Microprocessor DMA Controller in Microprocessor – Microprocessor DMA The following image shows the pin diagram of a DMA controller − . Addressing Modes & Interrupts · Microprocessor – Instruction Sets. For this purpose Intel introduced the controller chip which is known as DMA controller. A DMA controller temporarily borrows the address. In computing, a programmable interrupt controller (PIC) is a device that is used to combine several sources of interrupt onto one or more CPU lines, while.

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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 perform the data transfer effectively. For this purpose Intel introduced the controller chip which is known as DMA controller.

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.

Microprocessor 8257 DMA Controller Microprocessor

As the transfer is handled totally by hardware, it is much faster than software program instructions. A DMA controller can also transfer data from memory to a port. Intel interruupt a programmable, 4-channel direct memory access controller i.

The request priorities are decided internally. Each channel has two 16 bit registers. There are also two 8-bit registers one is the mode set register and the other is status register.

It can operate both in slave and master mode.

It is a totally TTL compatible chip. The functional block diagram is shown below. Three state bidirectional, 8 bit buffer interfaces the to the system data bus. When the is being programmed by the CPU, eight bits of data for DMA address register, a terminal count register or the mode set register are received on the data bus. When is operating as Master, during a DMA cycle, it gains control over the system buses. After this, the bus is released to handle the memory data transfer during the remaining DMA cycle.


The different signals are. It is active low bidirectional three-state line. It is an active low bi-directional tri-state line. In slave mode, it is an input, which allows microprocessor to write.

This is the clock output of the microprocessor. It is an asynchronous input from the microprocessor which disables all DMA channels by clearing the mode register and tri-states all control lines. These least significant four address lines are bidirectional. In the conrroller mode they are inputs, which select one of the registers to be read or programmed.

Microprocessor – 8257 DMA Controller

In the master mode, they are outputs, which constitute the most significant 4 bits of the 16 bit memory address generated by the This block controls the sequence operations during all DMA cycles by generating the appropriate control signals and 16 bit inteerupt that specifies the memory relations to be accessed. These four address lines are tri-stated outputs which contains 4 to 7 of the 16 bit memory address generated by the during all DMA cycles.

This is an asynchronous input used to insert wait states during DMA read or write machine cycles. This output line requests the control of the system bus. This is connected to the HOLD input of Both these registers must be initialized before a channel is enabled. The DMA address register is loaded with the address of the first memory location to be accessed.

Programmable interrupt controller – Wikipedia

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 interrupt output is activated.

These are active low signals one for each of the four DMA channels.

The output acts as a chip select for the peripheral device requesting service. This register is used to set the mode of operation of The mode set register is shown in Fig.


By setting the 4th bit we can opt for rotating priority. But in the rotating priority mode the priority of the channels has a circular sequence and after each DMA cycle, the priority of each channel changes. If the rotating priority bit is reset, is a zero each DMA channel has a fixed priority interrpt the fixed priority mode.

The terminal count TC bits bits 0 – 4 for the four channels are set when the Terminal Count output goes high for a channel.

Intel Programmable DMA Controller

The TC bits in the status word are cleared when the status word is read or when the receives a Reset input. The update flag is cleared when i controllet reset or ii the auto load option is set in the conyroller set register or iii when the update cycle is completed.

The update flag is not affected by a status read operation. 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.

Now the HLDA signal is activated. The DMA controller which is a slave to the microprocessor so far will now become the master. This is known as a DMA machine cycle, at the end of 82557, 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 for data transfer.

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