SECTION G - PRIMER                                    FILE PRIMER.G
                                       CONTROL CODES
                                       *************

          In this section we introduce some code combinations which affect the 
          screen display and the output to the printer. 

          The American Standard Code for Information Interchange (ASCII) is 
          the recognised standard for exchanging information between computers 
          and allied equipment. The standard ASCII table consists of the 
          numbers 0 to 127 (Hexadecimal #00 to #3F), each number representing 
          a symbol, among which are the letters of the alphabet, both in upper 
          and lower case,numbers 0 to 9, punctuation marks, the space and many 
          others. Your PCW has added to this with extra symbols up to 255 
          (Hex.No #FF).   Symbols with numbers in the range 0 to 31 are also 
          interpreted as CONTROL CODES, that is they can, when used correctly, 
          affect the output to screen or printer.   

          A full table as applied to your PCW can be seen in the CP/M manual 
          supplied with the machine in appendix I.4, page 113, and control 
          codes are discussed in appendix III, page 139. 

                    =============================================
                    = In PRIMER we shall begin to see how these =
                    = codes and Command codes are used in BASIC.= 
                    ============================================= 

          Many of the symbols on the ASCII table can be typed to screen or 
          printer direct from the keyboard. These include, of course, letters, 
          spaces, numbers etc., but some cannot be typed directly. For these 
          the function CHR$ is used. For example, the command line 

                  PRINT CHR$(35) 

          would print # on the screen since 35 is the ASCII code for # in 
          decimal.

          Conversely the command line 

                  PRINT ASC(#)    would print the number 35

          The programs following this text demonstrate this.

          ESCAPE SEQUENCES
          =================
          The number 27 in the ASCII table has a particular significance for 
          us as it represents ESCAPE. In CP/M it can be accessed simply by 
          pressing the EXIT key, but in BASIC we must use the CHR$(27) form. 
          This used in conjunction with other symbols can produce many 
          interesting effects. In PRIMER, as an introduction we shall see how 
          to use it to clear the screen.This is done by typing the line 
                  PRINT CHR$(27)+"E" and then pressing [RETURN].Similarly, to 
          'HOME' the cursor i.e. to place it in the top left-hand corner type
                  PRINT CHR$(27)+"H" and press [RETURN].
          These two commands can be, and usually are, combined to produce a 
          blank screen with the cursor homed. The command line is:-
                  PRINT CHR$(27)+"E"+CHR$(27)+"H" or you may also see it 
          written
                  PRINT CHR$(27);"E";CHR$(27);"H" : 
          Either way it is a pain in the **** to keep typing this line. 
          Luckily, it can be typed once in a program and represented by a 
          variable. This can be seen in line 10 of TUTOR where it is made 
          equal to cls$, so to clear the screen you type 'PRINT cls$ 'only. 
          Other 'Escape sequences' can be treated similarly. Following the 
          CHR$ and ASC programmes you will see lines 10 to 90 listed which 
          show the screen clearing codes plus some others used in TUTOR. 

              Although this section may seem complicated, in a short time you 
          will think it child's play. For the moment it is sufficient to be 
          aware of the existence of these codes and to recognise the CHR$(27) 
          as the ESCAPE code. We shall return to them in greater detail in the 
          next part,INTER   

          End of file PRIMER.G
CAPE code. We shall return to them in greater detail in the 
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