Direct Memory Access ppt
- 1. www.oeclib.in Submitted By: Odisha Electronics Control Library Seminar On Direct Memory Access
- 2. Content Introduction History What is DMA? What is actually DMA? Application DMA Channels Different Modes of DMA operation DMA Operational Modes and Settings Types DMA Signals Advantages Disadvantages Conclusion References
- 3. Introduction In many I/O interfacing applications and certainly in data acquisition systems, it is often necessary to transfer data to or from an interface at data rates higher than those possible using simple programmed I/O loops. Microprocessor controlled data transfers within the PC (using the IN (port) and OUT (port) instructions) require a significant amount of CPU time and are performed at a significantly reduced data rate. Further to this, the CPU cannot perform any other processing during program controlled I/O operations.
- 4. History An IBM (International Business Machines) compatible computer system includes two Intel 8237 compatible DMA controllers. A complete description of the 8237 DMA controller is found in the 8237A High Performance Programmable DMA Controller datasheet published by Intel Corporation, and hereby incorporated by reference.
- 5. What is DMA? The transfer of data between a fast storage device such as magnetic disk and memory is often limited by the speed of the CPU. Removing the CPU from the path and letting the peripheral device manage the memory buses directly would improve the speed of transfer .this transfer technique is called Direct memory access(DMA).
- 6. What is actually DMA? When data is ready for transfer, the board directs the system DMA controller to put it into in system memory as soon as possible. As soon as the CPU is able (which is usually very quickly), it stops interacting with the data acquisition hardware and the DMA controller moves the data directly into memory. The DMA controller gets ready for the next sample by pointing to the next open memory location. The previous steps are repeated indefinitely, with data going to each open memory location in a continuously circulating buffer. No interaction between the CPU and the board is needed.
- 7. Application DMA has been a built-in feature of PC architecture since the introduction of the original IBM PC. PC-based DMA was used for floppy disk I/O in the original PC and for hard disk I/O in later versions. PC-based DMA technology, along with high- speed bus technology, is driven by data storage, communications, and graphics needs–all of which require the highest rates of data transfer between system memory and I/O devices.
- 8. DMA Channels The 8237 DMA controller provided by IBM (International Business Machines) is a peripheral interface circuit for allowing peripheral devices to directly transfer data to or from main memory. It includes four independent channels and may be expanded to any number of channels by cascading additional controller chips. In the IBM architecture, two DMA controllers are used. One DMA controller is used for byte transfers, and the second DMA controller is user for word (16-bit) transfers.
- 9. Different Modes of DMA operation There are three different modes of DMA operations: Continuous DMA Cycle stealing interleaved DMA
- 10. DMA Operational Modes and Settings Single A single byte (or word) is transferred. The DMA must release and re-acquire the bus for each additional byte. This is commonly-used by devices that cannot transfer the entire block of data immediately. The peripheral will request the DMA each time it is ready for another transfer. Block/Demand Once the DMA acquires the system bus, an entire block of data is transferred, up to a maximum of 64K. If the peripheral needs additional time, it can assert the READY signal to suspend the transfer briefly.
- 11. Types Bus-Master DMA Devices Bus-master DMA devices are by far the most common type of DMA devices on Windows systems. A bus-master DMA device contains all the electronics and logic necessary to take control of, or “master,” the bus on which it is located and to autonomously transfer data between the device’s buffer and the host’s system memory. System DMA Devices System DMA devices are vestiges of the original IBM PC design. These devices rely on a DMA controller chip on the motherboard to perform data transfers.
- 12. DMA Signals DREQ0-DREQ3-DMA Request: These are four independent, asynchronous input Signals to the DMA channels through peripherals such as floppy disks and the hard disk. DACK0-DACK3-DMA Acknowledge: These are output lines to inform the individual peripheral that a DMA is granted, DREQ and DACK are equivalent to handshake signals in I/O devices. o AEN and ADSTB—Additives Enable and Address Length: These are active high output signals that are used to latch a high-order address byte to generate a 16-bit address.
- 13. Advantages DMA is fast because a dedicated piece of hardware transfers data from one computer location to another and only one or two bus read/write cycles are required per piece of data transferred. DMA is usually required to achieve maximum data transfer speed, and thus is useful for high speed data acquisition devices. DMA also minimizes latency in servicing a data acquisition device because the dedicated hardware responds more quickly than interrupts, and transfer time is short.
- 14. Disadvantages Cost of DMA hardware DMA is useful only for DATA commands. All non-data commands have to be executed by CPU. Data has to be stored in continuous locations in memory. CPU's intervention is required for initializing DMA logic for every continuous data block transfer. In other words, DATA CHAINING is not possible.
- 15. Conclusion National Instruments uses DMA hardware and software technology to achieve high throughput rates as well as to increase system utilization. These achievements are accomplished by using a background mechanism of data transfer that minimizes CPU usage. Data acquisition users are highly aware of the advantages of background data acquisition, and DMA solutions have been very popular. Lab Driver double-buffered data acquisition features are
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