Cambridge O Level Computer Science · Syllabus 2210 · Hardware
ROM
What is ROM?
Read only memory: non-volatile primary storage that holds instructions or data which must remain when the power is off, typically the instructions a computer needs to start up; in the syllabus model it is read from during normal operation rather than used as working memory.
This definition is part of the Hardware chapter in Cambridge O Level Computer Science.
ROM in context
Primary storage is storage that is directly accessed by the CPU. That single property is what puts RAM and ROM in this category and keeps hard disks out of it. The two kinds of primary storage do opposite jobs: RAM holds the programs and data currently in use and can be written to, but is volatile — its contents are lost when the power is removed. ROM holds instructions and data that must survive being switched off, and is non-volatile. A computer needs both: without RAM there is nowhere to work, and without ROM there is nothing to tell the machine what to do when it is first switched on.
Common mistakes with ROM
- 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.
- 19. “ROM is ordinary working memory.” Why wrong Working memory has to be written to constantly as programs run. That is not what ROM is for in this model. Correct model ROM is non-volatile primary storage holding instructions and data that must always be present, such as the start-up instructions. It is read from during normal operation; RAM is the working memory. Exam-safe “ROM is non-volatile and stores the instructions the computer needs at start-up; it is read from during normal operation rather than used as working memory.” Check: which of RAM and ROM is usually much larger, and why does that make sense?RAM, because it has to hold every program and file currently in use, whereas ROM only has to hold a fixed set of start-up instructions.
Questions students ask about ROM
What is the difference between RAM and ROM?
RAM (random access memory) is volatile primary storage that holds the programs and data currently in use, and can be both read from and written to; its contents are lost when power is removed. ROM (read only memory) is non-volatile primary storage that holds instructions, such as the start-up instructions, which must remain when the power is off, and is read from during normal operation. A computer needs both: without RAM there is nowhere to work, and without ROM there is nothing to tell the machine what to do when it is first switched on.