Computer Memory

Main memory consists of a number of storage locations, each of which is identified by a unique address. The ability of the CPU to identify each location is known as its addressability.  Each location stores a word i.e. the number of bits that can be processed by the CPU in a single operation. Word Length may be typically 16,24,32 or as many as 64 bits. A large word length improves system performance, though may be less efficient on occasions when the full word length is not used.

Types of main memory

There are two types of main memory, Random Access Memory (RAM) and Read Only Memory(ROM).

• Random Access Memory (RAM)

Holds its data as long as the computer is switched on.

All data in RAM is lost when the computer is switched off

Described as being volatile.

It is direct access as it can be both written to or read from in any order

Its purpose is to temporarily hold programs and data for processing in modem computers it also holds the operating system.

• Read Only Memory (ROM)

ROM holds programs and data permanently even when computer is switched off

Data cam be read by the CPU in and order so ROM is also direct access.

The contents of Rom are fixed at the time of manufacture.

Stores a program called the bootstrap loader that helps start up the computer.

Access time of between 10 and 50 nanoseconds.