Patent GB-839246-A (GB)
Title: Improvements in or relating to electronic computing apparatus
Inventor: BIRD RAYMOND
Filed: 1956-02-14
Abstract: 839,246. Digital electric calculating-apparatus. INTERNATIONAL COMPUTERS & TABULATORS Ltd. Feb. 12, 1957 [Feb. 14, 1956], No. 4574/56. Class 106 (1). An electronic calculating-apparatus using a random access time storage device such as a magnetic drum with a plurality of storage locations which are successively accessible includes an immediate access store, a first control means operable to enter data registered in the immediate access store into any specified one of the storage locations and a second control means to enter the same data into one or more of the storage locations immediately succeeding the specified location. The need for this arises in performing operations such as multiplying in which the same number is required repeatedly in successive operations. In multiplication it is necessary to read out the multiplicand under the control of each digit of the multiplier in turn. The track on the magnetic drum is divided up into word lengths in some of which an instruction is recorded and in others of which an operand number is recorded. In operations where the same operand is used repeatedly in successive operations controlled by a single instruction it slows down the calculation if it is necessary to wait for the drum to complete a full revolution before the required operand is again available. This loss of time is avoided by recording the operand in successive word spaces on the track as many times as it will be required. This is a preliminary operation carried out under the control of the instruction governing the calculation. Repetitive recording.-The head 32, Fig. 1, senses a track containing a single pulse which is amplified and applied to line ER. A clock pulse track is sensed by head 33, the pulses being amplified and passed to line CP. A number of heads such as head 34 sense tracks bearing the instructions and operand numbers. These are connected through read/write amplifiers 2 to produce an output on line Read In." Instructions are stored in a control register and a trigger 35 determines whether the machine is operating under the control of the stored instruction or whether it is in process of entering the next instruction into the control register. If the trigger is in the latter state it opens gate 36 to allow a section 37 containing the address of the next instruction to be applied to gate 8, the other input to this gate being from a counter 30 controlled by a word counter 3 operated by the clock pulses. The number registered in the counter 30 represents the storage location next to be sensed and when this coincides with the number registered in section 37 of the control register the gate 8 opens to allow a pulse to pass through a gate 12 which is held open by trigger 13, the pulse then setting the trigger 13 to close gate 12. Gate 38 controlled by triggers 13 and 35 opens to allow the signals on the read-in line to be entered into the control register. The entered instruction is assumed to be one which is to control repeated recording, starting at a specified location, of a number held in the immediate access store MR. The repetitive recording instruction is held in section 43 of the control register. The control register is connected to a function matrix 44 which produces outputs on lines adapted to govern the operation of the machine. As a result of the repeat instruction, the function matrix gives an output which is transmitted over line 5 to close a gate 4. A resetting pulse normally passes from the word counter 3 through gate 4 and gate 14 to reset the trigger 13 after the completion of the recording of the contents of the store MR in the storage location specified by section 31 of the control register. The signal from the function matrix closing the gate 4 prevents the trigger 13 from being reset, so that the contents of store MR will be recorded again in the next following storage location. This is repeated until the trigger 13 is reset by a pulse on line ER. If it is necessary to stop the recording before the end of the drum revolution, the resetting pulse may be taken from a counter 21 preset according to the number of repetitions required and operated by end-of-word pulses. The trigger 13 controls the recording by applying a signal to gate 42, other inputs of which come from the function matrix and store MR. When trigger 13 is set the gate 42 allows the clock pulses from line CP to gate out the contents of store MR which are amplified and recorded on the drum in the selected storage location. Multiplication.-The use of the repetitive recording function is described as applied to a binary multiplication operation. First the multiplicand value is transferred from the drum 1 to the store MR and then recorded on the drum again in a number of successive locations. The next instruction causes the multiplier value to be read from the drum and entered in the immediate access store MR to displace the multiplicand value. The next instruction conditions the machine to make the multiplication using a method in which successive pairs of multiplier digits are compared and according to the result of the comparison, an additive or subtractive entry into the accumulator is made of the multiplicand value and also the value in the accumulator is shifted one place upwards in significance for every comparison made except the last. These operations are made according to the following rules: if the two multiplier digits are unequal and the more significant is " one," the multiplicand is entered subtractively into the accumulator and if the multiplier digits are unequal and the more significant is zero the multiplicand is entered additively. This method depends upon the fact that a multiplication by 3, say, may be obtained by shifting the multiplicand two places (i.e. multiplying by 4) followed by subtraction of the multiplicand. The multiplier digits in store MR are shifted one by one on to a trigger and the setting of the trigger is compared at each step with the last digit in the store. By this means the store is gradually emptied as the operation proceeds, the empty stages being used to hold overflow digits from the accumulator. The steps of the calculation are controlled by circuit 20 which contains a comparison circuit connected to the trigger mentioned above and to the last stage of store MR. Shift pulses are applied from the control circuit 20 to the immediate access store, to the accumulator and to the counter. The counter terminates the operation after the correct number of steps. If as the result of the comparison it is necessary to enter the multiplicand into the accumulator a signal from the control circuit on line 49 opens gate 10 so that the next end of word pulse from counter 3 passes through to set trigger 13. This opens gate 66 so that the next recording of the multiplicand is read from the drum and passes to the accumulator. The accumulator has an adder/subtractor unit which is controlled over line 55 from circuit 20 to cause the entered number to be added or subtracted according to the result of the comparison. Specifications 839,241, 839,242, 839,243, 839,244 and 839,245 are referred to.