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International Business Machines Corporation

David George Caffo

James Nolan Hardage

Stephen Douglas Weitzel

Gilles Gervais

Christopher Anthony Freymuth

A bus interface apparatus and method are implemented. A pair of data streams is generated from the stream of data to be launched onto a data bus. Each stream is staged along a corresponding data path that includes a plurality of storage elements. Each path feeds an input of a multiplexer (MUX). The output of the MUX drives the bus, and the MUX selects a data value for launching onto the bus in response to a signal derived from an internal bus clock. The internal bus clock is also used to generate a bus clock that is output to the bus along with the data. The period of the bus clock may be a preselected multiple of the period of a processor clock. The data is staged along the two data streams in response to clocking signals derived from the processor clock. Each of the clocking signals is qualified by a corresponding hold signal, that, when asserted, holds the clocking signals in a predetermined state. The hold signals are generated in response to a plurality of control signals that are used to select the ratio of bus clock period to processor clock period. The bus interface may be asynchronously started in response to a signal from the startup logic in the central processing unit (CPU).

System for launching data on a bus by using first clock for alternately selecting data from two data streams and using second clock for launching data thereafter

A load queue is provided in a load/store unit of a superscalar processor that includes a real page number buffer for storing a real page number for each instruction entry in the load queue. The load queue also includes a real page number comparator coupled to the real page number buffer for comparing executing load instruction entries with queued load instruction entries in the load queue. The load queue further includes a cache line modified register coupled to the data cache. The cache line modified register marks the queued load instruction entries when a cache line of the data cache addressed by the queued load instruction entry has been modified. In a preferred embodiment, when the executing load instruction is out of program order with respect to one of the queued load instructions, and the modified cache line register has marked the queued load instruction, the load queue signals a sequencer unit to cancel the queued load instruction. The load queue further includes an instruction identification buffer coupled to a sequencer unit for storing an instruction identifier for each entry in the load queue and a program order comparator coupled to the instruction identification buffer. The program comparator compares the ordering of instruction entries in the load queue with the executing load or store instruction.

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Austin, TX, United States of America

David George Caffo