This is accomplished by subtracting the contents of the B register from {HEAD ~ the contents} of the A register and storing the result in the B register while forcing the A input of {TAIL ~ the ALU} equal to zero 
This is accomplished by subtracting the contents of the B register from { HEAD ~ the contents } of the A register and storing the result in the B register while forcing the A input of { TAIL ~ the ALU } equal to zero
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH