In this video, we take a look at the need for secondary storage. Storage in a computer system can be thought of in three broad terms, primary, secondary and tertiary. Primary storage is the immediate memory that the process has access to.
This includes the registers, the cache, the ROM and RAM. Programs and data that are not in use by the processor are held on secondary storage media. This includes the hard disk drive and other drives on the computer system.
Secondary storage is needed because the memory in a computer system is either ROM, which is read-only and cannot be changed, or the RAM, which is volatile and is lost when the power is switched off. Tertiary storage refers to the longer term backup or archiving of files. Backups are kept in case the main data file is lost or corrupt. Archives are data that is moved from the main files to free up space, but still have to be kept for long periods of time. Tertiary devices are not covered in the GCSE specification, so we can ignore those.
We've just included them here for completeness. Here we see four important internal components of a computer system. ROM, the CPU, RAM and a hard drive of some description.
The symbol in the centre represents if the power is currently turned on or off. When you very first turn your computer on and your CPU receives power, it gets its initial startup instructions from the bootstrap contained on the ROM. Once this is complete, the operating system and other software and data stored on non-volatile secondary storage devices, such as the hard disk, can be loaded by the CPU into main memory, RAM, so it can be accessed quickly.
When you shut down your computing device and it loses power, any instructions and data currently in use need to be stored safely. They can't be stored on ROM. as this type of memory has very little size and is read-only, and they can't be stored in RAM as this type of memory is volatile and so can't hold data when it has no power. Therefore all instructions and data need to be transferred back to a secondary storage device such as the hard drive, which is both non-volatile and writable.
So just to recap, secondary storage is needed because ROM is read-only and RAM is volatile. Secondary storage is needed for the storage of programs and data when the power is turned off, semi-permanent storage of data that can change, backing up of data files and archiving of data files.