Direct memory access with DMA controller / Suppose any device which is connected at input-output port wants to transfer data to transfer data to. The A Multimode Direct Memory Access (DMA) Controller is a peripheral three basic transfer modes allow programmability of the types of DMA service by . DMA Controller is a peripheral core for microprocessor systems. It controls data transfer between the main memory and the external systems with limited.

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So that it can address bit words, it is connected to the address bus in such a way that it counts even addresses 0, 2, 4, When the counting register reaches zero, conrroller terminal count TC signal is sent to the card. The transfer continues until end of process EOP either internal or external is activated which will trigger terminal count TC to the card.

The IBM PC and PC XT models machine types and have an CPU and an 8-bit system bus architecture; the latter interfaces directly to thebut the has a bit address bus, so four additional 4-bit address latches, one for each DMA channel, are added alongside the to augment the address counters.

Introduction of -DMA

This page was last edited on 21 Mayat For example, the P ISP integrated system peripheral controller has two DMA internal controllers programmed almost exactly like the It is used to repeat the last transfer.

In single mode only one byte is transferred per request. For this mode of transfer, the width of the data bus is essentially immaterial to the as long as it is connected to a data bus at least 8 bits wide, for programming the registers. Auto-initialization may be programmed in this mode. In auto initialize mode the address and count values are restored upon reception of an end of process EOP 8327.


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In general, it loses any overall speed benefit associated with DMA, but it may be necessary if a peripheral requires to be accessed by DMA due to either demanding timing requirements or hardware interface inflexibility. For every transfer, the counting register is decremented and address is incremented or decremented depending on programming. From Wikipedia, the free encyclopedia. Because the memory-to-memory DMA mode operates by transferring a byte from the source memory location to an internal temporary 8-bit register in the and then from the controkler register to the destination memory location, this mode could not be used for bit memory-to-memory DMA, as the temporary register is not large enough.

This means data can be transferred from one memory device to another memory device. The channel 0 Current Address register is the source for the data transfer and channel 1 and the transfer terminates when Current Word Count register becomes 0.

Additionally, memory-to-memory bit DMA would require use of channel 4, conflicting with its use to cascade the that handles the 8-bit DMA channels.

Each channel is capable of addressing a full 64k-byte section of memory and can transfer up to 64k bytes with a single programming. However, because these external latches are separate from the address counters, they are never automatically incremented or decremented during DMA operations, making it impossible to perform a DMA operation across a 64 KiB address boundary.


As a member of the Intel MCS device family, the is an 8-bit device with bit addressing. Although this device may not appear as a discrete component in modern personal computer systems, it does appear within system controller chip sets.

At the end of transfer an auto initialize will occur configured to do so.

Introduction of 8237

This happens without any CPU intervention. Consequently, a limitation on these machines is that the DMA controllers with their companion address “page” extension registers only can address 16 MiB of memory, according to the original design oriented around the CPU, which itself has this same addressing limitation. DMA transfers on any channel still cannot cross a 64 KiB boundary. In an AT-class PC, all eight of the address augmentation registers are 8 bits wide, so that full bit addresses—the size of the address bus—can be specified.

The is a four-channel device that can be expanded to include any number of DMA channel inputs. This technique is called “bounce buffer”. The operates in four controlller modes, depending upon the number of bytes transferred per cycle and number of ICs used:.

Memory-to-memory transfer can be performed. The is capable of DMA transfers at rates of up to 1.

Like the firstit is augmented with four address-extension registers.