Question 1:
Marks
10
Can we use DMA in protected mode or
virtual mode of memory? Explain your answer with a solid reason.
Solution:
The process for performing DMA
transfers in the protected mode Windows environment should be similiar to the
process for performing DMA transfers in the real mode MS-DOS environment. In
both environments, you must initialize the DMA controller with the starting
base address, page address, transfer count, and mode. In both environments, you
would usually install a hardware interrupt handler to handle terminal count
interrupts when a DMA transfer is complete.
In enhanced mode Windows, the
Virtual DMA Device (VDMAD) attempts to hide the virtual memory issues from
device drivers that are not fully aware of the virtual memory environment. The
main purpose of VDMAD is to convert linear addresses virtually programmed into
the DMA controller into physical addresses. This is necessary because Windows
applications and DLLs that are not fully aware of the virtual memory
environmentonly deal with linear addresses, but the DMA controller only deals
with physical addresses.
DMA is the hardware mechanism that allows peripheral components
to transfer their I/O data directly to and from main memory without the need to
involve the system processor. Use of this mechanism can greatly increase
throughput to and from a device, because a great deal of computational overhead
is eliminated
Question 2:
Marks
10
Compare all accessing mechanisms of a
magnetic disk with limitations and uses, and which mechanism will be
used for accessing of 1 Terabyte disk?
Solution:
Magnetic storage media can be
classified as either sequential access memory or random access
memory although in some cases the distinction is not perfectly clear. The
access time can be defined as the average time needed to gain access to stored
records. In the case of magnetic wire, the read/write head only covers a very
small part of the recording surface at any given time. Accessing different
parts of the wire involves winding the wire forward or backward until the point
of interest is found. The time to access this point depends on how far away it
is from the starting point. The case of ferrite-core memory is the opposite.
Every core location is immediately accessible at any given time.
Hard disks and modern linear
serpentine tape drives do not precisely fit into either category. Both have
many parallel tracks across the width of the media and the read/write heads
take time to switch between tracks and to scan within tracks. Different spots
on the storage media take different amounts of time to access. For a hard disk
this time is typically less than 10 ms, but tapes might take as much as 100 s.
The optical disc revolution started with CDs and then moved on to DVDs, and
we’re in the midst of the next-gen battle between HD DVD and Blu-ray. Since the
birth of the CD 25 years ago, we’ve gone from 600MB to a whopping 50GB of storage
capacity on these little, convenient and versatile discs. The company claims
that they can store up to 1TB (1,000GB) on an optical disc with the same
dimensions—only slightly thicker—than a regular DVD and will be able to store
5TB once the jump to blue lasers is made. The 1TB disc is divided into 200
different layers, each comprising 5GB of storage space. Unlike standard
multilayer DVDs, the layers aren’t physically stacked and stuck together.
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