128  Pascal User Manual  first character of each line as a "cal riage control" character, which is not  printed but may cause some actioJl such as a page eject or overprinting.  When a textfile is used to represmt a particular device, the program  must obey the conventions for us  i  ng that device.  Textfiles may be accessed through the predeclared file procedures  Get  and  Put.  This can, of course, be quite cumbersome as these  procedures are defined for  single-~:haracter  manipulation. To illustrate,  suppose we have a natural number stored in a variable x  and wish to  write it on the file output. Note that the pattern of characters denoting  the decimal representation of the \ alue will be quite different from that  denoting the value written as a Roman numeral (see Program 4.9). But  as we are usually interested in del imal notation, it appears sensible to  offer  built-in,  standard,  transf0nnation  procedures that  translate  abstract numbers (from whatever  I~omputer-internal  representation is  used) into sequences of decimal dl,gits and vice versa.  The two predeclared procedures  Read  and  Write  are thereby  extended in several ways to facilit.:tte the analysis and the formation of  textfiles.  The standard textfiles  Input  and  Output  usually represent the  standard I/O media of a computer s:vstem (such as the keyboard and the  video display).  Hence, they are  the  principal communication line  between the computer system and Its human user.  Because  these  two  files  an  used  very  frequently,  they  are  considered as "default values" in tl 	file operations when the textfile  F  is not explicitly indicated. That is  12.A.  The Standard Files Inpu1 and Output  Write (Ch)  Read(Ch)  Writeln  Readln  Eof  Eoln  Page  WritE: '1Itput,Ch)  Read(  'l},ut,Ch)  WritE  '(Output)  (See Section 12.B.)  Readl I (  nput)  (See Section 12.B.)  Eof (1 ,:'llt)  (See Section 12.B.)  Eoln (  :-put)  (See Section 12.B.)  Page (lulput)  (See Section 12.D.) 