Past exam of the year 1 of the computer science course of the University of Oxford / 2015 / Digital systems, linear algebra, introduction to formal proof (resit) / 3 / a

In the pictured single-cycle MIPS architecture, RegDst selects the write address of the register file: zero selects instruction bits $20\dots 16$, the rt field used by a load; one selects bits $15\dots 11$, the rd field used by an MIPS R-type instruction. RegWrite enables the clocked write to the selected processor register.

ALUSrc selects the second ALU input: zero uses the second register file read output, while one uses the sign-extended 16-bit immediate field. The zero signal is produced by the ALU, rather than being an opcode-decoder output: it is one exactly when the ALU result is zero. During beq, the ALU subtracts its operands, so zero tests equality. The diagram combines it with Branch to select the branch target instead of $PC+4$.