To access program memory above 64 KB, the Address Pointer must be larger than 16 bits. This is done by concatenating one of the address extension I/O registers (RAMPZ) with the internal Z-pointer. The RAMPZ register then holds the Most Significant Byte (MSB) in a 24-bit address or Address Pointer.
This address extension register is available only on devices with more than 64 KB of program memory. For the devices where extension pointers are required, only the number of bits required to address the whole program and data memory space in the device is implemented.
Figure 1. Combined RAMPZ + Z Register
