Slide 14.9: The forwarding unit

The Forwarding Unit

The following figure shows the forwarding unit in a pipelined datapath.

The unit controls the ALU multiplexors to replace the value from a general-purpose register with the value from the proper pipeline register. It includes two tasks:

Generate two control signals ForwardA and ForwardB, which are used to control the two forwarding multiplexors, and

Use Rs and Rt in IF/ID and Rw in ID/EX and EX/MEM.

A Forwarding Example
Consider the following instruction sequence:

     lw   $4, 100($9)    # $4 = Mem[100+[$9]]
     add  $7, $5, $6     # $7 = $5 + $6
     sub  $8, $4, $7     # $8 = $4 - $7

When lw reaches the MEM stage, add will be in the ALU stage and sub will be in the Decode stage.

ForwardA=10 ⇒ Forward data from MEM stage and ForwardB=01 ⇒ Forward ALU result from ALU stage.