Primary memory

Primary storage unit is used for four activities – Input/Output operations; Manipulations of text and calculation operation; Logic/Comparison operation; Storage and retrieval operations. Primary memory is the memory is place, which is used to store the currently running program and the data being processed and their results. This is generally a temporary storage area. It is used when small amount of data have to be exchanged quickly and used repeatedly. It is a very fast memory but is of small storage capacity. Examples of Primary memory are RAM, ROM, Cache Memory etc.

RAM (Random Access Memory)

RAM is used by the computer as a temporary storage area for the things it is using. Any data that you are using or inputting is temporarily stored in the RAM, where it can be manipulated and used by the computer. Its capacity is usually measured in MB (Megabyte). It is volatile and holds data until the power is on.

Types of RAM

ROM (Read Only Memory)

It is where your computer stores instructions that it uses when it boots (starts) up. ROM is non-volatile memory. That is to say, any information stored in ROM remains intact even when the computer is powered off. ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to. 

Types of ROM

Cache Memory

A buffer, smaller and faster than main storage, used to hold a copy of instructions and data in main storage that are likely to be needed next by the processor and that have been obtained automatically from main storage. It is pronounced cash, a special high-speed storage mechanism. It can be either a reserved section of main memory or an independent high-speed storage device. Two types of caching are commonly used in personal computers: memory caching and disk caching.

A memory cache, sometimes called a cache store or RAM cache, is a portion of memory made of high-speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is effective because most programs access the same data or instructions over and over. By keeping as much of this information as possible in SRAM, the computer avoids accessing the slower DRAM.

Some memory caches are built into the architecture of microprocessors. The Intel 80486 microprocessor, for example, contains an 8K-memory cache, and the Pentium has a 16K cache. Such internal caches are often called Level 1 (L1) caches. Most modern PCs also come with external cache memory, called Level 2 (L2) caches. These caches sit between the CPU and the DRAM. Like L1 caches, L2 caches are composed of SRAM but they are much larger.

Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory. The most recently accessed data from the disk (as well as adjacent sectors) is stored in a memory buffer. When a program needs to access data from the disk, it first checks the disk cache to see if the data is there. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.

When data is found in the cache, it is called a cache hit, and the effectiveness of a cache is judged by its hit rate. Many cache systems use a technique known as smart caching, in which the system can recognize certain types of frequently used data. The strategies for determining which information should be kept in the cache constitute some of the more interesting problems in computer science.