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The last time I was dealing with the 8085 has been about 20 years ago but in general, many processor architectures use multiple clock cycles to execute one instruction. It is easier to generate an internal symmetrical signal for once, some instructions are internally divided into smaller chunks that can be executed in one cycle and so on. Even today most architectures that claim sungle cycle execution need multiple cycles, now it is done in parallel and called a pipeline, the 8085 does everything in sequential form.
Hope I got your question right, otherwise my answer is void too ;-)
you can even consider a VLIW processor architecture in which a single clock cycle is enuff to execute a multi-op which has several instructions clubbed together. well as for the clock generated it has to be double the internal clock frequency for the fact that lots of spatial variations could be encountered.. so as to avoid that external frequency is nearly double the internal frequency.!! as you guys know the heart of any micro-processor is the clock generator..crystal osciallator..so its made sure that the external clock frequency generated is steady and since we use crystal (other reason why Crystal osciallot is prefered to LC circuits) .. he Q factor is high and frquency doesnt fade with time..
all the more reason why we have double the external frequency compared to the internal frequency..
let me find some other conclusive answer to the above query.. gimem sometime.