CPU PRIMARY AND SECONDARY STORAGE The  CPU  contains  circuits  that  control  and  execute instructions by using some type of MEMORY. Memory is referred to by size, such as 16K, 32K, 64K, and so on. The "K" represents the value of 1,000. Therefore 16,000 is 16K. Semiconductor  memory  consists  of  hundreds  of thousands of tiny electronic circuits etched on a silicon chip. Each of these electronic circuits is called a BIT CELL and can be in either an OFF or ON state to represent a 0 or 1 bit. This state depends on whether or not current is flowing in that cell. Another name used for semiconductor memory chips is integrated circuits (ICs).  Developments  in  technology  have  led  to large-scale  integration  (LSI)  that  allows  more  and  more circuits to be squeezed onto the same silicon chip. Some of the advantages of semiconductor storage are  fast  internal  processing  speeds,  high  reliability,  low power consumption, high density (many circuits), and low cost. However, there is a drawback to this type of storage. It may be VOLATILE, which means it requires a constant power source. When the power for your system fails and you have no backup power supply, all of the stored data is lost. Primary  Storage Two classifications of primary storage with which you  should  become  familiar  are  read-only  memory (ROM) and random-access memory (RAM). READ-ONLY MEMORY (ROM).—In computers, it is useful to have instructions that are used often, permanently stored inside the computer. ROM enables us to do this without loosing the programs and data when  the  computer  is  powered  down.  Only  the computer manufacturer can provide these programs in ROM; once done, you cannot change it. Consequently, you cannot put any of your own data or programs in ROM. Many complex functions, such as translators for high-level  languages,  and  operating  systems  are  placed in  ROM  memory. Since these instructions are hardwired, they can be performed quickly and accurately. Another advantage of  ROM  is  that  your  imaging  facility  can  order programs tailored for its specific needs and have them installed permanently in ROM. Such programs are called microprograms or firmware. RANDOM-ACCESS  MEMORY  (RAM).—RAM is another type of memory found inside computers. It may be compared to a chalkboard on which you can scribble down notes, read them, and erase them when finished.  In  the  computer,  RAM  is  the  working memory.  Data  can  be  read  (retrieved)  or  written (stored) in RAM by providing the computer with an address location where the data is stored or where you want  it  to  be  stored.  When  the  data  is  no  longer requited, you may simply write over it. Thus you can use the storage location again for something else. Secondary  Storage Secondary  storage,  or  auxiliary  storage,  is  memory external to the main body of the computer (CPU) where programs and data can be stored for future use. When the computer is ready to use these programs, the data is read  into  primary  storage.  Secondary  storage  media extends the storage capabilities of the computer system. Secondary storage is required for two reasons. First, the working memory of the CPU is limited in size and cannot  always  hold  the  amount  of  data  required. Second, data and programs in secondary programs do not disappear when the power is turned off. Secondary storage  is  nonvolatile  memory.  This  information  is  lost only when you erase it. Magnetic disks are the most common type of secondary storage. They may be either floppy disks or hard disks (hard drives). PERIPHERAL DEVICES Peripheral devices include all the input and output devices used with a computer system. When these devices are under control of the CPU, they are said to be   on   line.   When   they   perform   their   function independently, not under direct control of the CPU, they are said to be off line. The following peripheral devices are used commonly for input and output. Those that perform only input are marked (I), those that perform only output are marked (O), and those that perform both input and output are marked (I/O). Optical Character Reader (I) An optical character reader reads printed data (characters) and translates it to machine code. Keyboard  (I) The keyboard is used by a computer operator to communicate with a computer system. 3-4