Hitachi, Ltd.

Sako Ishikawa

Toru Ohtsuki

Yoshio Oshima

Masaharu Fukuta

Hideaki Yabe

Akira Yamagiwa

Koji Matsuda

Kenichi Kurosawa

Yoshihiro Miyazaki

Shinichiro Yamaguchi

Soichi Takaya

Kenji Kashiwagi

Masao Inoue

The present invention concerns clock source switchover between dual clock sources in the event of failure of any of them without affecting the clock output in the dual system, thereby preventing malfunctioning of processors therein. In the fault tolerant computer system of the invention, each of the plural processing units comprises a clock source, a clock selector, a clock stop detection unit, a clock phase adjusting unit, and a phase coincidence detection/operation suppression/resetting unit, whereby when switching over from a faulty clock source to a normal clock source in the event of clock failure, the clock phase adjusting unit ensures continuity in the output clock signals. The clock phase adjusting unit provided in the subsequent stage of the clock selector inserts the PLL circuit having an overdamping response characteristic obtained by lowering the gain of its loop filter.

Uninterruptible clock supply apparatus for fault tolerant computer system

An apparatus for decimal multiplication divides a multiplier of binary coded decimal (BCD) into plural groups, generates plural partial products of which are multiplied a multiplicand of BCD and the plural groups of multiplier over successive cycles and adds them to an intermediate product which is a summation of the previously generated partial products. The addition of the partial product and the intermediate product is made by a carry save adder. At a first cycle, the intermediate product is set to zero, and the addition of 6 is made to each digit of either one of the intermediate product sum and the partial product, and the addition of the partial product and the intermediate product is made by a carry save adder loop over successive cycles. At a final cycle, the sum and carry from the carry save adder are added by a full adder, and the subtraction of 6 is made for each digit according to the existence of carry transfer in each digit of the full adder and the resultant value is output as a multiplication result.

Apparatus for decimal multiplication

A binary coded decimal number division apparatus in which a quotient represented in a binary coded decimal notation is determined on digit-by-digit basis by using a quotient prediction table and a group of multiple value registers and in which a predicted quotient read out from the quotient prediction table is used intact when the predicted quotient is correct while otherwise the predicted quotient is decremented by one, wherein the values stored in the quotient prediction table together with redundant bit are previously modified to (0110).sub.2 to (1111).sub.2 in the binary coded decimal representation. The multiple value register is selected by using three of the four bits of the modified predicted quotient, while upon determination of the quotient, the value used for modification is subtracted from the output value of the quotient prediction table to thereby derive the predicted quotient of one digit. With this arrangement, three of the four bits of the predicted quotient of one digit read out from the quotient prediction table can be used directly as the selection signal for selecting the relevant divisor multiple register.

Binary coded decimal number division apparatus

In preparation of addresses of a quotient prediction table used in a binary coded decimal number division scheme with predetermined bits of a dividend and a divisor in binary coded decimal representation, the addresses are modified with the redundant bits. The absolute bit number for the addresses is thus decreased, whereby data quantity and hence capacity of RAM required for implementing the quotient prediction table can be significantly reduced, while satisfactory function of the quotient prediction table being assured. The apparatus for the binary coded decimal number division is implemented inexpensively in a small size.

A multiplier device comprising hold means for holding the result of addition, block product means for producing k block products each having 2n bits, where n is an integer equal to or greater than 2, the k block products being formed by multiplying each block by n bits, k blocks being obtained by dividing a multiplicand at intervals of n bits from the least significant bit of the multiplicand, and adder means for adding two groups of block products to the output of the hold means, the two groups of block products consisting of alternate block products out of the k block products from the block product means.

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Hadano, Japan

Sako Ishikawa