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Understanding Direct Memory Access (DMA) in Computer Systems

This excerpt elaborates on the concept of Direct Memory Access (DMA) as a method for efficiently transferring large amounts of data from devices like disk drives without overburdening the CPU. By employing a specialized DMA controller, systems can offload data transfer tasks, allowing the CPU to continue processing other tasks. The text outlines how DMA operates, including the steps to initiate a transfer, the role of device drivers, and the advantages and disadvantages of using DMA versus programmed I/O. Overall, DMA enhances system performance but comes with considerations for security and stability.

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Understanding Direct Memory Access (DMA) in Computer Systems

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Presentation Transcript

  1. 7.2 Peripheral Supplementary 2 DMA

  2. Reference • Siberschatz, “Operating System Concepts Sixth Edition”, John Wiley and Sons, Inc

  3. Programmed I/O • For a device that does large transfers, such as disk drive, it seems wasteful to use an expensive general-purpose processor to watch status bits and to feed data into a controller register 1 byte at a time. • This process termed “programmed I/O”

  4. Direct Memory Access (DMA) • Many computers avoid burdening the main CPU with PIO by offloading some of this work to a special-purpose processor called a direct memory-access (DMA) controller.

  5. DMA • To initiate a DMA transfer: • The host writes a DMA command block into memory • This block contains • A pointer to the source of a transfer • A pointer to the destination of the transfer • A count pf the number of bytes to be transferred. • The CPU writes the address of this command to the DMA controller, then goes on with other work. • The DMA controller proceeds to operate the memory bus directly, placing addresses on the bus to perform transfer without the help of the main CPU.

  6. During DMA • Device driver is told to transfer disk data to buffer at address X. (in memory) • Device driver tells disk controller to transfer C bytes from disk to buffer at address X • Disk controller initiates DMA transfer • Disk controller sends each byte to DMA controller • DMA controller transfers bytes to buffer X, increasing memory address and decreasing C until C = 0 • When C =0, DMA interrupts CPU to signal transfer completion

  7. Performance • When the DMA controller seized the memory bus, the CPU is momentarily prevented from accessing main memory, although it can still access data items in its primary and secondary cache. • Although this cycle stealing can slow down the CPU computation, offloading the data-transfer work to a DMA controller generally improves the total system performance.

  8. Direct Virtual-memory access (DVMA) • DVMA can perform a transfer between two memory-mapped devices without the intervention of the CPU or the use of main memory.

  9. Pros and Cons • If system prevents process from issuing device commands directly (e.g. access memory directly): • Pros: • protect data from access-control violations • Protect eh system from erroneous use of device controllers that could cause a system crash. • Cons: • Relatively low system performance

  10. Pros and Cons • Allow direct access: • Pros: • Obtain high performance, since avoid kernal communication, context switches, and layers of kernal software. • Cons: • System security • System stability.

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