Data General Corporation

Roger W March

Edward M Buckley

David H Bernstein

Michael B Druke

Ronald I Gusowski, Deceased

A data processing system using microcode architecture in which a two-level microcode system comprises one or more first, or 'horizontal', microinstructions and a plurality of second, or 'vertical', microinstruction portions in a vertical microcontrol store. In a preferred embodiment the vertical microinstruction portions include one or more 'modifier' fields, a selection field for selecting a horizontal microinstruction and a sequencing field for selecting the next vertical microinstruction portion of a sequence thereof, one or more fields of the horizontal microinstructions being capable of modification by the vertical modifier fields in order to form output microinstructions for performing data processing operations. Unique bus protocol signals are generated to prevent simultaneous access to the system bus by two competing system components and to permit substantially immediate control of the systems bus by a component without requiring a CPU decision thereon. Further, a unique system I/O interface unit permits access to certain I/O components via other I/O buses, such unit utilizing a unique polling technique to identify on an updated basis, all components present on one of such other buses. The system I/O interface unit also includes a unique frequency synthesizer unit for providing at least one clock signal having a substantially constant frequency which can be generated in response to any one of a plurality of input clock signals each having a different frequency.

Data processing system

A unique frequency synthesizer unit for use in a data processing system I/O interface unit for providing at least one clock signal having a substantially constant frequency which can be generated in response to any one of a plurality of input clock signals each having a different frequency. The system I/O interface unit may be used, for example, to control communication between a system bus and a plurality of I/O buses having different timing and operating characteristics. The system I/O interface unit may be used, for example, to control communication between a system bus and a plurality of I/O buses having different timing and operating characteristics.

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Santa Clara, CA, United States of America

Roger W March