In the standard textbook classification, the two types of main memory are:

Concept

Main memory is the CPU-accessible memory that holds instructions and data needed during processing.

In the standard textbook classification, it includes volatile read-write RAM and non-volatile ROM used for fixed instructions.

Application

The pair RAM and ROM names these two main-memory categories: RAM supplies temporary working space, while ROM retains fixed contents without power.

Contrast

• Primary and Secondary: Primary and secondary describe levels of the broader storage hierarchy, not two components within main memory.

• Random and Sequential: Random and sequential describe access methods rather than memory-component categories.

• Hard Disk and Pen Drive: Hard disks and pen drives are persistent secondary-storage devices.

• All of these: All of these cannot apply because the listed pairs mix classification levels, access methods, and storage devices.

Result

Therefore, the two types are RAM and ROM.