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6.2: Mid Term Exam 1

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    Mid Term Exam 1(20 %)


    Answer the following question be as detailed as possible Question: Explain the concept of interrupts and DMA.

    Grading Scheme

    Marks to be awarded based on key issues mentioned in the explanation. maximum is 10 marks for correct answers


    When an interrupt occurs the CPU issues commands to the I/O module then proceeds with its normal work until interrupted by I/O device on completion of its work.

    if an interrupt occurs due to the input device, the device interrupts the CPU when new data has arrived and is ready to be retrieved by the system processor. The actual actions to perform depend on whether the device uses I/O ports, memory mapping.

    if it occurs due to the output device, the device delivers an interrupt either when it is ready to accept new data or to acknowledge a successful data transfer. Memory-mapped and DMA- capable devices usually generate interrupts to tell the system they are done with the buffer.

    An Interrupt relieves the CPU of having to wait for the devices, but it is still inefficient in data transfer of large amount because the CPU has to transfer the data word by word between I/O module and memory. Below are the basic operations of Interrupt:

    • CPU issues read command

    • I/O module gets data from peripheral whilst CPU does other work

    • I/O module interrupts CPU

    • CPU requests data

    • I/O module transfers data

    Direct Memory Access (DMA)

    Direct Memory Access (DMA) means CPU grants I/O module authority to read from or write to memory without involvement. DMA module controls exchange of data between main memory and the I/O device. Because of DMA device can transfer data directly to and from memory, rather than using the CPU as an intermediary, and can thus relieve congestion on the bus. CPU is only involved at the beginning and end of the transfer and interrupted only after entire block has been transferred.

    Direct Memory Access needs a special hardware called DMA controller (DMAC) that manages the data transfers and arbitrates access to the system bus. The controllers are programmed with source and destination pointers (where to read/write the data), counters to track the number of transferred bytes, and settings, which includes I/O and memory types, interrupts and states for the CPU cycles.

    DMA increases system concurrency by allowing the CPU to perform tasks while the DMA system transfers data via the system and memory busses. Hardware design is complicated because the DMA controller must be integrated into the system, and the system must allow the DMA controller to be a bus master. Cycle stealing may also be necessary to allow the CPU and DMA controller to share use of the memory bus.

    This page titled 6.2: Mid Term Exam 1 is shared under a CC BY-SA license and was authored, remixed, and/or curated by Harrison Njoroge (African Virtual University) .

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