= Solution
Use the <MIPS zero register> as a <write-suppression destination>: encode $r_d=0$. There are only 31 stored <processor registers>, and address zero denotes the constant-zero source rather than a writable storage cell. Its writes are discarded, so
$$
\boxed{r_d=0\quad\Longrightarrow\quad\text{no visible register change}.}
$$
This permits a `store` to write memory while its unavoidable <register file> write has no effect. It also implements a no-operation by selecting no memory write and destination zero. The statement that reads of register zero return zero is realized consistently by this standard constant-zero, discard-writes implementation.
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