Subtraction is accomplished by forming the 2's complement of TMP before feeding it into the adder. All these circuits are inside the ALU box.

After the ALU forms its results, the CNVZ bits are set as discussed in Chapter 6.

The shifter is a separate box that takes the output of the ALU and performs either a logical shift or passes through the value unchanged. It is controlled by a 2-bit mini-opcode in the control wires S1 and S0:

  S1  S0
------------------------------------------------------------------
  0   0     do nothing           pass input to output unchanged
  0   1     right shift 1 bit    logical right shift
  1   0     left shift 1 bit     logical left shift
  1   1     unused