Compact Disk (CD) is which type of memory?

Concept

Computer memory is divided by role into two families. Primary (main) memory is directly accessible by the CPU and holds programs and data while they run; it is built from semiconductor chips (RAM and ROM) and is fast but limited in capacity. Secondary memory (also called auxiliary or backup storage) is not directly addressed by the CPU; it stores programs and data permanently on external media for long-term retention and large capacity, and the data must first be copied into primary memory to be used.

Application

A Compact Disk (CD) is an optical medium: data is written and read with a laser as microscopic pits on a reflective surface. It sits outside the CPU's directly addressable space, retains its contents after power is removed (non-volatile), and is used for bulk, long-term storage and distribution of data. By the definition above, a medium that stores data permanently on external media and is not directly accessed by the CPU is secondary memory. Therefore a CD is Secondary Memory.

Contrast

• RAM is semiconductor primary memory, volatile (loses contents on power-off) and directly accessed by the CPU; it is the working memory, not a storage disk.

• ROM is semiconductor primary memory, non-volatile but built into the machine to hold firmware/boot code; it is a chip, not an optical disk.

• Primary Memory is the umbrella term for the CPU's directly addressable chip memory (RAM plus ROM); a CD is not chip memory and is not directly addressed by the CPU.

• Secondary Memory is external, non-volatile, high-capacity media (CD, DVD, hard disk, USB flash) read and written via the I/O system; a CD fits this category exactly.