3.4.5 - Magnetic and solid-state storage
Explain why a computer needs persistent secondary storage, how magnetic and solid-state devices store data, and how their properties affect a storage choice.
Main memory and secondary storage
Secondary storage retains programs and data that must remain available after power is removed.
The specification defines secondary storage as non-volatile storage that is not directly accessible by the CPU. When a file or application is used, needed data or instructions are transferred into main memory. The CPU then works on the accessible data; saving writes a persistent copy back to secondary storage.
RAM is suitable for changing working data but loses it when power is removed. ROM is persistent main memory, but its mainly read-only, specialised startup role does not make it a replacement for a large writable file store. Secondary storage gives the computer a place to keep installed programs, documents, photographs and other files between sessions.
An unsaved edit in RAM and the older saved file on a drive can differ. Closing the application after saving permits the latest version to be loaded next time. Merely having a non-volatile drive in the computer does not automatically preserve every unsaved change.
How magnetic storage works
Magnetic storage represents binary data using the magnetic state of regions on a storage surface.
In a hard disk drive, platters coated with magnetic material rotate. Read/write heads move to access the required region. Writing changes its magnetic state; reading detects the stored state and obtains the corresponding data. The arrangement persists without power, so a stopped drive can still retain files.
Rotation and movement take time. A request may need the head to reach the right track and the required region to rotate underneath it. These mechanical operations help explain why access is generally slower than an SSD's electronic access. Moving components can also be damaged by shocks, particularly during operation.
Magnetic tape is another magnetic example. Data is stored along the tape, which must move past a head. It suits some large sequential backup uses, but reaching a distant item can require passing earlier tape sections. Do not assume that all magnetic devices have a spinning disk; the shared feature is the magnetic representation.
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