Interface Design
Secondary Storage
Omid Fatemi
University of Tehran 1
Outline
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PC disk drives
• 1st were diskette
• Then hard drives
• Then removable drives
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Diskette Drives for a PC
• Magnetic
• One or more circular disks that are coated
with material that responds to magnetic fields
• Disks are mounted on spindle and turn under
head(s) that move radially in and out to
read/write data
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Disk Organization
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Formats
• Original drives 5 ¼, today 3 ½ -- 160K to
1.44/2 M
• Microsoft Distribution Format (2 M)
• IBM – eXtended Diskette Format (2M) XDF
• Even have 2.88 diskette (but requires special
drive)
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Disk Size
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Hard Disks
• Disks are harder material – platters
• Platter – rigid disks made of aluminum or
glass
• Data is encoded before storage to ensure that
the patterns of 0s and 1s changes enough for
heads to read/write them correctly
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Access Time
• In the range of 10ms
• Seek time
• Settling time
• Latency time
– relates to rotation speed (RPM)
• Controller overhead
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Encoding Formats
• MFM – Modified Frequency Modulation
encoding
• RLL – Run Length Limited encoding
• ESDI (enhanced small device interface)
• SCSI (small computer system interface)
• IDE (integrated device electronics)
• E-IDE
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Encoding
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Run Length Limit
Data: 0011 10 10
Code: 00001000 0100 0100 University of Tehran 12
ESDI, IDE
• Like MFM and RLL, but allows for larger
capacity drives and speed
• Died quickly when IDE (integrated device
electronics) drives appeared. EIDE is
extension of IDE
• Controller electronics are on circuit card on
the side of the drive and not on a plug in I/O
card.
• AT attachment packet interface (ATAPI)
– Extended EIDE
– Accommodates other devices besides hard drives – CD
ROMs for example.
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Hard Disks
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Hard Disk Interleaving
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SCSI
• Tiny one-computer local area network
• Requires SCSI host adapter
• Multiple SCSI devices can be connected
together (drives and other peripheral devised)
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DOS Disk Overview
• Fundamental unit of storage is sector
– Usually 512 bytes of information
• Each sector is actually 571 bytes
– remaining is header and trailer for each footer
– checksum data for verification of data accuracy
• All sectors around a disk at a given distance
form a track.
• With multiple disks, tracks form cylinders
• Most disk can be recorded on both sides
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Physical Versus Logical
Formatting of Disks
• Physical – set up the sectors/tracks that will
be used
• Logical– add in the operating system
component (such as DOS header/trailer
information)
– Boot record (if disk is bootable)
– FAT – File Allocation Table
– Root directory
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Bad Sectors
• Diskette
– Sectors are marked and capacity id reduced by number
of bad sectors
• Hard Disk
– Spare sectors are held in reserve and used to replace the
bad ones
» Defragmentation of drive will optimize performance
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Integrity Maintenance
• Cyclical Redundancy Check (CRC)
• Diskettes try and then report „Abort, Retry, or
Fail‟
• Hard Drives also have ECC, so if can, errors
are fixed
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Logical Structure
• FAT 12 or FAT16
– DOS boot sector
– FAT #1
– FAT #2
– Root Directory
– Data area
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• FAT 32
– DOS boot record (3 sectors)
– Reserved sectors
– Copy of boot
– Reserved sectors
– FAT #1
– FAT #2
– Data area
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Clusters
• Minimum amount of space that can be
allocated to a file
• Means that if cluster is 4K, and file is 1K, then
to store file use 4K
• Unused portion is called slack space
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FAT usage
• FAT 12
– Floppy diskettes, hard disks of less than 16M
– (0 to 4086 values)
– For hard disks, the cluster size is 4K
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• FAT 16
– 16 bit numbers
– Up to 2 GB drive
– Cluster size of 2K for drives less than 128M
– Cluster size doubles every time capacity doubles, so 2G
means cluster size of 32K
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• FAT 32
– 32 bits (4 bytes)
– Minimum 512K, up to 2048G
– (32 bit number, but 28 are cluster, 4 are reserved – so
only 268,435,456 clusters max)
– FAT can be up to 1G
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Root directory
• Part of FAT 12 and 16
• Not part of FAT 32
• Is the “directory board”
• Subdirectories are pointed to by their parents,
themselves and their children
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FAT table
• Huge table of numbers
• Starts at address 2
• Each number is twelve bits
• 0 means it is unused
• EOF means that it is end of file
• 3rd for bad sector
• Any other number means address of next
cluster as part of file
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Floppy Layout
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Boot Record Layout
L CS:100 0 0 1
D CS:100 17F
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Directory Layout
L CS:200 0 13 1
D CS:200 27F
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FAT Layout
L CS:100 0 1 1
D CS:100 L100 University of Tehran 32
Logical Differences between
diskettes and hard disks
• MBR – Master Boot Record
• Hard drives have an MBR that not only
contains boot information, but also table of
the drive partitioning
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Drive Partitioning
• Primary logical volume.
• Hidden partition.
• Extended partition.
• Each disk can have only 1 active primary partition
– all others are hidden.
• Each disk can also only have 1 extended partition.
• A logical volume is all the storage space referred
to by a single drive letter. An extended partition
can have any number of logical volumes.
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LBA vs. CSH
• Partitioning involves separating blocks of
storage in terms of cylinder, head and
section numbers.
• Table may actually use logical block address
(LBA) numbering, so the division numbers
shown in the table listing (as seen in your
disk editor) may have no relationship with an
actual cylinder start on the physical drive.
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Partitions
• Hidden partitions can house other operating
systems
• When booting, you choose between the
operating systems
• When booted DOS can only see one active
partition and generally does not see the other
hidden partitions; however, some OS s like
Unix and Linux can see their partition as well
as the DOS partitions
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Managing Multiple Disk Drives
• Floppy diskettes
– Connect via 34-wire ribbon cables typically. These cables
can have 0, 1, or 2 drives.
– Drive connected to middle connection is B:, one at end
will be A:
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IDE Hard Disks
• IDE
– Connect via special connectors or option cards
– Current versions of the IDE allow four IDE channel, each
channel supporting 2 devices
– Most PCs support two EIDE
– Jumpers determine if drive is master or slave, the first
drive is usually the master, and the rest are slaves
– Hard drive wires are wider and not twisted with 3
connectors typically
– On each IDE channel, you have one master and then 1
slave
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IDE vs. EIDE
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IDE Connections
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IDE Registers
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SCSI
• SCSI
– All their controller electronics are on the device
– Operate over a SCSI bus with SCSI host adapter
– Some PCs (like Macintosh computers) have SCSI
host adapter built in
– Host adapter typically plugs into a PCI bus
» Adaptec 2940W/UW
» Has 2 internal connectors (one for narrow, one
for ultra wide)
» External connector (ultra wide)
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SMART Drives
• Self-Monitoring and Reporting Technology
• Set flags that the BIOS can read and then
report
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RAID
• Redundant arrays of inexpensive disks
• Data is stored on more than one disk and
then you can avoid or minimize downtime
• 9 levels
• In some cases, allows hot swaps of drives or
power supplies when they have failed
• Formerly used only on mainframe computers
and large servers
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Tape Drives
• Earliest type converted digital to audio and
stored it on audio cassettes
• Digital Audio Tape – special purpose drives
meant just for digital data recording
• Capacities go up to 24G
• Downside – sequential access only, and
sometimes not reliable in terms of
functionality
• Advantage – great for drive backups
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Removable disks
• Uses a docking bay
• Plug in drive, put data into it, then remove it
• Can hot swap
• Alternative is external drive (through parallel
port)
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Zip Drives
• Larger than 3 ½
• 100M and 250 M
• Internal and external modes
• ZIP drives appear as additional logic drives
• Over parallel port, performance is slow
• Over SCSI, approaches performance of a
hard drive
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Optical PC
• CD ROM, CD-R, CD-RW
• Only one side of CD is typically used
• Binary data stored and read optically
• Bits are stored more densely
• Pits burned to alter reflection of laser light
– Encode the data
– Signal where spiral track goes
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Optical Data
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CD ROM drives
• Differ in in standard, but can share some
parts and design
• Red, Orange, Yellow, Green, White, and Blue
Book
• Storage capacity of around 650M or better
• Can only be read, not written
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CD – R
• Disk are gold or silver and deep green or
cyan on recordable side
• Silver/cyan is Type II (to reflect red lasers
used in DVD
• Can be read in ROM drives, but written in CD-
R
• Laser operates at three or more power levels
– at highest, it burns pits
• Write once read many (worm)
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CD-RW
• Has two more layers on disk than CD-R
• Recordable layer is made of alloy of several
metals
• Melts regions to switch from crystalline form
to data form
• Enough temperature, and they go back to
crystalline form, allowing for a rewrite
• Marks are not as good at light-scattering, so
not all CD drives can read them
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Digital Versatile Disc
• DVD
• Higher capacity
• Tuned to red band of visible light spectrum
• Data is more densely packed
• Can be double sided – 17G or more
• MPEG2 standard
• Requires special decoders and copy-
protection circuitry
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DVD Drive
• Can read CD-ROMs but not as fast as best
CD-ROM drives
• DVD ram is still under discussion, but there
is one standard in place
• Allows for 2.6G or more per side as storage
• DVD RAM discs are in sealed cartridges and
can only be used in DVD RAM drives
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Data Compression
• Compressing data so that storage is
increased without changing the disks
• Two forms
– Compressing individual files through utility (such as
WINZIP)
– OS compression – all data automatically compressed by
the OS
• Downside: data has to be compressed then
uncompressed to be read
• Useful for archiving data
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OS Data Compression
• DoubleSpace, DriveSpace for example
• Requires extra driver
• OS takes care of compression
• While tested, errors can occur; so keep
backups
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Problems
• 1, 22, 24
• 25, 29
• 31, 33, 34
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