Modify the Single-Cycle Datapath to implement the following instruction: add 3 R

Question

Modify the Single-Cycle Datapath to implement the following instruction: add 3 Rd, Rs, Rt, Rx which is defined as R[Rd] = R[Rs] + R[Rt] + R[Rx] You do not need to redraw the entire datapath. Rather, redraw those components that need to be changed and draw those components that need to be added. Clearly show all connections to and from the changed and added components. Identify any additional control signals that are required, and define their behavior. The 'add3' instruction will require a new instruction format. Define this new instruction format so that it is as similar to existing instruction formats as possible. Clearly show all fields of this instruction format, and the bits associated with each field.

Explanation / Answer

This instruction can be executed using the existing blocks. This instruction uses instruction memory, both existing read ports of Registers, the ALU, and the write port of Registers. The new functional blocks are needed for this instruction is another read port in Registers (to read Rx) and either a second ALU (to add Rx to Rs + Rt) or a third input to the existing ALU. The new control signals are used to tell the new ALU what to do or if we extended existing ALU, need to add new ADD3 operation.

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