A computer's memory stores data and program that controls the processing. They are stored in cells at one byte per cell. This storage is called memory, main memory, primary memory, main storage, primary storage, random access memory etc. The processor has access to each cell, but it cannot think. The primary memory is used for four purposes:
- To hold data into input storage area until ready for processing;
- To hold intermediate results into a working storage space.
- To hold finished results in output storage area until released.
- To hold processing instructions in program storage area.
The above areas are not fixed by built-in boundaries. It varies from application to application.
Random Access Memory: The primary storage is referred to as RAM Chips. Random because the locations on a chip can be selected randomly to store/retrieve. These chips are classified as dynamic or static.
Dynamic chips contain (a) a transistor much the same way as a mechanical on/off switch (b) A capacitor capable of storing an electric charge. A no charge=0 bit of hold charge=1 bit. It is Volatile. To locate a particular cell, rows and column addresses are required.
Static RAM chips are also volatile. It takes more transistors, more space and more complicated than Dynamic RAM chips.
Due to more storage at lower-cost compactness and faster performance, semi-conductor chips are used in modern computers. Within limits, memory can be added by adding chips.
Note: In February 1991, the IBM (Dr. Rajiv Joshi), developed a chip named “Lightening” that can send or receive 8 billion bits (the fastest “data rate”) of information per second. It has a Static Random Access Memory (SRAM) that holds 524,288 bits and reads information in 4/Billionth of a second.
Magnetic Drums Prior to RAM, magnetic drums were used. It revolved under a set of Read/Write head to store or retrieve data. This was relatively slow. In the 50's a memory that recorded bits magnetically on small iron pieces called Cores was deviced. Known as Core Memory, it stored and retrieved data electronically and randomly.
Read Only Memory (ROM) With special programs higher level operations are performed. These are called micro-programs. They deal with low level machine functions. The micro-programs are held in special control storage elements in the processor and are called Read Only Memory chips (ROM). It stores programs and data that are essential for proper operations of the system. These form integral part of the computer. It is non-volatile and supplied by the vendor. The users have no control over it.
Programmable Read Only Memory (PROM) Lengthy operations that are executed slowly by a software, can be converted into micro-programs and fused into ROM chips called PROM. In hardware form, execution becomes faster. Once written into PROM, they cannot be altered. These are also supplied by vendors.
Erasable & Programmable Read Only Memory (EPROM) These are ROM chips that can be erased and reprogrammed. These are to be removed from the processor and exposed to ultraviolet light for accepting new contents. This is not useful for application programs.
The USB Flash Drives are now built on this technology.
Electrically Erasable & Programmable Read Only Memory (EEPROM) Chips that are programmed with special electric pulses are called EEPROM.