Electrical – Implement BGEZAL instruction-MIPS-32 in verilog

The bits from 16 to 20 are given to register file. It is 10001 which is decimal 17. But the description of BGEZAL says it writes to register 31. Should the values Intruction[16:20] not be 11111? So PC+8 must be written to GPR[17] or GPR[31] ?

If bits [26:31] are equal to 000001, then this is a REGIMM instruction, and bits [16:20] are not given directly to the register file. For example, BGEZ and BGEZAL both specify bits [26:31] to be 000001, but BGEZ has bits [16:20] as 00001, while BGEZAL has bits [16:20] equal to 10001.

And also, the instruction does PC + 8, but PC is not given as input to ALU in this lecture notes. So I have planned to have a MUX in the first input (upper input) of ALU. If the instruction is BGEZAL, the MUX will feed PC else, it will feed data1 from register_file. Is this fine?

If you're implementing a 5-stage pipeline like MIPS usually uses, then by the time the BGEZAL instruction has reached the "execute" phase of the pipeline (stage 3 of the pipeline), the PC will have incremented by 8. You can just store the current PC to register 31 at that point. For more information on what I mean by this, see these slides.

If you're trying to implement a single cycle non-pipelined MIPS architecture, then you could do something similar to this (slide 56): a fixed PC+4 block is inserted after the regular PC+4 for advancing to the next instruction, then this value is written to the register file for register 31. For future reference, this is the design I'm referring to (from the link):