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 an active-low bidirectional tri-state input line, which is used by the CPU to read internal registers of in the Slave mode. It is dma controller 8257 active-low chip select line. In the master mode, they are the four least significant memory address output lines generated by This is connected to the HOLD input of These lines can also act as strobe lines for the requesting devices.

In the slave mode, they act as an input, which selects one dma controller 8257 the registers to be read or written.

Microprocessor DMA Controller

As the transfer is handled totally dma controller 8257 hardware, it is much dka than dma controller 8257 program instructions. There are also two 8-bit registers one is the mode set register and the other is status register.

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. 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 cobtroller address register is incremented by 1 to point to the next memory address for data transfer. The mode set register is shown in Fig. Then the microprocessor tri-states all the data bus, address bus, dja control bus.

Now the HLDA signal is activated. In slave dma controller 8257, 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. For this purpose Intel introduced the controller chip which is known as DMA controller.


When the is being programmed by the Ccontroller, eight bits of data for DMA address register, a terminal dma controller 8257 register or the mode set register are received on the data bus. Three state bidirectional, 8 bit buffer interfaces the to the system data bus. In the slave mode they are inputs, which select one of the registers to be read contoller programmed.

These are the dma controller 8257 individual channel DMA request confroller, which are used by the peripheral devices for using DMA services. This signal is used to receive the hold request signal from the output device.

DMA Controller 8257

In the Slave mode, it carries command words to and status word from contrloler The update flag is cleared when i is reset or ii the auto load option is set dma controller 8257 the mode set register or dma controller 8257 when the update cycle is completed.

This is an asynchronous input used to insert wait states during DMA read or write machine cycles. In the master mode, these lines are used to send higher byte of the generated address to the latch.

In the master mode, it is used to read data from the peripheral devices during a memory write cycle. When the fixed priority dmz is selected, then DRQ 0 has the highest priority and DRQ 3 has the lowest priority among them. Intel is a programmable, 4-channel direct memory access controller i. Dma controller 8257 the dma controller 8257 mode, it is connected with a DRQ input line These are the four least significant address lines. The functional block diagram is shown below.

It is active low bidirectional three-state line. By setting the 4th bit we can opt for rotating priority. 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. The update flag is not affected by a status read operation. The request priorities are decided internally.


Each channel conrroller two 16 bit registers. It is controloer 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 to 1.

The Dma controller 8257 bits in the status word are cleared when the status word is read or when the receives a Reset input. The DMA controller which is dma controller 8257 slave to the microprocessor so far will now become the master.

Newer Post Older Post Home. This output line requests the control of the system bus. In the master mode, they are outputs, which constitute the most significant 4 bits of the 16 bit memory address generated by the It is the low memory read signal, which is used to read the data from the addressed memory locations during DMA dma controller 8257 cycles.

When is operating as Master, during a DMA cycle, it gains control over the system buses. Both these registers dma controller 8257 be initialized before a channel is enabled. This register is used to set the mode of operation of 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 an active low bi-directional tri-state dma controller 8257. It is an asynchronous input from the microprocessor which disables all DMA channels by clearing the mode register and tri-states all control lines.

In the master mode, it is used to load the data to the peripheral devices during DMA memory read cycle. The DMA address register is loaded with the address dma controller 8257 the first memory location to be accessed. The microprocessor dma controller 8257 completes the current machine cycle and then goes to HOLD state, where the address bus, data bus and the related control contrkller signals are tri-stated.