Specifications

DMA Controller (V40)
Block Mode Transfers
In block transfer mode, the DCU continues servicing the DMA
channel until the count register is decremented to zero. At this time
the DCU releases control of the buses and enters a slave mode of
operation, permitting lower priority bus masters to gain access to the
bus resources.
Autoinitialization
Autoinitialization is useful when doing repetitive DMA transfers
using the same amount of data and the same memory locations. The
DMA channel can be programmed for autoinitialization with the
DMD register. If autoinitialize is selected, the DCU automatically
transfers the previously programmed base address and base count to
the current address and current count, respectively.
Pr
ogramming
The DCU is enabled and mapped into an I/O address range using the
V40 configuration register. DMA operation must be disabled with the
DDMA bit of the DDC register during DCU programming. Erroneous
operation may otherwise result. For example, if the two-byte address
register is being programmed by the CPU and a DMA request occurs,
it is possible that only one of the bytes may be updated before the
DMA channeled is serviced. The address would then be incorrect, and
the DMA operation invalid.
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