The ROM window area is a program area where data can be directly read by an instruction. The program memory and data memory spaces are separated in Segment 0. Therefore, the program memory is read through the ROM window of the data memory space. The areas other than the ROM window of segment 0 (program memory space in the area overlapped with RAM or SFR) can be read from segment 8 (mirror area of segment 0). For segment 1 and later, data can be directly read because the program memory and data memory spaces are not separated.