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Concept of Direct Memory Access (DMA)
The microcomputer system basically consists of three blocks
- The microprocessor
- The memories of microprocessor like EPROM and RAM
- The I/O ports by which they are connected.
The possible data transfers are indicated below.
Between the memory and microprocessor data transfer occurs by using the LDA and STA instructions.
Between microprocessor and I/O ports also data transfer occurs by the help of two instructions IN and OUT.
Through DMA data transfer, data is transferred between the Input Output ports and the memory.
Fig: The figure shows possible ways of data transfers in a microcomputer system.
For performing the data transfer either between the microprocessor and memory or between I/O ports and the processor instructions for the programs are executed such that, the data transfers which are called as programmed data transfer, are already in used when a small amount of data transfer is involved in the process.
Both the microprocessor and the memory are made of semiconductor chips that work in comparable to electronic speeds. So it is not all a problem to transfer the data between microprocessor and memory. If the case arises such that the memory is slow than the microprocessor, the processor used has to insert wait states. No problem other than this can occur.
The ports like 8255, 8212 and others are also made by semiconductor chips which work at electronic speeds. We use them for interfacing the INPUT–OUTPUT devices which are electromechanical in nature, and hence the speed of the microprocessor turns slower which results the transfer of data between microprocessor and INPUT-OUTPUT ports becomes very complex in nature.
If the data transfer which is used for reading from the memory location is 3456H and then writing to output port number is 50H which takes 13 clocks only for reading from memory location 3456H using the LDA 3456H instruction and ten clocks for writing to the output port number 50H. Therefore 23 clocks in total. If the processor works at an internal frequency of 3-MHz with a clock period of 0.33μs, it takes a fraction of 7.66μs to work externally. By using the program data transfer scheme, input port 40H is read and written to the memory location input port 2345H
If we use DMA data transfer to read from memory location 3456H and write to the output port number 50H only four clocks are needed, which results a fraction of only 1.33μs. Some devices like hard disk and floppy disk can perform data transfer at a very fast speed. A floppy disk of memory 1.44 MB which rotates at a speed of 360 rpm, having 18 sectors per track, can store up to 512 bytes of memory, when the rate of data transfer becomes 54K bytes in a second, or about 19μs per byte. Hard disks can easily transfer data at least ten times faster. Hence it turns out to be 1.9μs per byte. This is the situation when DMA data transfer becomes compulsory. In the programmed data transfer process where we need 7.66μs per byte only the 4 bytes comes out from the hard disk in the same time repeatedly. Hence it can be concluded that I/O devices plays an important role in the DMA data transfer process.
To obtain statistics like average, the largest, and the smallest values, we use Analog to Digital converter. Using the programmed data transfer scheme 1,000 × 7.66 = 7,660μs of processor time is needed. But surprisingly using the DMA data transfer scheme only 1,000 × 1.33 = 1,330μs of time is needed. So when large amount of data is to be transferred between the memory and I/O port by routing each byte through microprocessor, it takes a lot of time, so to do the work in less time the I/O port can be directly accessed to the memory for data transfer. This scheme is termed as DMA data transfer.
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