Extending MIPS datapath to implement SLL and SRL

[izelkay]

Homework Statement

Here's the datapath:

So this seems like a pretty common question but I can't seem to find any answers on how to extend the datapath to implement SLL and SRL.

The Attempt at a Solution

This is how I would think to do it but I'm not entirely sure:

It would need another mux right next to Read data 1 next to the register file. This mux would take Read data 1 (rs) and Read data 2 (rt) as inputs. It would select Read data 1 if we're not doing a shift operation, and it would select rt if we ARE doing a shift operation (since sll and srl use rt, not rs). This would then be fed into the ALU.

Next, we would need to branch Instruction[10:6] (the shift amount) off of Instruction[15:0], and Instruction[10:6] would then be fed into the other port of the ALU. Is this correct thinking?

 

[izelkay]

It looks like I'd actually need to feed Instruction[10:6] and Instruction[15:0] into another Mux and have it select Instruction[10:6] when I want to shift, which will then be sent through the sign extender and into the ALU when ALUSrc is set to 1. Right? Anyone?