Direct Memory Access

Published on Nov 12, 2015


The Idea Of Direct Memory Access

By providing circuitry to generate addresses and memory control signals independently of the processor we see that it is possible to improve performance considerably. All that is necessary is to provide a signal to tell the processor to remove itself from the bus so there is no contention on the memory control, address, or data lines.

This is the entire concept behind direct memory access (DMA). To complete our picture, it is important to note that there are actually several ways that DMA can be done. In particular, there are three common choices:-

burst mode DMA :- In burst mode the DMA controller transfers an entire block of data without interruption. During the time of the transfer, the DMA controller retains complete control of the processor's bus.

Cycle Stealing :- If it is desirable to let the processor do some processing while the DMA controller transfers data cycle stealing can be used. In this case, the DMA controller will periodically request use of the bus until all of its data are transferred. This slows down the processor and the DMA transfer.

Transparent DMA :- It is possible for the DMA controller to monitor the internal status of the processor. In those cases where the processor does not need bus access (internal data moves and such) the DMA controller uses the bus. This slows down the DMA transfer, but not the processor.

The proper transfer mode to use is determined by the system requirements.

Burst Mode DMA

Burst mode DMA is a typical way of handling bulk data transfers (like the idea we described for our video controller). The other modes operates in a similar fashion.

For a burst mode transfer:-

1) The processor loads the DMA controller with the start address of the destination in memory and the number of words to transfer.

2) When the device delivering or requesting data is ready, the DMA controller is signaled. In turn, the
controller signals the processor that a DMA transfer is pending.

3) The processor acknowledges the request, finishes its current instruction, and then floats the address, control, and data buses.

4) The DMA controller now provides address and control signals to memory while the device requesting the transfer supplies or receives the data.

5) Once the transfer is complete, the DMA signals the processsor to resume its normal operation.

The signaling of the 8088 processor is done through the hold and holda lines. When the DMA controller wants the bus, it signals the processor by raising hold. After the processor completes the current instruction, it tri-states the data, address, and control lines, and raises hold acknowledge (holda). Once the DMA controller sees holda, it knows it is free to use the bus.

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