Slide 14.10: Forwarding control signals

Forwarding Control Signals

The following table shows the control values for the forwarding multiplexors that select either the register file values or one of the forwarded values.

Control signal	Explanation
`ForwardA` = 00	First ALU operand comes from the register file.
`ForwardA` = 01	Forwarded from the previous ALU result
`ForwardA` = 10	Forwarded from data memory or the second previous ALU result
`ForwardB` = 00	Second ALU operand comes from the register file.
`ForwardB` = 01	Forwarded from the previous ALU result
`ForwardB` = 10	Forwarded from data memory or the second previous ALU result

The conditions for finding hazards and the control signals to resolve them are

   if ( IF/ID.Rs == ID/EX.Rw ≠ 0 and ID/EX.RegWrite)
      ForwardA = 01        // EX hazard

   elseif ( IF/ID.Rs == EX/MEM.Rw ≠ 0 and EX/MEM.RegWrite )
      ForwardA = 10        // MEM hazard

   else  ForwardA = 00     // No hazard
 
   if ( IF/ID.Rt == ID/EX.Rw ≠ 0 and ID/EX.RegWrite )
      ForwardB = 01        // EX hazard

   elseif ( IF/ID.Rt == EX/MEM.Rw ≠ 0 and EX/MEM.RegWrite )
      ForwardB = 10        // MEM hazard

   else  ForwardB = 00     // No hazard