[Solved]: MIPS with one read port in the register file?

Problem Detail: 

I'm studying the MIPS multi-cycle datapath, and I'm wondering what would happen if there was only one read port in the register file for certain instructions.

First, for R-type instructions, I know that it takes in 2 registers(for example, with 'add', rs and rt are both passed into read-port 1 and read-port 2 in the register file) and then added in the ALU. With only one read-port, the fetch step will be the same amount of time (fetching the instruction won't be affected), but the execution step will take 2 clock cycles instead of 1, because there will be a clock cycle for each time a register is passed in. Then, the other steps will also take the same amount of time after that.

But what about the load and store instructions? The register file takes rs and rt into the read-ports normally, but for example, sw STORES it's value in rt. Will this mean that the execution step won't be affected because rt will be passed into the "write register" port? Could anyone clear this up for me?

Asked By : user3472798

Answered By : TEMLIB

For your MIPS CPU with 1read + 1write register file :

• ALU instructions will last two cycles as you describe.
• Load instructions can be executed in 1 cycle (with data in cache, etc...)
• Store instructions must be split in two cycles to get the two source registers.

Another example, in SPARCs, STORE R1,[R2+R3] require two cycles with a 2R1W register bank, whereas LOAD [R1+R2],R3 can be executed in 1 cycle.

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Question Source : http://cs.stackexchange.com/questions/33586