Types of Memory

Human beings memorise many things and recall them to make decisions — but we do not rely on memory alone; we also make notes and store important information in notebooks, manuals, journals and documents. Computers mirror this exactly: they have two types of memory — primary (the fast working memory) and secondary (the permanent record). Between them sits a third, special helper: cache memory.

(A) Primary memory

Primary memory is an essential component of a computer system: programs and data must be loaded into primary memory before processing, and the CPU interacts directly with primary memory for its read and write operations. It comes in two types:

RAM (Random Access Memory)

• RAM is volatile: it retains data only as long as power is supplied. The moment the power supply is turned off, all contents of RAM are wiped out.
• It stores data temporarily while the computer is working. Whenever the computer starts, or a software application is launched, the required program and data are loaded into RAM for processing.
• RAM is usually what is meant by main memory, and it is faster than secondary memory (storage devices).

ROM (Read Only Memory)

• ROM is non-volatile: its contents are not lost even when the power is turned off.
• It serves as a small but fast permanent store for contents that are rarely changed.
• Classic example: the startup program (boot loader) — the program that loads the operating system into primary memory when the computer switches on — is stored in ROM.

(B) Cache memory

RAM is faster than secondary storage, but it is still not as fast as the processor — so a CPU waiting on RAM may have to slow down. The remedy is cache: a very high speed memory placed between the CPU and primary memory.

How cache works:

• It stores copies of data from frequently accessed primary-memory locations.
• When the CPU needs data, it first examines the cache. If the required data is there, it is read from the cache; otherwise the primary memory is accessed.
• The overall effect is to reduce the average time needed to access data from primary memory.

(C) Secondary memory

Primary memory has two limitations: its storage capacity is limited, and it is either volatile (RAM) or read-only (ROM). So a computer system needs auxiliary or secondary memory to store data and instructions permanently for future use.

Properties of secondary memory:

• Non-volatile — contents survive power-off.
• Larger capacity than primary memory.
• Slower and cheaper than main memory.
• Not directly accessible by the CPU — contents of secondary storage must first be brought into main memory before the CPU can work on them. …