Data Bus D 0 -D 7 : These are bi-directional tri-state signals connected to the system data bus. When CPU is having control of system bus it can access contents of address register, status register, mode set register, and a terminal count register and it can also program, control registers of DMA controller, through the data bus. During DMA cycles these lines are used to send the most significant bytes of the memory address from one of the. In the idle cycle they are inputs and used by the CPU to address the register to be loaded or read. In the Active cycle they output the lower 4 bits of the address for DMA operation.
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Actions Shares. Embeds 0 No embeds. No notes for slide. Microprocessor then executes the program until it needs to read a block of data from the disk. Asserting the IOR signals enables the disk controller to output the data from the disk on the data bus and MEMW signal enables the addressed memory to accept data form the data bus.
Each channel can be programmed independently to transfer up to 64kb of data by DMA. The priority logic can be programmed to work in two modes, either in fixed mode or rotating priority mode. The functional block diagram consists of 1. DMA channels 2. Data bus buffer 3. Control logic 5. Mode set Register 6. Status Register Eight bits of data for DMA address register, terminal count register or the Mode Set register are received on the data bus. After this, the bus is released to handle the memory data transfer during the remaining DMA cycle.
HRQ must conform to specified setup and hold times. So this pin is used to split data and address line from the DMA. So we need to demultiplex these data lines for which AEN pin is used. When the rotating priority mode is selected, then DRQ0 will get the highest priority and DRQ3 will get the lowest priority among them. These lines can also act as strobe lines for the requesting devices. In the master mode, the lines which are used to send higher byte of the generated address are sent to the latch.
In the master mode, it also helps in reading the data from the peripheral devices during a memory write cycle. In the master mode, it is used to load the data to the peripheral devices during DMA memory read cycle.
In the slave mode, they perform as an input, which selects one of the registers to be read or written. In the master mode, they are the outputs which contain four least significant memory address output lines produced by In the slave mode, it is connected with a DRQ input line You just clipped your first slide!
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Direct memory access with DMA controller 8257/8237
Direct Memory Access (DMA) in Computer Architecture
It holds the ability to directly access the main memory for read or write operation. DMA controller was designed by Intel , to have the fastest data transfer rate with less processor utilization. We know in order to execute an operation, the microprocessor first fetches the instruction and then decodes it, then further execute it. But individually if the processor is performing all the task inside the system then it unnecessarily keeps the processor busy all the time. So, to enhance the performance of the processor, an external device is used that can manage data transfer operation between peripherals and memory with least CPU utilization. Basically, it is nothing but hardware controlled data transfer, where the address and control signals required for transferring the data is generated by an external device. This method of data transfer is known as direct memory access and the external device used for this purpose is known as DMA controller.