2/17 how disk storage works
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2/17 How Disk Storage Works. Roll call. Video: disk storage. Step-by-step lecture. 1. Formatting a disk: Why bother?. Formatting provides a “street map” for the data to be laid out on. Where’s that data?. Image courtesy of Mapquest.com. 1A. Formatting a disk. - PowerPoint PPT PresentationTRANSCRIPT
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2/17 How Disk Storage Works
• Roll call.
• Video: disk storage.
• Step-by-step lecture.
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1. Formatting a disk: Why bother?
• Formatting provides a “street map” for the data to be laid out on.
• Where’s that data?
Image courtesy of Mapquest.com
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1A. Formatting a disk
• A grid of wedges and tracks, made with magnetic “signposts”is laid out across the disk.
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1B. Formatting a disk: wedges
• Wedges are wedge-shaped areas on a disk.
• Divided radially.
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1C. Formatting a disk: tracks
• Tracks are ring-shaped sections of the disk.
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1D. Formatting a disk
• The read/write heads check these signposts as it reads the disk to know where it is.
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2. Sectors
• The junction of a wedge and a track is called a sector.
• Sectors can be thought of as city blocks: the 3600 block of Lindell Boulevard -- 1 sector, 1 block.
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2A. Sectors make up clusters
• A cluster is two or more sectors.
• A cluster is the smallest unit of storage for the disk.
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2B. How many sectors to a cluster?
• EX: a file needs 8K of space. Each sector is 12K big.
• How much disk space will the file take up?ANS: 24K (2 sectors is the minimum space)
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2C. Operating systems differ
• Different operating systems divide up the disk in different portions.
• Some are more efficient than others.
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3. The FAT and the VFAT
• A directory file -- File Allocation Table is created in sector 0 of the disk.
• DOS: FAT
• Win95: VFAT -- Virtual File Allocation Table
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3A. Differences: FAT and VFATname length bits
read
FAT <=11 char. 16 bits
VFAT <=255 32
VFAT is faster at
accessing data than
FAT.
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4. Writing a file to a disk
• The program is holding the file in the RAM.
• When you click “Save”, it asks the CPU to move the file from RAM to a disk.
RAM
HARD DRIVE
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5. First, list it in the VFAT
• The operating system changes the VFAT to list where the file will be stored in the folders in the directory.
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6. Second, find a spot for the file
• The VFAT also tells the operating system where an available cluster is on the disk.
• The cluster address lists the track and sectors that make up the cluster.
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7. The VFAT to the OS to the BIOS
• The VFAT tells the OS what the address is of the cluster.
• The OS hands off the info to the BIOS.
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8. The BIOS sends the file
• The BIOS grabs the file out of the RAM. It tells the disk controller to save the file, starting at the beginning point of the cluster.”
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9. Oops! The file’s too big
• If the file is bigger than the cluster, the VFAT tells the OS where another available cluster is.
• The BIOS continues sending the file, but now directed to the next cluster.
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9A. Oops! The file’s too big
• The VFAT keeps track of the chain of clusters that the file uses.
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10. File’s done!
• An end of file marker tells the BIOS and VFAT that the transfer is complete.
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11. Update the VFAT
• The file’s record in the VFAT is updated with the various clusters that make up the file.
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On 2/19:
• Reading a file from a disk
• How a hard drive works
• 2nd individual assignment discussion