Cambridge O Level Computer Science · Syllabus 2210 · Hardware
Secondary Storage
What is Secondary Storage?
Storage that is not directly accessed by the CPU and is used for the more permanent storage of data; it is generally non-volatile, so data is retained when the power is off, and data needed for processing must first be transferred into RAM.
This definition is part of the Hardware chapter in Cambridge O Level Computer Science.
Common mistakes with Secondary Storage
- 1. “The CPU is the entire computer.” Why wrong A CPU on its own cannot receive a key press, keep a file after a power cut, or show a result. It is one component among several. Correct model The CPU processes instructions and data and controls the computer's operations. The computer is the whole system: CPU, memory, input devices, output devices and secondary storage. Exam-safe “The CPU processes the instructions and data input into the computer so that a result can be output, and controls the computer's operations.” Check: name three things a computer needs that are not the CPU.Any three of: memory (RAM and ROM), input devices, output devices, secondary storage, buses.
- 18. “RAM is permanent storage.” Why wrong RAM is volatile. Anything in it disappears the instant the power is removed — which is why an unsaved document is lost in a power cut. Correct model RAM holds the programs and data currently in use and can be read from and written to, but it is volatile. Permanent storage is the job of secondary storage. Exam-safe “RAM is volatile primary storage holding the programs and data currently in use; its contents are lost when power is removed.” Check: a file is edited but not saved, and the power fails. Where was the file, and why is it gone?It was in RAM, which is volatile, so its contents were lost. It had not yet been written to secondary storage.
- 21. “Virtual memory is extra RAM.” Why wrong No physical memory has been added. Virtual memory is space on a drive being used as if it were RAM. Correct model Virtual memory is an area of secondary storage used when there is not enough RAM. Pages are transferred between RAM and virtual memory as they are needed, and the CPU still only works with RAM. Exam-safe “Virtual memory uses part of secondary storage when there is insufficient RAM, with pages of data transferred between the two as they are needed.” Check: where does virtual memory physically exist?In secondary storage — on the hard disk drive or solid-state drive.
- 22. “Virtual memory always improves performance.” Why wrong Every page transfer costs time, because secondary storage is slower than RAM. Using virtual memory heavily makes a machine slower, not faster. Correct model Virtual memory lets programs continue running when RAM is full, instead of stopping. The correct comparison is “slow but still working” against “unable to continue” — never against “faster”. Exam-safe “Virtual memory allows programs to continue running when RAM is insufficient, but because secondary storage is slower than RAM, frequent page transfers reduce performance.” Check: a computer is slow because it is using virtual memory heavily. Does a bigger hard disk fix it?No. More RAM is needed, so that fewer pages have to be transferred at all. A bigger disk only makes the slow store larger.
Questions students ask about Secondary Storage
Why is virtual memory not simply extra RAM?
Virtual memory is an area of secondary storage used as though it were RAM when there is not enough physical RAM for the programs and data in use; pages of data are transferred between RAM and virtual memory as they are needed. No physical memory has actually been added, and because secondary storage is much slower than RAM, frequent page transfers reduce performance rather than improving it. Virtual memory lets programs keep running when RAM is full instead of stopping, but it does not make the computer faster.