Random Access Memory
Random access memory (RAM), also called main memory or primary storage, is the temporary memory in which programs and data are stored while the computer is in use. Programs must first be entered, or input, into RAM before they are executed or data is processed. The CPU then moves information from RAM into its registers for processing. RAM performs these three functions:
- accepts and holds program instructions and data
- acts as the CPU's source for data and instructions and as a destination for operation results
- holds the final processed information until it can be sent to the desired out- put or storage devices, such as a printer or disk drive
The CPU must be able to find programs and data once they are stored in RAM Therefore, program instructions and data are placed at specific location within RAM known as an address. Each location has its own unique address, just as each person has an individual postal mailing address. When the CPU needs an instruction or data from RAM, an electronic message is sent to the instruction's address and the instruction is transferred to the appropriate register in the CPU
Random access means that because each RAM location has an individual address, the computer can go directly to the instructions and data it needs. rather than search each individual location one after another (sequentially). RAM memory is both readable and writable, meaning that the contents of any RAM location can be changed and/or read at any time, RAM memory is also volatile memory, meaning that it requires a constant charge to keep its contents intact. It a computer loses power, the contents of its memory are lost. Therefore, it is important to save any valuable work frequently to a permanent storage medium
The temporary nature of RAM is its most important characteristic. When the computer is finished with one set of instructions and data, it can store another set in the first set's place RAM is reusable, much like a chalkboard. Instructions and data can be written on the chalkboard (or into RAM) and then erased to make room for new instructions and data to be written in the same space.
Types of RAM Two types of RAM used with early PCs were Dynamic RAM
(DRAM pronounced dee-ram) and Static RAM (SRAM, pronounced essram). Some personal computers contained either, or both, types. Without a continuous supply of electrical energy. DRAM chips eventually lose their contents. Because of this, DRAM chips must be constantly refreshed by receiving a fresh supply of energy. SRAM is a static type of RAM that is faster and more reliable (and more expen- sive) than the more common DRAM. The term static refers to the fact that SRAM doesn't need to be refreshed like DRAM and it therefore allows a faster access time.
Over the years, newer computers have been introduced that contain faster microprocessors. To accommodate the increased speed, chip manufacturers have designed and built faster RAM chips Synchronous DRAM (SDRAM) divides RAM into two separate memory banks to increase the processing of memory requests. Double Data Rate SDRAM (DDR SDRAM) can transfer data twice as fast as SDRAM because it reads data twice during each clock cycle. DDR2 and DDRS are newer versions of DDR SDRAM offering significant improvements in speed and performance.
The amount of main memory in a computer is important. Large programs, such as desktop publishing and computer-aided design applications, require a lot of main memory. A computer may be unable to use a program if the computer's main memory is insufficient. Additional RAM chips can be installed inside the system unit on most computers.
Measuring RAM Capacities RAM storage capacities are measured in bytes.
billions, of bytes, it is common to refer to storage capacity in terms of kilobytes Since most personal computers have enough memory to store millions or even equal to one thousand bytes), megabytes (equal to one million bytes), and even (one billion bytes). Storage capacities of personal computers are typi #cally quoted as 512 megabytes, or as one or more gigabytes. By contrast, today's mainframe computer storage is often measured in terabytes, or trillions of bytes, and the most powerful supercomputers offer storage capacities expressed in peta- bytes, each of which is approximately 1,000 terabytes. The prefix tera- is derived from the Greek word for monster, an apt association to the tremendous size of a terabyte.