Below we have the diagram of DMA controller that explains its working:. Below we have a block diagram of DMA controller. Whenever a processor is requested to read or write a block of data, i. Thus the DMA controller is a convenient mode of data transfer. Your email address will not be published.
Save my name, email, and website in this browser for the next time I comment. Key Takeaways DMA is an abbreviation of direct memory access. Then the microprocessor tri-states all the data bus, address bus, and control bus. These are the four individual channel DMA request inputs, which are used by the peripheral devices for using DMA services. When the fixed priority mode is selected, then DRQ 0 has the highest priority and DRQ 3 has the lowest priority among them. These are the active-low DMA acknowledge lines, which updates the requesting peripheral about the status of their request by the CPU.
These lines can also act as strobe lines for the requesting devices. These are bidirectional, data lines which are used to interface the system bus with the internal data bus of DMA controller. In the Slave mode, it carries command words to and status word from The system design may also need pullup resistors or terminators on control signals such as read and write strobes so the control signals don't float to the active state during the brief period when neither the processor nor the DMA controller is driving them.
DMA controllers require initialization by software. Typical setup parameters include the base address of the source area, the base address of the destination area, the length of the block, and whether the DMA controller should generate a processor interrupt once the block transfer is complete. It's typically possible to have the DMA controller automatically increment one or both addresses after each byte word transfer, so that the next transfer will be from the next memory location.
Transfers between peripherals and memory often require that the peripheral address not be incremented after each transfer. When the address is not incremented, each data byte will be transferred to or from the same memory location. Some DMA controllers support both. In burst mode, the DMA controller keeps control of the bus until all the data buffered by the requesting device has been transferred to memory or when the output device buffer is full, if writing to a peripheral.
In single-cycle mode, the DMA controller gives up the bus after each transfer. This overhead can result in a drop in overall system throughput if a lot of data needs to be transferred. In most designs, you would use single cycle mode if your system cannot tolerate more than a few cycles of added interrupt latency. Likewise, if the peripheral devices can buffer very large amounts of data, causing the DMA controller to tie up the bus for an excessive amount of time, single-cycle mode is preferable.
Note that some DMA controllers have larger address registers than length registers. For instance, a DMA controller with a bit address register and a bit length register can access a 4GB memory space, but can only transfer 64KB per block.
If your application requires DMA transfers of larger amounts of data, software intervention is required after each block. This eliminates the need for external bus buffers and ensures that the timing is handled correctly. Also, an internal DMA controller can transfer data to on-chip memory and peripherals, which is something that an external DMA controller cannot do. Because the handshake is handled on-chip, the overhead of entering and exiting DMA mode is often much faster than when an external controller is used.
If an external DMA controller or processor is used, be sure that the hardware handles the transition between transfers correctly. To avoid the problem of bus contention, ensure that bus requests are inhibited if the bus is not free. Sign up. Term of the Day.
Best of Techopedia weekly. News and Special Offers occasional. Techopedia Explains Direct Memory Access DMA A defined portion of memory is used to send data directly from a peripheral to the motherboard without involving the microprocessor, so that the process does not interfere with overall computer operation. Memory addresses. Interrupt request numbers IRQ. Direct memory access DMA channels. Share this Term.
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