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Jan's Computer Basics:

Storage: Accessing Data

Accessing Data

The process of accessing data has 4 steps.
1.  Seek
2.  Rotate
3.  Settle
4.  Data transfer

Step Measured as: Click to start and stop animations
1. seek
move the read/write head to proper track		 seek time
(ms)		 read/write head seeks track
2. rotate
rotate disk under the head to the correct sector		 rotational delay
(ms)		 disk rotates under head
3. settle
head lowers to disk;
wait for vibrations from moving to stop
(actually touches only on floppies)		 settling time
(ms)		 head settles toward disk
4. data transfer  
 
copy data to main memory		 data transfer rate
(kbit/s)		 data is transferred

where ms stands for millisecond = .001 second and kbit/s is kilobits per second. Historically, one kbit has always been 1000 bits instead of 1024 bits. Go figure!

Total time to transfer a kilobyte:

for floppies, 175 - 300 ms
for hard drive, 15 - 80 ms
    new hard drives,       
   [2009]		 .0032 ms   (at 300 MB per sec).
This is seriously fast!!

Clearly, getting data from a hard disk is immensely faster than from a floppy.