This is to convert tokenized BBC BASIC for Windows files to a human-readable (plain text) format. Note that this will probably not handle tokenized files from other versions of BBC BASIC -- specifically, I know that the RISC OS version uses a different format (see this page and this page at the BB4W wiki).

Two pages that may be of interest at the BB4W wiki are Tokeniser and Detokeniser, both of which discuss this topic using BBC BASIC code.

I believe that the previous issues with line numbers as the targets of GOTO and the like has been fixed, using a QB translation of code on the above-mentioned Detokeniser page at the BB4W wiki. (View the page history for details, if you're interested.)

The VBA code that generates the below program has been moved to its own page.

Licensing

The GNU General Public License (GNU GPL) v2 or, at your option, any later version.
The GNU Free Documentation License (GNU FDL) v1.2 or, at your option, any later version.

The two licenses are not 100% compatible; which license you use, I leave up to you.

Exception

Any source code that is generated by the generated QBasic program (only) is specifically excluded from the above licensing terms, and is instead covered by whatever license(s) cover the original, tokenized sources.

Detoken.bas

This is the code that actually does the work. I chose QBasic for a number of reasons, the most important of which are:

interpreter (no compile time)
simple
ease of including in batch files

Note that since I expect the code to run under QBasic, you don't specify the file(s) to be detokenized on the command line, but instead do it one file at a time, via the DETOKEN environment variable. (This can easily be changed, but I don't care to.)

DATA 1,CIRCLE,2,ELLIPSE,3,FILLED,4,MOUSE,5,ORIGIN,6,QUIT,7,RECTANGLE,8,SWAP,9,SYS,10,TINT
DATA 11,WAIT,12,INSTALL,14,PRIVATE,15,BY,16,EXIT,128,AND,129,DIV,130,EOR,131,MOD,132,OR
DATA 133,ERROR,134,LINE,135,OFF,136,STEP,137,SPC,138,TAB(,139,ELSE,140,THEN,142,OPENIN,143,PTR
DATA 144,PAGE,145,TIME,146,LOMEM,147,HIMEM,148,ABS,149,ACS,150,ADVAL,151,ASC,152,ASN,153,ATN
DATA 154,BGET,155,COS,156,COUNT,157,DEG,158,ERL,159,ERR,160,EVAL,161,EXP,162,EXT,163,FALSE
DATA 164,FN,165,GET,166,INKEY,167,INSTR(,168,INT,169,LEN,170,LN,171,LOG,172,NOT,173,OPENUP
DATA 174,OPENOUT,175,PI,176,POINT,177,POS,178,RAD,179,RND,180,SGN,181,SIN,182,SQR,183,TAN
DATA 184,TO,185,TRUE,186,USR,187,VAL,188,RND,189,CHR$,190,GET$,191,INKEY$,192,LEFT$,193,MID$(
DATA 194,RIGHT$,195,STR$,196,STRING,197,EOF,198,SUM,199,WHILE,200,CASE,201,WHEN,202,OF,203,ENDCASE
DATA 204,OTHERWISE,205,ENDIF,206,ENDWHILE,207,PTR,208,PAGE,209,TIME,210,LOMEM,211,HIMEM,212,SOUND,213,BPUT
DATA 214,CALL,215,CHAIN,216,CLEAR,217,CLOSE,218,CLG,219,CLS,220,DATA,221,DEF,222,DIM,223,DRAW
DATA 224,END,225,ENDPROC,226,ENVELOPE,227,FOR,228,GOSUB,229,GOTO,230,GCOL,231,IF,232,INPUT,233,LET
DATA 234,LOCAL,235,MODE,236,MOVE,237,NEXT,238,ON,239,VDU,240,PLOT,241,PRINT,242,PROC,243,READ
DATA 244,REM,245,REPEAT,246,REPORT,247,RESTORE,248,RETURN,249,RUN,250,STOP,251,COLOUR,252,TRACE,253,UNTIL
DATA 254,WIDTH,255,OSCLI,0

DIM lineNum AS LONG, encoded AS STRING * 1
DIM tmp AS LONG, lineNumDec AS LONG
DIM keywords(255) AS STRING
DO
    READ t%
    IF 0 = t% THEN EXIT DO
    READ keywords(t%)
LOOP
nm$ = ENVIRON$("DETOKEN")
OPEN nm$ + ".bbc" FOR BINARY AS 2
OPEN nm$ + ".bas" FOR OUTPUT AS 3
'skip first char
GET #2, , encoded
GET #2, , lineNum
SEEK #2, SEEK(2) - 2
lineNum = (lineNum AND &HFFFF&)
IF (CHR$(0) = encoded) AND (&HFFFF& = lineNum) THEN
    CLOSE : SYSTEM
ELSEIF lineNum <> 0 THEN
    PRINT #3, LTRIM$(STR$(lineNum)); " ";
END IF
DO UNTIL EOF(2)
    GET #2, , encoded
    i% = ASC(encoded)
    SELECT CASE i%
        CASE 0
            'do nothing
        CASE 13
            PRINT #3,
            'skip first char of next line
            IF NOT EOF(2) THEN
                GET #2, , encoded
                GET #2, , lineNum
                SEEK #2, SEEK(2) - 2
                lineNum = (lineNum AND &HFFFF&)
                IF (CHR$(0) = encoded) AND (&HFFFF& = lineNum) THEN
                    CLOSE : SYSTEM
                ELSEIF lineNum <> 0 THEN
                    PRINT #3, LTRIM$(STR$(lineNum)); " ";
                END IF
            END IF
        CASE &H8D
            'deal with encoded line numbers (GOTO targets)
            IF NOT EOF(2) THEN
                GET #2, , encoded
                IF NOT EOF(2) THEN
                    GET #2, , lineNum
                    SEEK #2, SEEK(2) - 2
                    lineNum = (lineNum AND &HFFFF&)
                ELSE
                    'should NEVER get here
                    lineNum = 0
                END IF
                lo& = ((ASC(encoded) * 4&) AND &HC0) XOR (lineNum AND 255&)           'LEFT SHIFT ASC(encoded), 2
                hi& = ((ASC(encoded) * 16&) AND &HC0) XOR ((lineNum \ 256&) AND 255&) 'LEFT SHIFT ASC(encoded), 4
                tmp = lo& + (hi& * 256&)
                PRINT #3, LTRIM$(RTRIM$(STR$(tmp)));
            END IF
        CASE ELSE
            IF keywords(i%) <> "" THEN
                PRINT #3, keywords(i%);
            ELSE
                PRINT #3, encoded;
            END IF
    END SELECT
LOOP
CLOSE
SYSTEM