Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 16 | Low Level
16.3 Describe direct memory access (DMA). Can a user level buffer / pointer be used by
kernel or drivers?
SOLUTION
pg 82
Direct Memory is a feature which provides direct access (read/write) to system memory without
interaction from the CPU. The “DMA Controller” manages this by requesting the System
bus access (DMA request) from CPU. CPU completes its current task and grants access by asserting
DMA acknowledgement signal. Once it gets the access, it reads/writes the data and
returns back the system bus to the CPU by asserting the bus release signal. This transfer is
faster than the usual transfer by CPU. Between this time CPU is involved with processing task
which doesn’t require memory access.
By using DMA, drivers can access the memory allocated to the user level buffer / pointer.
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Cracking the Coding Interview | Knowledge Based