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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It is active low bidirectional three-state line. It is the active-low three state signal which is used to write the data to the addressed memory location during Dma controller 8257 write operation.

DMA Controller 8257

A DMA controller temporarily borrows the address bus, data bus and control bus from the microprocessor and transfers the data bytes directly from dma controller 8257 port to memory devices.

When the is being programmed by the CPU, eight bits of data for DMA address register, a terminal count register or the mode set dmaa are dma controller 8257 on the data bus. If the rotating priority bit is reset, is a zero each DMA channel has a fixed priority in the fixed priority mode. It is the hold acknowledgement signal which indicates the DMA controller that the bus has dma controller 8257 granted to the requesting peripheral by the CPU contoller it is set to 1.

In the master mode, it is used to read data from the peripheral devices during a memory write cycle. The mode set register is shown in Fig. This signal is used to receive the hold request signal from the output device. Intel is a programmable, 4-channel direct memory access controller i. When the fixed priority mode is selected, then DRQ 0 dma controller 8257 the highest priority and DRQ 3 has the dmx priority among them.


A DMA controller can also 857 data from memory to a port. When is operating as Master, during controllef DMA cycle, it gains control over the system buses.

These dma controller 8257 the four individual channel DMA request inputs, which are used by the peripheral devices dma controller 8257 using DMA services. In the slave mode, they act as an input, which selects one of the registers to contrloler read or written. The value loaded into the low order 14 bits of the terminal count register specifies the number of DMA cycles minus one before the terminal count output is activated.

This is connected to the HOLD input of In the master mode, they are the four least significant memory address output lines generated by By setting the 4th bit we can opt for rotating priority.

Both these registers must be initialized before dma controller 8257 channel is enabled. The mark will be activated after each cycles or integral multiples of it from the beginning. Each channel has two 16 bit registers. These are active low signals one for each of the four DMA channels. It is designed by Intel to transfer data at the fastest rate. The TC bits in the status word are cleared when the status word is read or when the receives a Reset input. These lines can also dma controller 8257 as strobe lines for the requesting devices.

Dma controller 8257 dam flag is not affected by a status read operation.



But in the rotating priority mode the priority of the channels has a circular sequence and after each DMA cycle, the priority of each dma controller 8257 changes. Three state bidirectional, 8 bit buffer interfaces the to the system data bus.

In the master mode, it is used to load the data to the peripheral devices during DMA memory read cycle. This output line requests the control of the system bus. It is an asynchronous input from the microprocessor which disables all DMA channels by clearing the mode register and dma controller 8257 all control lines. Now the HLDA signal is activated.

Microprocessor – 8257 DMA Controller

The DMA address register is loaded with the address of the first memory location to be accessed. It can operate both in slave and master mode.

After this, the bus is released to handle dma controller 8257 memory data transfer during the remaining DMA cycle. 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 to the next memory address dma controller 8257 data transfer.

These are bidirectional, data lines which are used to interface the system dma controller 8257 with the internal data bus of DMA controller. This is an asynchronous input used to insert wait states during DMA read or write machine cycles.