Suppose any device which is connected at input-output port wants to transfer data to transfer data to memory, first of all it will send input-output port address and control signal, input-output read to input-output port, then it will send memory address and memory write signal to memory where data has to be transferred. In normal input-output technique the processor becomes busy in checking whether any input-output operation is completed or not for next input-output operation, therefore this technique is slow. This problem of slow data transfer between input-output port and memory or between two memory is avoided by implementing Direct Memory Access DMA technique. Suppose a floppy drive which is connected at input-output port wants to transfer data to memory, the following steps are performed:.
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No notes for slide. CPU having the control over the bus: 9. When DMA operates: Programmable DMA controller. The TC bits indicate if the channel has reached its terminal count transferred all its bytes.
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Direct memory access with DMA controller 8257/8237
The is a four-channel device that can be expanded to include any number of DMA channel inputs. The is capable of DMA transfers at rates of up to 1. Each channel is capable of addressing a full 64k-byte section of memory and can transfer up to 64k bytes with a single programming. The operates in four different modes, depending upon the number of bytes transferred per cycle and number of ICs used:. Memory-to-memory transfer can be performed. 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.
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