RAM: Random-Access Storage

RAM, a read-and-write memory, enables data to be both read and written to memory. RAM (primary storage) is electronic circuitry with no moving parts. Electrically charged points in the RAM chips represent the bits (Is and Os) that comprise the data and other information stored in RAM. With no mechanical movement, data can be accessed from RAM at electronic speeds. Over the past two decades, researchers have given us a succession of RAM technologies, each designed to keep pace with ever-faster processors. Existing PCs have FPM RAM, EDO RAM, SRAM, DRAM, BEDO RAM, and other types of memory. However, most new PCs are being equipped with synchronous dynamic RAM (SDRAM). SDRAM is able to synchronize itself with the processor, enabling data transfer at more than twice the speed of previous RAM technologies. With the next generation of processors, we'll probably move to Rambus DRAM (RDRAM), which is six times faster than SDRAM.
A state-of-the-art SDRAM memory chip, smaller than a postage stamp, can store about 128,000,000 bits, or more than 12,000,000 characters of data! Physically, memory chips are installed on single in-line memory modules, or SIMMs, and on the newer dual in-line memory modules, or DIMMS. SIMMS are less expensive, but have only a 32-bit data path to the processor, whereas DIMMs have a 64-bit data path.
There is one major problem with RAM storage: It is volatile memory. That is, when the electrical current is turned off or interrupted, the data are lost. Because RAM is volatile, it provides the processor only with temporary storage for programs and data. Several nonvolatile memory technologies have emerged, but none has exhibited the qualities necessary for widespread use as primary storage (RAM). Although much slower than RAM, nonvolatile memory is superior to SDRAM for use in certain computers because it is highly reliable, it is not susceptible to environmental fluctuations, and it can operate on battery power for a considerable length of time. For example, it is well suited for use with industrial robots.
READIN', 'RITIN', AND RAM CRAM All programs and data must be transferred to RAM from an input device (such as a keyboard) or from secondary storage (such as a disk) before programs can be executed and data can be processed. Therefore, RAM space is always at a premium. Once a program is no longer in use, the storage space it occupied is assigned to another program awaiting execution. PC users attempting to run too many programs at the same time face RAM cram, a situation in which there is not enough memory to run the programs. Programs and data are loaded to RAM from secondary storage because the time required to access a program instruction or piece of datum from RAM is significantly less than from secondary storage. Thousands of instructions or pieces of data can be accessed from RAM in the time it would take to access a single piece of datum from disk storage. RAM is essentially a high-speed holding area for data and programs. In fact, nothing really happens in a computer system until the program instructions and data are moved from RAM to the processor.

Interaction between Computer System Components
During processing, instructions and data are passed between the various types of internal memories, the processor's control unit and arithmetic and logic unit, the coprocessor, and the peripheral devices over the common electrical bus. A system clock paces the speed of operation within the processor and ensures that everything takes place in timed intervals. Refer back to this figure as you read about these components throughout the remainder of this chapter.
illustrates how all input/output (I/O) is "read to" or "written from" RAM. Programs and data must be "loaded," or moved, to RAM from secondary storage for processing. This is a nondestructive read process; that is, the program and data that are read reside in both RAM (temporarily) and secondary storage (permanently) .
The data in RAM are manipulated by the processor according to program instructions. A program instruction or a piece of datum is stored in a specific RAM location called an address. RAM is analogous to the rows of boxes you see in post offices. Just as each P.O. box has a number, each byte in RAM has an address. Addresses permit program instructions and data to be located, accessed, and processed. The content of each address changes frequently as different programs are executed and new data are processed.