CD-ROM is a

Correct — A, (a) secondary memory. A CD-ROM — Compact Disc Read-Only Memory — is an optical disc whose contents are impressed at manufacture and afterwards only read, by a laser that senses the pits and flat lands along a spiral track. What the question is really asking is where the device sits in the memory hierarchy, and the hierarchy has two halves. Primary or main memory is what the processor addresses directly: it is made of chips, it is fast, it is small, and the working part of it (RAM) empties the moment the power goes. Secondary or auxiliary storage is everything that keeps data when the power is off and that the processor cannot reach into directly — a hard disk, a magnetic tape, a pen drive, a CD or a DVD. Before the CPU can work on anything held there, the operating system must copy it into main memory.

A CD-ROM answers to every part of that second description. It is non-volatile, so a disc pressed years ago still reads today. It is removable, which is exactly why the format was used to distribute software, encyclopaedias and installation media. Its usable capacity, around 650 to 700 megabytes, is enormous beside a processor's registers and cheap per byte beside RAM. And its access time is measured in milliseconds against the nanoseconds of main memory, which is the price of the mechanical spin and the moving optical head. Slow, large, cheap, permanent and not directly addressable is the signature of secondary storage, and it is what option (a) names.

Why the others are wrong

• (b)magnetic memory — A CD-ROM is optical, not magnetic. Its bits are physical features — pits burnt or pressed into a reflective layer under clear polycarbonate — and they are read by the change in reflected laser light as the beam crosses from a land into a pit. Nothing on the disc is magnetised and no head flies over it. Magnetic media are the hard disk, the floppy disk and the tape cartridge, which hold each bit as a direction of magnetisation in a coating and can be corrupted by a strong magnetic field — a hazard an optical disc is immune to. A candidate who chooses this option has the level of the hierarchy right, because magnetic media are secondary too, and the recording technology wrong. The stem asks what class of memory the device is, and on the recording technology a CD parts company with the hard disk.
• (c)memory register — A memory register is the smallest and fastest storage in the machine, and it is inside the processor rather than attached to it. A register holds one word — the operand the arithmetic and logic unit is working on this instant, the address of the next instruction, the instruction currently being decoded — and a processor has only a handful of them, so the whole register file is measured in bytes. Registers are built from the fastest logic on the chip because everything the CPU does passes through them. A CD-ROM sits at the opposite end of the same hierarchy: hundreds of megabytes, removable, mechanical and the slowest link in the chain. Naming a register here confuses the top of the pyramid with its base, and the two differ by a factor of many millions in both size and speed.
• (d)semiconductor memory — Semiconductor memory means memory built as an integrated circuit on silicon — the RAM modules, the ROM and EPROM chips, the flash cells in a pen drive or a solid-state drive. A CD-ROM is not a chip at all: it is a disc of polycarbonate with a thin reflective coating, and it has no transistors. The distinction worth carrying away is that semiconductor and secondary are not opposites in general — a flash drive is both, being a chip and being auxiliary storage — so this option cannot be dismissed by saying that secondary storage is never semiconductor. It is dismissed on the particular device: a compact disc is read optically from a moulded plastic surface, and there is no semiconductor anywhere in it.

Computer memory is taught as a hierarchy, and the whole of this question is a request to place one device in it. At the top are the processor registers, a few dozen words of the fastest storage in the machine, sitting inside the CPU. Below them is cache, a small, fast buffer that holds the lines of main memory the processor has just used or is likely to want next. Below that is primary or main memory — the semiconductor RAM the processor addresses directly, together with the ROM that holds the firmware — fast, volatile in its RAM part, and limited in size. At the bottom is secondary or auxiliary storage: hard disks, solid-state drives, magnetic tape, pen drives and optical discs such as the CD-ROM, DVD and Blu-ray. Every step down the hierarchy is slower and larger and cheaper per byte than the step above it, and secondary storage is distinguished by two properties in particular — it survives a power cut, and the processor cannot operate on it directly, so its contents must first be read into main memory. A CD-ROM belongs to that bottom tier. It was standardised by Philips and Sony in 1985 in the specification known as the Yellow Book, holds about 650 to 700 megabytes, and is read by a laser tracking a spiral of pits and lands from the centre outwards.

Key facts

• CD-ROM stands for Compact Disc Read-Only Memory: the contents are written once at manufacture and can afterwards only be read.
• It is secondary (auxiliary) storage — non-volatile, removable, and not directly addressable by the processor; data must be copied into main memory first.
• A CD-ROM is read optically. A laser senses pits and lands on a reflective layer beneath clear polycarbonate; nothing is magnetised.
• Standard capacity is about 650 to 700 megabytes, which is why the format carried software distributions and reference works.
• The CD-ROM specification is the Yellow Book, issued by Philips and Sony in 1985; ISO 9660 is the file system usually used on it.
• Primary memory is the semiconductor RAM and ROM the CPU addresses directly; RAM is volatile and loses its contents when power is removed.
• Registers are inside the processor and hold single words; cache sits between the registers and main memory as a high-speed buffer.
• Each step down the hierarchy — registers, cache, main memory, secondary storage — is slower, larger and cheaper per byte than the one above it.

Common traps

• Reading 'ROM' in the name and concluding that a CD-ROM must be primary memory because ROM chips are; the ROM here only means read-only.
• Assuming that anything read-only is a semiconductor device — a compact disc has no transistors anywhere in it.
• Treating optical and magnetic as interchangeable words for secondary storage; both are secondary, but a CD is optical.
• Confusing a memory register inside the processor with memory in general, which places the slowest device in the fastest tier.
• Forgetting that secondary storage cannot be operated on directly — its contents must be brought into main memory first.

• practice — not a real PYQ

  Which one of the following is not a characteristic of secondary storage in a computer system?

  • (a)It retains data when the power is switched off
  • (b)Its contents are addressed directly by the arithmetic and logic unit
  • (c)It is cheaper per byte than main memory
  • (d)Its contents must be copied into main memory before processing

  Answer(b) Its contents are addressed directly by the arithmetic and logic unit

• practice — not a real PYQ

  Data on a CD-ROM is read by which one of the following methods?

  • (a)A magnetic head sensing changes in magnetisation
  • (b)A laser beam sensing pits and lands on a reflective layer
  • (c)An electron beam scanning a phosphor coating
  • (d)Electrical charge trapped in floating-gate transistors

  Answer(b) A laser beam sensing pits and lands on a reflective layer