In this case source register R2 and destination register R3 has to be different, because the two operations R2in and R2out can not be performed together. If the registers are made of simple latches then only we have the restriction.
We may have another CPU organization, where three internal CPU buses are used. In this organization each bus connected to only one output and number of inputs. The elimination of the need for connecting more than one output to the same bus leads to faster bus transfer and simple control.
A simple three-bus organization is shown in the figure 5.9.
A multiplexer is provided at the input to each of the two working registers A and B, which allow them to be loaded from either the input data bus or the register data bus.
In the diagram, a possible interconnection of three-bus organization is presented, there may be different interconnections possible.
In this three bus organization, we are keeping two input data buses instead of one that is used in two bus organization.
Two separate input data buses are present – one is for external data transfer, i.e. retrieving from memory and the second one is for internal data transfer that is transferring data from general purpose register to other building block inside the CPU.