I must say I'd never heard of Domosys before. Anyway Jeff, the question your asking is particular to that chip. Whilst the instruction set may be same/similar, just because it says it uses an 8051 core doesn't tell us much. The SFRs etc are up to the device manufacturer so you just have to refer to their documentation. As such, the 8051 didn't have a choice of internal/external clocking - it only had external. Newer devices may have this feature, but it don't this there is any standardisation between manufactures regarding this.