chap6 - William Stallings Computer Organisation and...

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William Stallings Computer Organisation and Architecture Chapter 6 External Memory
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Types of External Memory Magnetic Disk RAID  P174 Removable Optical disk CD-ROM, CD-R, CD-R/W DVD-ROM, DVD-R, DVD-R/W Magnetic Tape Flash disk
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Magnetic Disk Range of packaging Floppy is obsolete Winchester hard disk Removable hard disk
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Magnetic Disk Disk  substrate  coated with magnetizable  material (iron oxide…rust) Substrate used to be aluminium Now glass Improved surface uniformity Increases reliability Reduction in surface defects Reduced read/write errors Lower flight heights (See later) Better stiffness Better shock/damage resistance , , , ,
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Read and Write Mechanisms Recording and retrieval via conductive coil called a head May be single read/write head or separate ones During read/write, head is stationary, platter rotates Write Current through coil produces magnetic field Pulses sent to head Magnetic pattern  recorded on surface below Read (traditional) Magnetic field moving  relative to coil  produces current Coil is the same for read and write Read (contemporary) Separate read head, close to write head Partially shielded  magneto resistive (MR) sensor Electrical resistance   depends on  direction of  magnetic field High frequency operation Higher storage density and speed
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Inductive Write MR Read 2007 Nobel Prize for physics T : T , ~ . , , ,
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Data Organization and Formatting Concentric rings or  tracks Gaps between tracks Reduce gap to increase capacity Same number of bits per track (variable packing  density) Constant angular velocity Tracks divided into  sectors Minimum block size is one sector May have more than one sector per  block
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Disk Data Layout 500-2000 tracks/side 100- ~ sectors/track 512- n bytes/sector 1-k sector/block
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Disk Velocity Bit near centre of rotating disk passes fixed point slower  than bit on outside of disk Increase spacing between bits in different tracks  Rotate disk at constant angular velocity (CAV) Gives pie shaped sectors and concentric tracks Individual tracks and sectors addressable Move head to given track and wait for given sector Waste of space on outer tracks Lower data density Can use  zones  to increase capacity Each zone has fixed bits per track More complex circuitry
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Disk Layout Methods Diagram 8-16 8-8
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Finding Sectors Format Must be able to identify  start  of track and sector Format  disk Additional information not available to user Marks tracks and sectors
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ST506 format (old!) Foreground reading Find others Sector 0
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chap6 - William Stallings Computer Organisation and...

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