PROGRAMMABLE DMA CONTROLLER – INTEL • It is a device to transfer the data directly between IO device and memory without through the CPU. 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 Intel* is a 4-channel direct memory access (DMA) controller. It is specifically designed to simplify the transfer of data at high speeds for the Intel®.

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When the is being programmed by the CPU, eight bits of data for Vma address register, a terminal count register or the mode set register are received on the data bus.

The value loaded into the low order 14 bits of the terminal count register specifies the number of Dma controller 8257 cycles minus one before the terminal count output is activated. The TC bits in the status word are cleared when the status word is contriller or when the receives a Reset input. There are also two 8-bit registers one is dma controller 8257 mode set register and the other sma status register.

The different signals are. The terminal count TC bits bits 0 – 4 for the four channels are set when the Terminal Count output goes high for a channel.

It is a totally TTL compatible chip.

In Direct Memory Access technique, the data transfer takes place without the intervention of CPU, so there must be a controller circuit which is programmable and which can perform the data transfer effectively.


When is operating as Master, during a DMA cycle, it gains control over dma controller 8257 system buses. The update flag is cleared when i dma controller 8257 reset or ii the auto load option is set in the mode set register or iii when the update cycle is completed.

Microprocessor – 8257 DMA Controller

The request priorities are decided internally. Each channel has two 16 bit registers.

In the Slave mode, it carries command words to and status word from It dma controller 8257 active low bidirectional three-state line. Dma controller 8257 signal is used to convert the higher byte of the memory address generated by the DMA controller into the latches.

It is an active low bi-directional tri-state line. Intel is a programmable, 4-channel direct memory access controller i.

These are active low signals one for each of the four DMA channels. But controloer the rotating priority mode the priority of the channels has a circular sequence and after each DMA cycle, the priority of each channel dma controller 8257.

In the slave mode controlller are inputs, which select one of the registers to be read or programmed.

DMA Controller 8257

In the slave mode, they act as an input, which selects one of the registers to be read or written. The microprocessor then completes the current machine dma controller 8257 and then goes to HOLD state, where the address bus, data bus and the related control bus signals are tri-stated.

In the slave dma controller 8257, it is connected with a DRQ input line This register is used to set the mode of operation of This is known as a DMA machine cycle, at the end of which, the number of bytes to be transferred is decremented by 1 in the count register and address register is incremented by 1 to point dma controller 8257 the next memory address for data transfer.


In slave mode, it is an input, which allows microprocessor to write. These are bidirectional, data lines which are used to interface the system bus with the internal data bus of DMA controller.


This signal is used to receive dma controller 8257 hold request signal from the output device. It is an asynchronous input from the microprocessor which disables all DMA channels by clearing the mode register and tri-states all control lines. It is the hold acknowledgement signal which indicates the DMA controller that the bus has been granted to the requesting peripheral by the CPU when it is set dma controller 8257 1.

It is designed by Intel to transfer data at the fastest rate. The update flag is not affected by a status read operation. The DMA controller which is a slave to the microprocessor so far will now become the master. When the fixed priority mode is selected, then DRQ 0 dma controller 8257 the highest priority and DRQ 3 has the dma controller 8257 priority among them.

After this, the bus is controllr to handle the memory data transfer during the remaining DMA cycle.