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DMA CONTROLLER
12.2.8 Cascade Mode
Cascade mode allows an external 8237A or another DMA-type device to gain bus control. A cas-
caded device requests bus control by holding a channel’s request input (DRQn) active. Once
granted bus control, the cascaded device remains bus master until it relinquishes bus control by
deactivating DRQn.
If a refresh request occurs while a cascaded device has bus control, the cascaded device must
deassert its request or the refresh cycle will be missed. The following steps take place in response
to a refresh request.
1. The channel deasserts its acknowledge signal (DACKn#) to the cascaded device.
— At this point, the cascaded device should relinquish bus control by removing DRQn.
2. As soon as DRQn is removed, the refresh cycle is started.
— At this point, if the cascaded device wants to regain bus control after the refresh cycle,
it must reassert DRQn.
3. If the cascaded device has reasserted DRQn when the refresh cycle is complete, the
channel reasserts DACKn#, giving bus control back to the cascaded device without bus
priority arbitration.
The following flowchart (Figure 12-17) shows this process flow.