38 lines
1.4 KiB
Markdown
38 lines
1.4 KiB
Markdown
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## Hard disks (HDDs)
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Spin platters of magnetically-coated material under moving RW heads
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- Rotate at 60 to 250 times/s
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- Transfer rate is rate at which data flows between drive and pc
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- Positioning time (Random access time) to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)
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### Controller
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Receives data/ command from the OS, controls I/O operation and sends back results to OS
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### I/O steps
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- Move head to track -> Seek Time (S)
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- Find sector -> Rotational Latency Time (r)
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- Transfer data to/from controller -> Block Transfer Time (btt)
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- Error checking and report to OS
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### Disk Formatting
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Each platter must receive a low-level format done by software, i.e. marking the cylinders and tracks, each containing a few sectors, with short gaps in between
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Each sector includes:
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- Preamble: starts with start of the sector bit pattern, the cylinder and sector numbers
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- Data: size of the data portion is determined by the formatting program -> 512 bytes usually
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- ECC: error correction, typically 16 bytes
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Numbering the sectors
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- Position of sector 0 on each track is offset from the previous track when the low-level format is laid down (cylinder skew)
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