GDB: Cannot access memory at address 0x40048000

I've got gdb up and running nicely with an LPCLINK to a LPC1114. You GDB works great with debugging the software, but I'm having a bit of a problem accessing the peripheral registers..

For example, my code executes:

LPC_SYSCON->MAINCLKSEL = 0x00000003;

in this case LPC_SYSCON-> MAINCLKSEL is mapped to 0x40048000, but when I run:

x 0x40048000

I get:

0x40048000: Cannot access memory at address 0x40048000

How can I make these registers accessible with the debugger?

Thanks,

Steve