RANDOM-ACCESS MEMORY (RAM).\ RAM, also called read/write memory, is like a chalkboard. You can write notes, read them, and erase them when you no longer need them. In the computer, RAM is the working memory. Data can be read (retrieved) from or written (stored) into RAM just by giving the computer the address of the RAM location where the data is stored or is to be stored. When the data is no longer needed, you can simply write over it. This allows you to use the storage again for something else. Core, semiconductor, and bubble storage have random-access memory (RAM) capabilities.

READ-ONLY MEMORY (ROM).\ In most computers, it is useful to have often used instructions, such as those used to bootstrap (initial system load) the computer or other specialized programs, permanently stored inside the computer. The memory that enables us to do this without the programs and data being lost even when the computer is powered down is read-only memory (ROM). Only the computer manufacturer can install these programs into ROM, and, once installed, they cannot be changed. Consequently, you cannot put any of your own data or programs into ROM. Many complex functions, such as routines to calculate square root, translators for high-level programming languages, and operating systems, can be stored into ROM. Because the instructions are permanently stored, they are quickly performed with accuracy. Also, your computer facility can order programs designed for its needs and have them permanently installed into ROM by the manufacturer. To describe these permanently installed programs, the term microprogram, or firmware, is used.

PROGRAMMABLE READ-ONLY MEMORY (PROM).\ Your computer facility can also buy programmable read-only memory (PROM) already programmed by the manufacturer or in a blank state. Using a blank PROM and a device designed to write (burn) a program into PROM, you can enter any program into the memory. However, you cannot make any changes to the program once it has been written into PROM. But, PROM does provide flexibility not available with ROM. Of course, you must be sure the program is error free before it is written into PROM.

ERASABLE PROGRAMMABLE READ-ONLY MEMORY (EPROM).\ The erasable programmable read-only memory (EPROM) was developed to overcome the drawback of PROM. Your

facility can buy blank EPROMs from the manufacturer, and you can write programs developed at your command/activity using a special device. The big difference with EPROM is that you can erase it if and when the need arises. The data and programs can be retrieved many times. If you want to reprogram memory, you first erase the EPROM with a burst of ultraviolet light. This not only enables you to reprogram when requirements change, but also, you can erase and write the program again if a mistake is made while programming the EPROM. In other words, a mistake is not fatal, as it is when using PROM. You have the flexibility to change programs to include improvements or modifications in the future.

SECONDARY STORAGE

The last type of memory we briefly introduce here is called secondary storage or auxiliary storage. This is memory outside the main body of the computer where we store programs and data for future use. When the computer is ready to use these programs and data, it reads them into primary storage. Secondary (auxiliary) storage media extends the storage capabilities of the computer. We need secondary storage for two reasons. First, because the computer's working memory (primary storage) is finite and limited in size, it cannot always hold all the data we need. Second, in secondary storage, data and programs do not disappear when power is turned off as they do when semiconductor ^{memories are used.} Secondary storage media are nonvolatile memories. This means the information is lost only if you or the users intentionally erase it. The three types of secondary storage we most commonly use are magnetic tape, magnetic disk/diskette, and magnetic drum.