DLXALU is the busiest block of the DLX processor. The ALU
opearations are executed by this block. Destbus is the 32-bit output bus
of the ALU. For forwarding purposes, the value stored in this bus is fed
back to the ALU as Destbus_a and Destbus_b which are actaully the values
of the Destbus in the Memory and Write Back stages repectively. Destbus_a
and Destbus_b are multiplexed with the S1 bus, S2 bus and the LMDR bus at
the input of the ALU.