Re: BASIC listing..

Hello,

* On Mon, Jun 10, 2013 at 07:36:00PM +0200 silverdr@wfmh.org.pl wrote:
> 
> On 2013-06-10, at 19:27, Spiro Trikaliotis wrote:
> 
> >> instead of pointing to 0x0000 _after_ the last EOL, why not point to
> >> the last byte of the last line?
> > [...]
> > Even on the PET (which does *not* relink the BASIC program), this would
> > not work, though. The interpreter intself does not care about the link
> > bytes (as long as no GOTO/GOSUB is involved), thus, you need the link
> > pointers HIGH BYTE==0 in order to end the program - or you MUST add an
> > "END" statement.
> 
> Pointing to the last byte of the last line would do, wouldn't it? If
> the PRG wasn't relinked, that is.. Why wouldn't it work if the
> pointers were not rewritten by LINKPRG? HI byte of the last line's
> next line pointer would be \0.

It would work on LISTing, but not on execution (at least, with the code
of VIC20/C64 BASIC2).

Take something like this:

0800 00 0C 08 0A 00 aa bb cc dd ee ff gg hh 00 xx yy
                                         /\
                                        080C

with aa .. hh being some BASIC command bytes that are not of interest
here.

On listing, BASIC outputs the line number ($000A = 10) and then the
command as specified by aa .. hh, until it reaches the $00 in $080D.
Then, it goes back to $0801, reads the link pointer (to $080C), checks
the high byte at $080D, and ends the listing.

On execution, things are different. The interpreter interprets aa .. hh,
whatever they mean. Then, it sees the $00 as end of the line.

Next, it examines the following link pointer (xx, yy). For the
interpreter to step here, yy must be $00 - otherwise, it will happily
interpret the rest of the memory.

Regards
Spiro

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