Direct Memory Access                                                                                                                            Print this page
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When the processor wishes to read or write a block of data, it issues a command to the DMA module, by sending to the DMA module the following information.

    • Whether a read or write is requested, using the read or write control line between the processor and the DMA module.
                    
    • The address of the I/O devise involved, communicated on the data lines.
                     
    • The starting location in the memory to read from or write to, communicated on data lines and stored by the DMA module in its address register.
                          
    • The number of words to be read or written again communicated via the data lines and stored in the data count register.

The processor then continues with other works. It has delegated this I/O operation to the DMA module.

The DMA module checks the status of the I/O devise whose address is communicated to DMA controller by the processor. If the specified I/O devise is ready for data transfer, then DMA module generates the DMA request to the processor. Then the processor indicates the release of the system bus through DMA acknowledge.

The DMA module transfers the entire block of data, one word at a time, directly to or from memory, without going through the processor.
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