storage device used to summative project cattle file to the teacher for evolution
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Secondary storage
Secondary storage is the name we give to any storage device that stores our programs and files, without needing power to do so. For example, a pen drive, an SD card, a micro SD card, a CD, a DVD, a Blu-Ray disk and a hard drive are all examples of secondary storage devices.
When they are used in a computer or camera or phone or any other computer-based equipment, they get their power from that equipment. The power is enough to enable them to work, so we can read from them and write to them. We can save videos, pictures, music, applications, word processed documents and any other digital file we like to them and then we can take them out of the camera, or phone, or computer and the contents will remain. We can plug them back into another piece of hardware later, and get back the files. The programs and files won't disappear just because the secondary storage device had no power connected to it for a while!
Non-volatile devices
Secondary storage devices are called non-volatile devices. That simply means that when the power is removed from them, they don't lose their contents.
Hard drives
When we turn the computer off, we don't want to lose all our files or we would have to do the work again! Neither do we want to have to reinstall software that we have already installed on our computer - that would use up a lot of our time. For these reasons, computer systems usually come with a hard disk. When you save files and applications to a hard disk, they are magnetically encoded onto disks so that when you turn the power off, the contents remain. When you power the computer back on, everything you saved onto the hard drive is still there for us to use. A hard drisk is a good example of a 'non-volatile' piece of hardware.
SuitcasePrimary memory versus secondary storage
Hard drives (hard disks) aren't very clever! A hard drive is really just like a big suitcase. It can store lots of stuff but it can't actually do anything. Very importantly, it can't work directly with the brain of the computer, the CPU. If the CPU wants to open a program that's on the hard drive (or indeed any secondary storage device), it first has to make a copy of the program and then move that copy to its RAM (Random Access Memory). Then the CPU can work with the program in RAM. In just the same way, if you want to open a personal file you've stored, the CPU first has to copy the file from the secondary storage device to RAM. Only then, can you work on it. When you want to save any file, the file is copied from RAM to the secondary storage device. We can draw a model of this.
Abstraction
Moving copies of a file or program from a secondary storage device to RAM and back again is in practice quite a complex process but in the diagram below, we have hidden all of the unnecessary detail and just shown the important idea, that the CPU only works with RAM, not a secondary storage device like a hard drive. When you draw a diagram that simplifies a complex process, we call this abstraction.
CPU model
Rebooting the computer and memory management
We have already said that secondary storage devices are non-volatile devices. Primary memory such as RAM is by contrast volatile. When the power is switched off, everything in RAM is lost. Sometimes, your computer starts to play up. Things don't work as they should or it slows right down. One possible reason for this is that the programs and files in RAM have interfered with each other. The solution is to switch your computer off (so that the entire contents of RAM are removed) and then on again. In other words, reboot the computer. Then you open up the applications and files you were using before. Hopefully this time, they will all be opened and organised properly by the operating system (which is in charge of opening and organising where applications and files go in RAM, a process called memory management), without disrupting each other.
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