Direct memory access with DMA controller 8257/8237

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. This is faster as the microprocessor/computer is bypassed and the control of address bus and data bus is given to the DMA controller.

Suppose a floppy drive which is connected at input-output port wants to transfer data to memory, the following steps are performed:

Modes of DMAC:

  1. Single Mode – In this only one channel is used, means only a single DMAC is connected to the bus system.

  2. Cascade Mode – In this multiple channels are used, we can further cascade more number of DMACs.

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