PC-1 - UNIVERSITY OF TOKYO - TOKYO, JAPAN

    The PC-1 is a pilot model, binary, single-address computer installed at Department of Physics, Faculty of Science, University of Tokyo, and one of the first general purpose computers using parametron logic and two frequency magnetic core memory. Its construction was started in September 1957 and completed in March 1958. Its successful operation and high performance have lead to the construction of several larger computers based on the same design principle. Standard machine running hours are from 9:00 a.m. to 9:00 p.m. from Monday to Saturday. It is primarily intended for exploratory work on logical design and programming of computers, but about half of the time is made available to other laboratories in the university.

    Memory. Magnetic core memory of the PC-1 uses sinusoidal waves rather than pulses for write-read operation. The core matrix consists of a 36 x 256 rectangular wire net. In each writing operation, a sinusoidal wave of frequency f/2 is put through the selected one of the 256 "row" wires, and the 36 information bits are applied to the 36 "column" wires in the form of the sinusoidal wave of frequency f, where the sign or phase of the latter wave represents each information bit. The core on the cross points of both wires are subjected to the magnetizing force of the form I₀ cos π ft ± I₁ cos 2π ft, and the asymmetry of this wave form causes magnetization of the core in one or other direction.

    The read-out of information is achieved by applying the current of frequency f/2 on a row wire and picking up the 2nd harmonic from the column wires.

    The current of frequency f/2 is generated by special parametron elements used as frequency dividers. The selection of a particular row out of 256 is made by the threshold action of these parametrons against variation of the exciting current. The exciting inputs to these parametron frequency dividers are derived from a special multi-winding transformer called "polyhybrid." Each of its output winding forms 256 different types of linear superposition of the outputs of 18 power amplifiers, and the phases of these power amplifiers are changed in a definite way according to the bits specifying the particular address. In this manner, one and only one of the 256 parametrons receives a high input current, and all remaining parametrons receive current not more than 1/3 of the maximum value. This system, which is based on the same principle as the error-correcting code, has the advantage that the machine operates correctly even in case of failure of 1 or 2 tubes in the power amplifier, and has proved superior both in economy and reliability.

    Control. The design of the whole computer is based on synchronous philosophy. Control of the PC-1 is characterized by overlapping operation of different stages of successive instructions. The standard control sequence starts with the end pulse of the previous instruction (i - 1). This causes reading of the operand of the next instruction (i), and decoding of the same instruction (i). At the end of the decoding operation, the operation pulse is emitted, which causes execution of the operation of the instruction (i), reading of the instruction (i + 1), and increasing the content of the sequence counter to i + 2. There are, of course, some exceptions in case of store instruction and other non-arithmetic operations. These overlapping operations have been very effective in increasing the speed of devices using perfectly synchronous circuit elements such as parametron.

    Arithmetic unit. The arithmetic unit contains three 36-bit registers, A, R, and M. A and R are shifting registers and form a double length register for some arithmetic operations. A has a parallel adder used in all kinds of arithmetic and logical operations. The adder has a special high-speed carry network which enables the complete assimilation of carries in 36-bit addition in only 6 logical stages.

    Overlapping input-output operation. Standard 6-unit teleprinter equipments (similar to Flexowriter) are used for input and output, together with a photoelectric tape reader. Since conversion to and from decimal scale is always done by program, special provision has been made to enable effective overlapping operation of input-output devices and the computer. The address part of every input-output instruction is a jump address, and when a particular input (or output) instruction is encountered and the input (output) device is not ready at the moment it is called for, the machine does not wait, but a jump takes place to that address. When any of these devices becomes ready, the normal program sequence is interrupted and control jumps to the address 511, while the address of the instruction which was to be executed net if the break-in did not occur is stored in the location 510. A special input-output executive program can take care of an effective overlapping operation of machine control and input-output devices by taking advantage of these facilities.

    Summary Specifications. 4200 parametrons; number word, binary 18 bits (short word) or 36 bits (long word); instruction word, binary 18 bits single address, 27 different instructions; memory, magnetic cores, 512 short words; operation speed: addition and subtraction 270 microsecond; multiplication 2970, store 540, unconditional jump 270, negative jump 540; power consumption, 3kva; floor area, 8 square meters.