It is designed by Intel to transfer data at the fastest rate. Then the microprocessor tri-states all the data bus, address bus, and control bus. Each channel has bit address and bit counter. Each channel can transfer data up to 64kb.
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Modes[ edit ] The operates in four different modes, depending upon the number of bytes transferred per cycle and number of ICs used: Single - One DMA cycle, one CPU cycle interleaved until address counter reaches zero. The CPU is permitted to use the bus when no transfer is requested. Actual bus signals is executed by cascaded chip. 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.
This happens without any CPU intervention. It is used to repeat the last transfer. At the end of transfer an auto initialize will occur configured to do so. Single mode[ edit ] In single mode only one byte is transferred per request. For every transfer, the counting register is decremented and address is incremented or decremented depending on programming. When the counting register reaches zero, the 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. Auto-initialization may be programmed in this mode. 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 the , but 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.
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. 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. 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, Like the first , it is augmented with four address-extension registers.
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.
DMA transfers on any channel still cannot cross a 64 KiB boundary. 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 temporary 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. 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.
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. This technique is called "bounce buffer".
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. Integration into chipsets[ edit ] Although this device may not appear as a discrete component in modern personal computer systems, it does appear within system controller chip sets.
For example, the P ISP integrated system peripheral controller has two DMA internal controllers programmed almost exactly like the
Microprocessor - 8257 DMA Controller