A undefined instruction exception!

A undefined instruction exception appeared when I'm using MDK to make a user bin,and run with my bootloaser!
I use a s3c2440 micro chip.
in my .c:

void ChangeClockDivider(int hdivn_val,int pdivn_val) //14 12
{
        int hdivn=2, pdivn=0;
    hdivn=2 ;
    if(pdivn_val==12){
        pdivn=1;
    }else if(pdivn_val==11){
        pdivn=0;
    }
        rCLKDIVN = (hdivn<<1) | pdivn;
   // MMU_SetAsyncBusMode();
   //If I invalid this ,no undefined instruction exception happens but i didnot means to do it.
}

in my asm,I code follow the datasheet of s3c2440:

   EXPORT MMU_SetAsyncBusMode
MMU_SetAsyncBusMode
   mrc p15,0,r0,c1,c0,0
   orr r0,r0,#R1_nF:OR:R1_iA
   mcr p15,0,r0,c1,c0,0
   MOV_PC_LR
   END

I don't know if it is caused by MDK syntax rules error.
Or something else?