The arithmetic-logic unit (ALU) of a central processing unit (CPU) is frequently called upon to use the output of its last computation as one of the two operands of its next computation. For example, in an expression such as "a = b x (c + d)," the result of the addition "c + d" must be used as an operand for multiplication by b. Feedback of the ALU's latest output to one of its inputs is also required by loops of the following commonplace form:

• for(i = 1; i < 100; i++){x = x + y(i);}

Execution of this line of C code requires 99 additions, in each of which x is involved both as operand (old value of x) and result (new value of x). The variable x might be described as accumulating the first 99 values of the vector y(i). Thus, any register to which the output of the ALU is fed, and...