BASE_AIC
        STR     lr, [r14, #AIC_EOICR]




        ; Restore interrupt context and branch back to calling code
        LDMIA   sp!, {r0,r1,lr}
       MSR     SPSR_cxsf, lr
        LDMIA   sp!, {pc}^

above I write several line in my start up file
can you help me?? pleas!