Chapter 8 The UNIVAC system 158

ister receives the multiplier from the memory and the comparator determines the sign of the final product by comparing the signs of the multiplier and multiplicand. During the next three steps the multiplicand, which has been stored in the L-register by some previous instruction, is transferred three times to the A-register through the algebraic adder. The result, three times the multiplicand, is then stored in the F-register. During the next 11 steps of multiplication, the successive multiplier digits, beginning with the least significant, are transferred from the X-register to the multiplier-quotient counter. The multiplier-quotient counter then determines whether each particular multiplier digit is less than three, or greater than or equal to three.

If the former, the L-register releases the multiplicand to the A-register via the adder, and the multiplier-quotient counter is stepped downward one unit. If the multiplier digit is equal to or greater than three, the multiplier-quotient counter sends a signal to the F-register which releases three times the multiplicand to the A-register and the multiplier-quotient counter is stepped three times. Thus a multiplier digit of seven would be processed as two transfers from the F-register to the A-register and one transfer from the L-register to the A-register, or a total of three transfers.

When the multiplier-quotient counter reaches zero, the next multiplier digit is brought in from the X-register, while the A-register, containing the first partial product, is shifted one position to the right.

During the final step of multiplication, the sign is attached to the product which has been built up in the A-register. One of the several available multiplication instructions causes the least significant digits, as they are shifted beyond the limits of the A-register, to be transferred to the X-register where they replace the multiplier digits as they are moved to the multiplier-quotient counter. Thus 22 place products can be obtained as well as 11 place.

The division operation is performed by a nonrestoring method. The divisor is stored in the L-register by some previous instruction and the dividend is brought from the memory and put in the A-register during the first step of the division instruction. As in multiplication, the signs of the two operands are compared in the comparator at this time and the sign of the quotient is then stored in the comparator pending completion of the division operation. The principal stages of division consist of transferring the divisor from the L-register to the A-register through the complementer and adder as many times as required to produce a quantity less than zero in the A-register, the dividend having been first shifted one position to the left. The multiplier-quotient counter counts each transfer, thereby building up the first quotient digit. As soon as the quantity in the A-register, (neglecting its original sign) goes negative, the digit in the multiplier-quotient counter, not counting the transfer which causes the remainder to go negative, is transferred to the X-register and the remainder in the A-register is shifted one place to the left. The divisor is then added to the A-register until the quantity becomes positive. This time the multiplier-quotient counter must give the complement of the number of transfers for the real quotient digit. Special complementing read-out gates provide this method of interpreting the multiplier-quotient counter.

The X-register therefore collects the quotient, digit by digit, from the multiplier-quotient counter until the full 11 digits have been obtained. The quotient is then transferred to the A-register and the sign from the comparator (CP) is affixed during the final stage of the divide instruction.

The other internal operations of the UNIVAC include many transfer instructions by which words may be moved among the registers and memory with and without clearing, the extraction instruction by which certain digits of a word may be extracted into another word according to the parity of the corresponding digits of an extractor word; shift instructions; and special control instructions such as breakpoint, transfer of control, (explained in subsequent paragraphs) and stop.

The basic operating cycle of the UNIVAC is founded upon single address instructions which specify the memory location of one word. In the case of the arithmetic instructions which require two operands, one of the operands must be moved into the proper register by some previous instruction. In order to control the sequence of instructions, a special counter, called the control counter (CC), retains the memory location from which the succeeding instruction word is to be obtained. Each time a new instruction word is received from the memory, the quantity in the control counter is passed through the adder where a unit is added to it. Therefore the normal sequence is to refer to successive memory locations for successive instruction words. Initially the control counter is cleared to zero and the first group of instructions must, therefore, be placed in memory locations from zero upward. A transfer of control instruction enables the programmer to change the control counter reading whenever desired and thus shift from one sequence to another. After a transfer of control takes place, the new number in the control counter is increased by unity each time a new instruction word is obtained from the memory.