The protection on the LPC2000 device consists of disabling the JTAG interface, which is the only way that you have to read out code from the device. This happens first, at power up, by modifying the PINSEL register, regardless of whether you have protection on or not. Then, the Boot loader has time to go in and check to see whether protection is supposed to be enabled. It enables the JTAG interface if the protection pattern is not found in the corresponding memory location. To protect the code, it is necessary to program a specific value into sector zero at the top of the interrupt vector. This allows room for the FIQ exception handler before you get to the address space for the application code.