Question
In: Computer Science
Direct memory access is used for high-speed I/O devices in order to avoid increasing the CPU's...
Direct memory access is used for high-speed I/O devices in
order to avoid increasing the CPU's execution load.
(a) What does the CPU do to interface with the device to
coordinate the transfer?
(b) How does the CPU know when the memory operations are
complete?
(c) The CPU is allowed to execute other programs while the DMA
controller is transferring data. Does DMA interfere with the
execution of the other programs? If so, describe what form of
interference are caused.
Solutions
Expert Solution
a) The hardware used for direct memory access is DMA controller and it is part of I/O device's interface circuit. DMA controller interfaces between the bus and I/O devices. The CPU sends the starting address, number of words in the data block, and direction of transfer of data to the DMA controller. The transfer of data is controlled by the processor. When DMA controller is free, it request for bus and CPU grants the bus to DMA by sending grant signal. Then data tranfer will be initiated.
b) When the memory operation (transfer) is completed, the DMA controller informs the processor by raising an interrupt.
c) When DMA controller is transferring data, the program requested the transfer can not continue, but the CPU is allowed to execute other programs.
A conflict may arise between the CPU and DMA controller when both of these want to use the bus at the same time to access the main memory. So, any other program that require all the memory bus cycles could run more slowly when DMA is active.
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