BBC BASIC Programmers' Reference

by Jon Ripley, January 2008, May 2011

The FNdetokenise function listed below accepts a string containing either a tokenised program line or tokenised code and returns a string containing detokenised code:

        DEF FNdetokenise(S$)
        IF S$ = "" OR S$ = CHR$0+CHR$255+CHR$255 THEN =""
        PRIVATE Keywd$()
        IF !^Keywd$()=0 THEN
          DIM Keywd$(160)
          Keywd$() = "AND","DIV","EOR","MOD","OR","ERROR","LINE","OFF",\
          \"STEP","SPC","TAB(","ELSE","THEN","","OPENIN","PTR",\
          \"PAGE","TIME","LOMEM","HIMEM","ABS","ACS","ADVAL","ASC",\
          \"ASN","ATN","BGET","COS","COUNT","DEG","ERL","ERR",\
          \"EVAL","EXP","EXT","FALSE","FN","GET","INKEY","INSTR(",\
          \"INT","LEN","LN","LOG","NOT","OPENUP","OPENOUT","PI",\
          \"POINT(","POS","RAD","RND","SGN","SIN","SQR","TAN",\
          \"TO","TRUE","USR","VAL","VPOS","CHR$","GET$","INKEY$",\
          \"LEFT$(","MID$(","RIGHT$(","STR$","STRING$(","EOF","SUM","WHILE",\
          \"CASE","WHEN","OF","ENDCASE","OTHERWISE","ENDIF","ENDWHILE","PTR",\
          \"PAGE","TIME","LOMEM","HIMEM","SOUND","BPUT","CALL","CHAIN",\
          \"CLEAR","CLOSE","CLG","CLS","DATA","DEF","DIM","DRAW",\
          \"END","ENDPROC","ENVELOPE","FOR","GOSUB","GOTO","GCOL","IF",\
          \"INPUT","LET","LOCAL","MODE","MOVE","NEXT","ON","VDU",\
          \"PLOT","PRINT","PROC","READ","REM","REPEAT","REPORT","RESTORE",\
          \"RETURN","RUN","STOP","COLOUR","TRACE","UNTIL","WIDTH","OSCLI",\
          \"","CIRCLE","ELLIPSE","FILL","MOUSE","ORIGIN","QUIT","RECTANGLE",\
          \"SWAP","SYS","TINT","WAIT","INSTALL","","PRIVATE","BY","EXIT"
        ENDIF
        LOCAL flag%, O$, i%, ch%
        IF ASCS$ = LENS$ AND ASCRIGHT$(S$,1) = 13 THEN
          ch% = ASC MID$(S$, 2)+256*ASC MID$(S$, 3)
          IF ch% O$=STR$ch%+" "
          S$ = MID$(S$, 4, LENS$-4)
          IF S$="" THEN =LEFT$(O$, LENO$-1)
        ENDIF
        FOR i%=1 TO LENS$
          IF ASCMID$(S$,i%)=34 flag%EOR=1
          IF flag% AND 1 THEN
            O$ += MID$(S$, i%, 1)
          ELSE
            ch% = ASCMID$(S$, i%)
            CASE TRUE OF
              WHEN ch% >= 17 AND ch% < 128:O$ += CHR$ch%
              WHEN ch% = &8D
                ch% = ASCMID$(S$,i%+1)
                lo% = ((ch% << 2) AND &C0) EOR ASC MID$(S$, i%+2)
                hi% = ((ch% << 4) AND &C0) EOR ASC MID$(S$, i%+3)
                O$+ = STR$(lo% + 256*hi%)
                i% += 3
              OTHERWISE O$ += Keywd$(ch% EOR 128)
            ENDCASE
          ENDIF
        NEXT i%
        =O$

FNdetokenise is useful to list the source code of a line when one occurs.

Using FNdetokenise it is possible to write a Quine, that is, a program which when run outputs its own listing:

        P% = PAGE
        WHILE (!P% AND &FFFFFF) <> &FFFF00
          line$ = ""
          FOR i% = P% TO P%+?P%-1
            line$ += CHR$?i%
          NEXT i%
          PRINT FNdetokenise(line$)
          P% += ?P%
        ENDWHILE
        END

In the author's opinion using this method or *LIST to write a Quine is cheating.

The BBC BASIC for Windows and BBC BASIC for SDL 2.0 interpreters have a built-in detokeniser accessed using the *LIST command that detokenises a specified file and outputs the listing on the current output device. Whilst it is possible to write a simpler looking detokeniser utilising *LIST the resulting routine is significantly slower due to the number of file operations required.

        DEF FNdetokenise(S$)
        IF S$="" OR S$=CHR$0+CHR$255+CHR$255 THEN =""
        IF LENS$>251 ERROR 100,"Line too long"
        LOCAL T%, fIn$, hFile%, hOut%
        DIM T% LOCAL 255
        SYS "GetTempPath", 256, T%
        SYS "GetTempFileName", $$T%, "tmp", 0, T%:fIn$=$$T%
        SYS "GetTempPath", 256, T%
        SYS "GetTempFileName", $$T%, "tmp", 0, T%
        hFile% = OPENOUT$$T%:IF hFile% = 0 ERROR 100,"Cannot create temp file"
        IF ASCS$=LENS$ AND ASC RIGHT$(S$,1)=13 THEN
          BPUT#hFile%, S$+CHR$0+CHR$255+CHR$255;
        ELSE
          BPUT#hFile%, CHR$(LEN(S$)+4)+CHR$0+CHR$0+S$+CHR$13+CHR$0+CHR$255+CHR$255;
        ENDIF
        CLOSE#hFile%
        hOut% = @vdu%?-130
        hFile%=OPENUPfIn$:IF hFile%=0 ERROR 100,"Cannot create temp file"
        OSCLI "Output "+STR$hFile%
        OSCLI "List """+$$T%+""""
        OSCLI "Output "+STR$hOut%
        PTR#hFile%=0:S$=GET$#hFile%:CLOSE#hFile%
        WHILE ASCS$=32:S$=MID$(S$,2):ENDWHILE
        SYS "DeleteFile", fIn$
        SYS "DeleteFile", $$T%
        =S$

Do not use this version in production code.