32-Bit Barrel Shifter

Carry In / Carry Out

-Carry in only used in RRX (rotate right extended) operations

-Carry out always computed, even though not needed in rotate operations

Carry Out Logic: Two Options

• Separate logic computes Cout early using input and shift amount

Pros:

-Cout signal ready much earlier, no need for propagation

-Simpler bit shifter designs

Cons:

-Many more gates needed

Carry Out Logic: Two Options

• Individual bit shifters compute and propagate Cout signal

Pros:

-Simpler overall design

-Fewer logic gates

Cons:

-Takes longer for Cout to be ready (propagation delay)

-More complicated bit shifters

Conclusion

• Went ahead and implemented Cout logic in the bit shifters

-Don’t really need the signal to be ready any earlier than the rest of the shifter output, especially not at the addition gate cost

-Each shifter computes Cout for its own shift amount and passes it on, or leaves Cout alone if it is disabled