The instruction cycle begins when an instruction source delivers an instruction word to the Executor and signals "instruction valid". (We will discuss where instruction words come from shortly). 

The Executor is an almost trivial circuit, consisting of one SR latch and sixteen ordinary latches. When an instruction word arrives, the 16 instruction bits are latched and put onto the bus. And the "copy start" signal is also asserted and put onto the bus.

And that is all.

The Frontend runs on three control signals, "copy start", "copy in progress", and "copy done".

The instruction cycle begins with copy start asserted and 16 valid instruction bits on the bus. These bits flow down the bus (so to speak) where all the registers will "see" them.

In the next few pages we will follow these bits as they "travel" down the bus. And finally, when "copy done" arrives back, we reset our SR latch and let the external source know we are ready for the next instruction.