read-only  memory  (ROM),  programmable  read-only memory  (PROM),  and  erasable  programmable  read- only memory (EPROM). 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. PERIPHERAL  DEVICES Peripheral devices include all the I/O devices used with a computer system. When these devices are under control of the CPU, we say they are online. When they perform  their  functions  independently,  not  under  direct control  of  the  CPU,  we  say  they  are   offline.  The peripheral   devices   described   in   the   following paragraphs  are  the  ones  we  commonly  use:  magnetic ink character readers, scanners, bar-code readers, key- to-online  data  entry  terminals,  magnetic  tape  units, magnetic  disk  drive  units,  floppy  disk  drive  units, 1-6