Date   : Mon, 27 Jun 2005 16:53:11 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: BASIC IV: Coded Line Number

Angus Duggan wrote:

> Because that would make scanning for end of line and end of program
harder;
> the MSB/LSB might conflict with other tokens. If I remember correctly,
it's
> designed so that none of the following three bytes are a legal end of
> program sequence.

Yep... even though Basic lines contain a 'line length' value, which allows
the interpreter to skip to the next line very quickly, without scanning,
sometimes it is necessary.  Take the following line:

10 IF A%=0 GOTO 139 ELSE PRINT"Banana"

If A% is non-zero, the interpreter has to scan forward, looking for either:
  * an ELSE clause
  * the end of the line

Clearly if line number 139 was coded as a hi/lo pair, a bogus ELSE token
(139) would be found and it would all go wrong.  Instead, by making the
line number look a bit like a variable name using the three-byte coding
method, the problem is eliminated.

Of course, during scanning forward, it could make a special case of the 8D
token and automatically skip the next two bytes.  But the interpreter
(Basic II at least) doesn't make special cases of *anything* when scanning
the line as you can see by this example:

10 IF A%=0 PRINT"<139>Works" ELSE PRINT"Broken"
(<139> is the ASCII character 139 inserted directly into the string)

When run with A%=0, the message '<139>Works' is printed as expected.
When run with A%=1, we get the less useful 'Mistake at line 10'.
The interpreter is failing to notice that the ELSE token (139) is within
quotes and should therefore be scanned past.  This is a BUG!  (bit unfair
admittedly)

A truly evil way to obfuscate a Basic program would be to actually put
valid interpretable code inside that string after the <139>, ending it with
:<244> (the token for REM, to tell it to skip the rest of the line, or that
closing quote would be a syntax error).  I've just tried this in Basic II
and it works perfectly, much to my horror!

Try it (type it exactly as shown):

>MODE 7
>NEW
>10A$="HELLO"
>20IFA%=0 PRINT"**A$:*" ELSE PRINT"GOODBYE"
>?(PAGE+&1A)=&8B
>?(PAGE+&1B)=&F1
>?(PAGE+&1F)=&F4
>
>LIST
   10A$="HELLO"
   20IFA%=0 PRINT" qA$:t" ELSE PRINT"GOO
DBYE"
>
>A%=0:RUN
 qA$:t
>A%=1:RUN
HELLO
>

I have a feeling this was corrected in Basic IV... anyone care to try it?



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