Please check the 8080/z80 code.. You can run and test this chip without any memory(program) to check the signals generated by the CPU (Adress, M1, MREQ, RD, ....etc). The code 00 -> NOP or the code 0FFH -> RST 7/RST38H So either short the data bus to ground or to V+, the CPU will run.. However the Addr bus patterns captured are different for the above two data bus shortting methods. How different?? Think it!