The mpc555 has 446kB internal flash and 26kB internal ram. On the NTB MPC555 headerboard there is an additional 4MB external flash and 2MB external ram. All internal memory can be accessed within 2 clock cycles (2 * 25ns = 50ns @40MHz operating frequency). The external memory (RAM and flash) needs 2 extra bus wait states. This gives a total access time of 4 clock cycles. The complete memory map is described in the document below.
memorymap_ntb-mpc555-headerboard.pdf