ISR is not executing in lpc2138

This is the code i used,

void extint(void) __irq ;

void main()
{
        PINSEL0 = 0X0000000C;                       //selecting EXT0
        IODIR0 = 0x0000ff00;
        VICVectCntl0 = 0x0000002E;

        VICVectAddr0 = (unsigned long)extint;
        VICIntEnable = 0x00004000;



        IODIR0 = 0x00ff0000;
        while(1)
        {
                IOSET0 = 0x00ff0000;
                IOCLR0 = 0x00ff0000;
        }

}

void extint(void) __irq
{
        IOSET0 = 0x0000ff00;
        EXTINT = 0x00000001;
        VICVectAddr = 0;
}

it compiles. but when I run it on the simulation it does not enters in to the ISR function.

when I checked it on debug mode, the 'int' bit in EINTO is getting set and the VICVectAddr

also updated with the address of VICVectAddr0.

I used an example and it worked fine.

In the disassembly, in the address 0x00000018"LDR PC, [PC, #-0x0FF0]"