• It is a device to transfer the data directly between IO device and memory without through the CPU. So it performs a high-speed data transfer between memory and I/O device.

The features of 8257 is,

  • The 8257 has four channels and so it can be used to provide DMA to four I/O devices

  • Each channel can be independently programmable to transfer up to 64kb of data by DMA.

  • Each channel can be independently perform read transfer, write transfer and verify transfer.

It is a 40 pin IC and the pin diagram is,
Functional Block Diagram of 8257:

  • The functional blocks of 8257 are data bus buffer, read/write logic, control logic, priority resolver and four numbers of DMA channels.

The functional block diagram of 8257 is shown in fig.

Functional block diagram
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