access Violation for sram at address 0x80000000

Dear all,
Do anyone has an idea regarding how i can access external sram.
i am using
LPC2468,Cpress 2MB External SRAM,Keil ARM (µvision3)compiler in my application.

when i am read/write data to onchip sram it works fine but when we try to access external Sram it shows access violation after debugging.
the routine is as follows
if anybody has an idea Pls. help me in this regards..

#define A0EXT_MEM 0x80000000
#define A0BLOCK_SIZE 512

Init_EMAC()
{
  //configuring EMC

  PINSEL9  = 0x10550055 ;//A[19:16]/P4[19:16 & 31:30 & 27:24]   enable CS0 ,BLS0,BLS1,WE,OE
  // select D0 -D15

 //select A0- A19


}
ReadFromSRAM
{
//Reading from the address 0x80000000(CS0) leads access violation no read permission
for(i = 0 ; i < A0BLOCK_SIZE ; i += sizeof(unsigned int)){
data = *(unsigned int*)((unsigned int)&A0EXT_MEM + i);
}
}

WriteToSram()
{
 //Writing to the address leads access violation no write perission
for(i = 0 ; i < A0BLOCK_SIZE ; i += sizeof(unsigned int)){
*(unsigned int*)((unsigned int)&A0EXT_MEM + i) = data;
}
}

Regards

Tripathi