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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-97F97D7B-D5A3-5471-A359-77B805ED198C" xml:lang="en"><title>Design</title><shortdesc>Describes different modes of operations of the DMA Framework .</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    13 <p>Some DMA controllers provide several modes of operation. For example, one

    14 ASSP provides both single-buffer mode and scatter-gather mode transfers. </p>

    15 <p>There are two options: </p>

    16 <ul>

    17 <li id="GUID-EAFDBEFC-1524-55C4-BBD3-AF4CD88ADEF1"><p>Select a single mode

    18 of operation. </p> </li>

    19 <li id="GUID-ED15AB92-3A9B-5968-A5C9-E49309F14EE1"><p>Select a multiple mode

    20 of operation, and split the physical DMA controller into several logical controllers,

    21 one for each mode to be supported. If this option is chosen, the PSL must

    22 include one concrete controller class per logical controller; a controller

    23 class is derived from <xref href="GUID-25398075-927B-36E4-B953-16785EC4086C.dita"><apiname>TDmac</apiname></xref>. </p> <p>If the DMA controller

    24 supports more than one mode of operation, implement the simplest one first.

    25 Implementing single-buffer mode first allows most of the the PSL (platform-specific

    26 layer) to be debugged before you start to implement the more complex scatter-gather

    27 functionality. </p> </li>

    28 </ul>

    29 <p>One reference implementation adopts a mixed strategy; the mode of operation

    30 is selectable at build-time by defining the macro <codeph>__DESFETCHMODE__</codeph> to

    31 exercise and demonstrate two different modes of operation. </p>

    32 <p>Scatter-gather support is essentially a superset of single-buffer support

    33 because a scatter-gather list is an ordered set of variable sized buffers,

    34 and means that code shown here demonstrates the implementation of a scatter-gather

    35 DMA controller. </p>

    36 <p>Almost all modern MCUs use scatter-gather capable DMA controllers. Note

    37 that we refer to descriptor fetch mode synonymously with scatter-gather mode,

    38 as scatter-gather is implemented by passing the DMA controller a linked list

    39 of descriptors describing the buffers to be transferred. Non–descriptor mode

    40 requires that the DMA controller registers are programmed by software to describe

    41 the buffer transfer required. Since each transfer must be handled by an interrupt

    42 service routine triggered when the DMA controller has completed the transfer,

    43 non-descriptor mode is not capable of performing transfers past buffer boundaries. </p>

    44 </conbody></concept>