The instruction register uses bits

The instruction register uses bits IR1, IR0 to select the data source. The 2-line to 4-line decoder side of the diagram decodes bits IR1, IR0 into one of four signals: E0, E1, E2, E3. The register enabled by the source code puts its data on the bus, which is fed to the D inputs of all registers. The 2-bit destination code IR3, is fed to the decoder to generate one of the clock signals C0, C1, C2, C3. All the AND gates in this decoder are enabled by a common clock signal, so that the data transfer does not take place until this line is asserted.