How do I use PinTool to find the offset from an address stored in a register?

This might be very easy but I'm new to pintool. Basically, my question is: for an instruction such as:

mov 0x28(%rax) %xmm1

How do I record the value 0x28 into the output trace file? Is it the difference between IARG_MEMORYREAD_EA and IARG_REG_VALUE?