A plurality of specialized controllers, each oneadapted to control a particular type of data transferoperation, control the flow of data between a system busand a local bus on a computer adapter card. When theDirect Memory Access (DMA) controller is controlling aDMA operation on the local bus, certain other controllerscan break-in to the current DMA operation, temporarilyhalting the DMA operation until the other controller hascompleted its data transfer operation. To break-in to aDMA operation, handshaking signals between the DMAcontroller and the local bus interface circuit aretemporarily blocked by blocking signals from a break-inlogic circuit. The break-in circuit includes afour-state state machine to block the handshaking signalsat the appropriate times, and to signal the interruptingcontroller to begin its data transfer operation. Whenbreaking-in to a DMA operation in this manner, theoperation of the DMA controller is not altered; instead,to the DMA controller, it appears that the local businterface circuit is merely slow to respond with itsacknowledge handshake.
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