What is primary storage and why does a computer need RAM, ROM and virtual memory?
The need for primary storage, the purpose and characteristics of RAM and ROM, the differences between them, and the need for virtual memory.
An OCR J277 1.2.1 answer on the need for primary storage, the purpose and characteristics of RAM and ROM, the differences between them, and why virtual memory is needed.
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What this dot point is asking
OCR wants you to explain why a computer needs primary storage, describe RAM and ROM (their purpose and characteristics), state the differences between them, and explain virtual memory and when it is used. The RAM-versus-ROM comparison is a classic exam question, so learn the contrasts precisely.
Why a computer needs primary storage
RAM
ROM
The differences between RAM and ROM
Virtual memory
Try this
Q1. State whether RAM is volatile or non-volatile, and what this means. [2 marks]
- Cue. RAM is volatile, meaning it loses its contents when the power is switched off.
Q2. State the purpose of ROM. [1 mark]
- Cue. It stores the fixed start-up program (bootstrap or BIOS) that runs when the computer is switched on.
Q3. Explain one drawback of using virtual memory. [1 mark]
- Cue. It is much slower than real RAM because secondary storage is slow to access, so the computer runs more slowly.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20204 marksState two differences between RAM and ROM, and explain the purpose of each.Show worked answer →
Differences (one mark each, up to two): RAM is volatile (loses its contents when the power is off) whereas ROM is non-volatile (keeps its contents without power); RAM can be both read from and written to whereas ROM is normally read-only; RAM is much larger than ROM.
Purpose (one mark each): RAM holds the programs, the operating system and the data that are currently in use, so the CPU can access them quickly while the computer runs. ROM stores the small fixed start-up program (the bootstrap or BIOS) that the computer runs first when it is switched on to load the operating system.
Markers reward "volatile versus non-volatile" and a correct purpose for each. Saying ROM "stores all the programs" is wrong.
OCR 20223 marksExplain what virtual memory is and why a computer may need to use it.Show worked answer →
Virtual memory is an area of secondary storage (the hard disk or SSD) that the operating system uses as if it were extra RAM. When the RAM becomes full, the operating system moves data and programs that are not currently being used out of RAM and into this area on the disk, freeing RAM for the active program.
It is needed when there is not enough RAM to hold all the programs and data the user has open at once, so the computer can keep running rather than stopping. The drawback is that virtual memory is much slower than real RAM, because secondary storage is far slower to access, which is why heavy use of it slows the computer down.
Markers reward "secondary storage used as extra RAM", "when RAM is full", and ideally a note that it is slower than RAM.
Related dot points
- The need for secondary storage, the common types (optical, magnetic, solid state) and how to choose a suitable device using capacity, speed, portability, durability, reliability and cost.
An OCR J277 1.2.2 answer on the need for secondary storage, the three common types (optical, magnetic, solid state), and how to choose a suitable device by capacity, speed, portability, durability, reliability and cost.
- Why data must be represented in binary, the units of information (bit, nibble, byte, kB, MB, GB, TB, PB) and how to convert between them.
An OCR J277 1.2.3 answer on why computers use binary, the units of information from bit and nibble up to petabyte, and how to convert between units of data capacity.
- The purpose of the CPU and the fetch-decode-execute cycle, the von Neumann architecture, and the function of common CPU components (ALU, CU, cache, registers including the MAR, MDR, Program Counter and Accumulator).
An OCR J277 1.1.1 answer on the purpose of the CPU, the fetch-decode-execute cycle, the von Neumann architecture, and the function of the ALU, control unit, cache and the named registers (MAR, MDR, Program Counter, Accumulator).
- The purpose and functions of the operating system: user interface, memory management and multitasking, peripheral management and drivers, user management, and file management.
An OCR J277 1.5.1 answer on the purpose and functions of an operating system: the user interface, memory management and multitasking, peripheral management and drivers, user management, and file management.
- The purpose of utility software, and the purpose of encryption software, defragmentation software, data compression and backup utilities (full and incremental).
An OCR J277 1.5.2 answer on the purpose of utility software and the specific roles of encryption software, defragmentation software, data compression and backup utilities, including full and incremental backups.
Sources & how we know this
- OCR GCSE (9-1) Computer Science (J277) specification — OCR (2020)