5/6/2023 0 Comments Secondary memory pictures![]() ![]() The construction differences between HDDs and SSDs notwithstanding, SSDs still operate more slowly than CPUs and main memory. SSDs do not have moving parts, so they are faster, quieter, use less power, and generate less heat, but they are also more expensive. ![]() Solid state drives (SDD) use various integrated circuits to store data. Rotational latency and seek time seem small in terms of human perception, but they are very long compared to the speeds of a modern CPU and main memory. The time it takes for the arm to move from its current position to a needed track is called the seek time. The time that a program waits while a needed sector rotates around to the read/write head is called the rotational latency. So, each sector takes more than 1/8000th of a second to make one complete rotation. In a typical HDD, the disks spin at 7200 rotations per minute (RPM) or 120 rotations per second. The drive reads or writes data from sectors as they pass under the read/write head. An arm moves a read/write head from one track to another. "The header and trailer contain information used by the disk controller" (Silberschatz, Galvin, & Gagne, 2011, p. Each sector consists of a header, a data area, and a trailer. Drives organize data on both sides of a platter as concentric tracks and further divide each track into small arcs called sectors. An HDD consists of one or more disks or platters covered in a magnetic material that can store digital data. Modern computers use many physical devices for secondary memory, but we'll restrict our discussion to disk drives. The "persistent" nature of secondary memory means that it doesn't lose its contents when the computer is powered off. Secondary memory is slower than main memory, but it can persistently store large amounts of data and programs. Main memory is fast but requires constant power to "remember" the stored data and programs. Our previous discussions involving variables, functions, pointers, and other data-related features all centered around main memory. ![]()
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