RAM :
Primary memory is commonly referred to by the acronym RAM, which stands for random access memory. Random access means that the storage has addresses and, therefore, the computer system can go to the programs and data it wants directly.
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This is in contrast to sequential access, where the computer system must check each storage location in turn. The term RAM can be slightly misleading in studying computers. Some kinds of secondary storage devices also have random access capabilities.
One of the most important traits of primary memory is that it is temporary- ‘ When the CPU finishes with one set of data and programs in primary memory, it writes another set in its place. In order to be used again, programs 1 and data must be kept in secondary storage, which is relatively slow.
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Also, primary memory is usually volatile. This means that when the computer’s power is shut off, the contents of primary memory are destroyed. This is in contrast to secondary storage, which is nonvolatile.
ROM:
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Because of advances in small semiconductor memories, there has been a trend in recent years to build some software functions directly into computer chips. Like RAM, these all-electronic random-access chips are mounted onto boards inside the system unit.
Once placed on these chips, programs can be accessed very rapidly. On many microcomputer systems, for example, certain operating-system routines are built onto a chip rather than stored on disk.
This kind of software-within-hardware is called firmware. Several kinds of firmware are available:
Read-only memory (ROM) is by far the most common form of firmware. A ROM module contains a prewritten program supplied by the firmware manufacturer.
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The program can be read from the module, but it is impossible for a user to destroy the module’s contents by accidentally or purposely writing over them (hence the term read-only).
Programmable read-only memory (PROM) is identical to ROM except that the buyer writes the program. In other words, a PROM module is like a blank ROM module.
Special equipment is needed to write a program onto a PROM module, and once the program is on it can’t be erased.
Erasable programmable read-only memory (EPROM) is like PROM except that its contents can be erased by exposure to ultraviolet light and a new program written on. The newest type of EPROM, which is electrically erasable, is called EEPROM. EEPROM modules are commonly used in supermarket cash registers to store product prices.
Firmware is usually supplied with the computer system purchased. It can also be bought separately.