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Communications of the ACM

Communications of the ACM

SABRAC, a time-sharing low-cost computer

The serial SABRAC computer designed and built in the Scientific Department of the Israel Defence Ministry has a 5000-location magnetic drum, main store. To avoid a need to resort to optimum programming techniques and to increase its overall efficiency the computer has also been given a 224-word ferrite core store from which the program is obeyed. Transfers between the core and drum stores and to and from the twin paper-tape input and output channels are all available autonomously (concurrently, time-shared). Multiplication and division orders are also autonomous, so that the machine may be executing up to three orders simultaneously. All functions naturally are interlocked. A number of other advanced orders and facilities are also incorporated. In particular, an “Execute” order permits a temporary jump for up to four orders and a second modifier register permits double modification in general and relative addressing of subroutines in particular. Thus the overall effective speed of the machine is much higher than its basic specification would lead one to expect and its design indicates one way in which the concepts of time sharing may be incorporated in “low-cost” computers.

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