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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This is known as a DMA machine dma controller 8257, 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.

By setting the 4th bit cotroller can opt for rotating priority.

Intel is a programmable, 4-channel direct memory access controller dma controller 8257. But in the rotating priority mode the priority of the channels controlle a circular sequence and after each DMA cycle, the priority of each channel changes. In the master mode, they are outputs, which constitute the most dma controller 8257 4 bits of the 16 bit memory address generated by the 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 dja the data transfer effectively.

These are the 82557 DMA acknowledge lines, which updates the requesting peripheral about the status of their request by the CPU. These are the four least significant address lines. The DMA controller which is a slave to the dma controller 8257 so far will now become the master. When is operating as Master, during a DMA cycle, it gains control over the system buses.

In the Slave mode, it carries command words to and status word from It dma controller 8257 an active low bi-directional tri-state line.

Microprocessor – 8257 DMA Controller

The terminal count TC bits bits 0 – 4 for the four channels are set when the Terminal Count dma controller 8257 goes high for a channel. This signal is used to receive the hold request signal from the output device. Now the HLDA signal is activated. 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 the active-low three state signal which is used to write the data to the dma controller 8257 memory location during DMA write operation.

It can operate both in slave and master mode. It is an active-low chip select line. If dma controller 8257 rotating priority bit is reset, is a zero each DMA channel has a fixed priority in the fixed priority mode.

Three state bidirectional, 8 bit buffer interfaces the to the system data bus.

Dma controller 8257 the master mode, they are the four least significant memory address output lines generated by This block controls the sequence operations during all DMA cycles by generating the appropriate control signals and 16 bit address that specifies the memory relations to be accessed. The mark will be activated after each cycles contorller integral multiples of dma controller 8257 from the beginning.

The update flag is not affected by a status read operation. The update flag is cleared when i is reset or ii the auto load option is set in the dma controller 8257 set register or iii when the update cycle is completed.

The output acts as a chip select for the peripheral device requesting service.


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. These least significant four address lines are bidirectional. This is the clock output of dma controller 8257 microprocessor. The different signals are.

This is connected to the HOLD input of 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. These lines can also act as strobe lines for the requesting devices. These are bidirectional, data dma controller 8257 which are used to interface the system bus with the internal data bus of DMA controller. It is a totally TTL compatible chip.

It is the low memory read signal, which is used to read the data from the addressed memory locations during DMA read dma controller 8257. The TC bits in the status word are cleared when the status word is read or when the receives a Reset input. This register is used to set the mode of operation of Both these registers must be initialized before a channel is enabled.

The request priorities are decided internally. Then the microprocessor tri-states all the data bus, address bus, and control bus. It is active low bidirectional three-state line. These are active low signals one for each of the four DMA channels. In the master mode, it is used to read data from the peripheral devices during a memory write cycle.

There are also two 8-bit registers one is the mode set register and the other is status register. The mode set register is shown in Fig. A DMA dma controller 8257 can also transfer data from memory to a port.

These four address lines are tri-stated outputs which contains 4 to dma controller 8257 of the 16 bit memory address generated by the during all DMA cycles.