6455 interrupt issue under DSP/BIOS

HI,DEAR TI,
//----------------------------------
DSP:6455;CCS:3.3.82.13;BIOS:5.41;CGT:6.1.13;NDK:2.0
//----------------------------------
my board also hava a piece of fpga and a ddc,both of them via gpio as interrupt pin link to dsp.
as follow:
fpga--gp8--dsp
in the tcf GUI:
interrupt selection num:59; function:_fpga_isr; use dispatcher;interrupt mask:all
in the tcf GUI:
ddc---gp9--dsp
interrupt selection num:60; function:_ddc_isr; use dispatcher;interrupt mask:all
//-----------------------------------
 in the main()
       IER |= 1<<8; //Enable fpga  interrupt
        IER |= 1<<9; //Enable ddc   interrupt
when running, i can run into ddc_isr and fpga_isr,i want to disable fpga interrupt and ddc interrupt
in ddc_isr,so in the
void ddc_isr() i use
        IER &=~(1<<8)
       IER &=~(1<<9)
to disable ddc and fpga interrupt ,but the ddc and fpga interrupt cannot be disabled.
why? how can i disabled interrupt in isr?
thank you!