Lec8_ExternalMemory_show_1011

Lec8_ExternalMemory_show_1011 - Lecture 8 External Memory...

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CC2202 1 Lecture 8 External Memory
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CC2202 2 1 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 Magnetoresistive (MR)
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CC2202 3 Step 2 Step 3 1. Circuit board controls movement of head activator and small motor 2. Small motor spins platters 3. When software requests disk access, read/write heads determine location of data 4. Head actuator positions read/write head arms over correct location on platters to read or write data Step 1 Hard Disks How does a hard disk work? Step 4
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CC2202 4 Hard Disks
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CC2202 5 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 (contemporary) Separate read head, close to write head Partially shielded magneto resistive (MR) sensor Electrical resistance depends on direction of magnetic field Bit representation by phase encoding
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CC2202 6 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 (CAV) Tracks divided into sectors Minimum block size is one sector May have more than one sector per block
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CC2202 7 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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CC2202 8 Disk Layout Methods Diagram Zone A Zone B Data Density Increase Same number of bit is each sectors Surface is divided into a number of concentric zones. Within a zone, the
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Lec8_ExternalMemory_show_1011 - Lecture 8 External Memory...

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