If I understand correctly, the Intel 151 trimmed the instruction cycle from 12 clocks to 4 clocks. The small app note pamphlet I have on it illustrates this in different external timing diagrams. My question is, however, are all internal instruction cycles trimmed to 4 clocks per cycle as well? This is a question of calculating code execution time. With a normal 12-clock/cycle 8051, I count up the instruction cycles then mutiply by the XTAL / 12. Is it correct to assume that for the 151 I should do the same but multiply by XTAL / 4 ??? Your feedback is appreciated.