3.4.5 - Memory types and CPU performance
Compare RAM, ROM, cache and registers by their purpose and persistence, then explain how clock speed, processor cores and cache size can affect CPU performance.
RAM, ROM, cache and registers
A computer uses different kinds of memory because capacity, access speed and purpose create different needs.
| Type | Purpose and reason it is needed |
|---|---|
| RAM (random access memory) | Holds programs and data currently in use. It can be read and written so applications can update working data. |
| ROM (read-only memory) | Holds persistent instructions, such as startup firmware, that must be available when the system begins operation. It is not normally rewritten during ordinary application use. |
| Cache | Small, fast memory close to or within the CPU, holding copies of frequently or recently needed instructions/data to reduce slower main-memory accesses. |
| Registers | Very small, very fast storage in the CPU holding immediately needed instructions, values or intermediate results. |
An image editor's working image is typically held in RAM because it is larger than immediate processor storage and changes while being edited. A cache may hold data the processor repeatedly uses. Registers hold the handful of values involved in a current operation. The capacities and roles differ; cache is not the place to save an entire photo collection permanently.
The specification defines main memory as directly CPU-accessible memory except cache and registers. RAM and ROM therefore belong to main memory in this model. Secondary storage such as a drive is a different category.
Volatile and non-volatile memory
Volatility describes whether stored information is retained when electrical power is removed.
Volatile and non-volatile
Volatile memory loses its contents when power is removed. Non-volatile memory retains its contents without power.
RAM is volatile. Its read/write behaviour makes it useful for changing working data, but the current contents disappear when the power is removed. Saving a document writes a persistent copy to secondary storage; merely leaving the document open in RAM does not preserve it through a power loss.
ROM is non-volatile, so startup instructions remain available before the system has loaded normal applications. It is intended mainly for reading during normal use; some modern firmware can be updated using a controlled procedure. This does not make it equivalent to RAM for routine changing application data.
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