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 the hold dma controller 8257 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.

In the master mode, they are the four least significant memory address output lines generated by When the fixed priority mode is selected, then Dma controller 8257 0 has the highest priority and DRQ 3 has the lowest priority among them. If the rotating priority bit is reset, is a zero each DMA channel has a fixed priority in the fixed priority mode. When is operating as Master, during a DMA cycle, it gains control over the system buses.

dma controller 8257

It is the low memory read dma controller 8257, which is used to read the data from the addressed memory locations during DMA read cycles. This is connected to the HOLD input of There are also two 8-bit registers one is the mode set register and the other is status register.

In the slave mode, it is connected with a DRQ input line In the master mode, it is used to load the data to the peripheral devices during DMA memory read cycle. A DMA controller can also transfer data from memory dma controller 8257 a port.


cotroller For this purpose Intel introduced the controller chip which is known as DMA controller. This is known as a DMA machine cycle, at the end of which, the number of bytes to be transferred dma controller 8257 decremented by 1 in the count register and address register is incremented by 1 to point to the next memory address for data transfer.

This signal is used to convert the higher byte of the memory address generated by the DMA dma controller 8257 into the latches.

DMA Controller 8257

In the master mode, these lines are used to send higher byte of the generated address to the latch. In slave mode, it is an input, which allows microprocessor to write. It is an active low bi-directional tri-state line. These are active low signals one dma controller 8257 each of the four DMA channels.

Microprocessor – 8257 DMA Controller

In the slave mode they are inputs, which select one of the registers to be read or programmed. The update flag is cleared when i is reset or ii the auto load option is set in the mode set register or iii dma controller 8257 the update cycle is completed.

The mode set register is shown in Fig. When the is being programmed by the CPU, eight bits dma controller 8257 data for DMA address register, a terminal count register contrkller the mode set register are received on the data bus.

Microprocessor DMA Controller

Dma controller 8257 cobtroller the four individual channel DMA request inputs, which are used by the peripheral devices for using DMA services. 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 are the dma controller 8257 least significant address lines. 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.

DMA Controller

The update flag is not affected by a status read operation. The TC bits in the status word are cleared when the dma controller 8257 word is read or when the receives a Reset input.

In the slave mode, they act as an input, which selects one of the registers to be read or written. The DMA controller which dma controller 8257 a slave to the microprocessor so far will now become the master. In the master mode, it is used to read data from the peripheral devices during a memory write cycle.

The request priorities are decided internally.

Three state bidirectional, 8 bit buffer interfaces the to the system data bus. It dma controller 8257 an asynchronous input from the microprocessor which disables all DMA channels by clearing the mode register and tri-states all control lines. The DMA address register is loaded with the address of the first memory location to be accessed.