Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Digital systems, linear algebra, introduction to formal proof / 3 / b / ii

Move the no-write decision into the destination-address path. If the original destination selection produced $d$ and the original write-enable decision was $W$, drive the new register file withA five-bit two-input multiplexer after the existing destination selector implements this, or the decoder can directly select among zero, the instruction's destination fields and register 31. Its control still distinguishes writing and non-writing instructions: removing the register-file enable pin does not remove that logical decision.

Read-address selection and the result-data path can remain unchanged. Instructions that formerly disabled RegWrite now select destination zero; their write data is irrelevant. Instructions writing an actual destination preserve the same address and data. Since writes to the MIPS zero register were already discarded, the architectural behaviour is unchanged for that case as well.