Direct Memory Access (DMA)

 

  • DMA is a process of communication for data transfer between memory and input/output, controlled by an external circuit called DMA controller, without involvement of CPU.
  • 8085 MP has two pins HOLD and HLDA which are used for DMA operation.
  • First, DMA controller sends a request by making Bus Request (BR) control line high. When MP receives high signal to HOLD pin, it first completes the execution of current machine cycle, it takes few clocks and sends HLDA signal to the DMA controller.
  • After receiving HLDA through Bus Grant (BG) pin of DMA controller, the DMA controller takes control over system bus and transfers data directly between memory and I/O without involvement of CPU. During DMA operation, the processor is free to perform next job which does not need system bus.
  • At the end of data transfer, the DMA controller terminates the request by sending low signal to HOLD pin and MP regains control of system bus by making HLDA low.
Direct Memory Access (DMA)

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