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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STUDY LIKE A PRO: DMA Controller – Intel /

This register is used to set the mode of operation of These lines can also act as strobe lines for the requesting devices. When the is being programmed by the CPU, eight bits of data for DMA address register, dma controller 8257 terminal count register or the mode set register are received on the data bus. This signal is used to receive the hold request signal from the output device. In Direct Memory Access technique, the data transfer takes place without the intervention of CPU, so there must be a dma controller 8257 circuit which is programmable and which can perform the data transfer effectively.

Intel is a programmable, 4-channel direct memory access controller i. The output acts as a chip select for the peripheral device requesting service. A DMA controller temporarily borrows the address bus, data bus and control bus from the microprocessor and transfers the data bytes directly from the port to memory devices. The TC bits in the status word are cleared when the status word is read dma controller 8257 when the receives a Reset input.


It is active low bidirectional three-state line. Controllre the Slave mode, it carries command words to and status word from In the master mode, they are outputs, which constitute the most significant 4 bits of the 16 bit memory address dma controller 8257 by the Now the HLDA signal is activated.

These are cojtroller, data lines which are used to interface the system bus with dma controller 8257 internal data bus of DMA controller.

Both these registers must be initialized before a channel is enabled. For this purpose Intel introduced the controller chip which is known as DMA controller. It is designed by Intel to transfer data at the fastest rate. These are active low signals one for each of the four DMA channels. This is the clock output of the microprocessor. When is operating as Master, during a DMA cycle, it gains control over the dma controller 8257 buses.

The terminal count TC bits bits 0 – 4 for the four dma controller 8257 are set when the Terminal Count output goes high for a channel. This block controls the sequence operations during all DMA cycles by generating the appropriate control signals and 16 bit address that specifies the memory dka to be accessed.

This is known as a DMA machine cycle, at the end of which, the number of mda to be transferred is decremented by 1 in the count register and address register is incremented by 1 to point to dma controller 8257 next memory address for data transfer. Then the microprocessor dma controller 8257 all the data bus, address bus, and control bus.


DMA Controller 8257

Dma controller 8257 are the four individual channel Dma controller 8257 request inputs, which are used by the peripheral devices for using DMA services. The different signals are. Three state bidirectional, 8 bit buffer interfaces the to the system data bus.

But in the rotating priority mode the priority dam the channels has a circular sequence and after each DMA cycle, the priority of each channel changes.

As the transfer is handled totally by hardware, it is much faster than software program instructions. This output conhroller requests the control of the system bus.

In dma controller 8257 slave mode they are inputs, which select one of the registers to be read or programmed. In the slave mode, it is connected with a DRQ input line This is connected to the HOLD input of The update flag is not affected by a status read operation.

dna These are the active-low DMA acknowledge lines, which updates the requesting peripheral about the status of their request dma controller 8257 the CPU. The microprocessor then completes the current machine cycle and then goes to HOLD state, where the address bus, data bus and the related control bus signals are tri-stated.

It is an active low bi-directional tri-state line.

By setting the 4th bit we can opt for rotating priority. It is an active-low chip select line. The functional block diagram is shown below.