Subj : Re: Something strange going on.
To   : Shaun Bebbington
From : ArcadeAge
Date : Wed Apr 11 2018 11:55:06

On Wednesday, April 11, 2018 at 12:33:17 PM UTC+2, Shaun Bebbington wrote:
> Hi,
>
> I found something odd today in the powerful CBM BASIC v2.0.
>
> Having worked with Sinclair BASIC quite a lot recently, I use GOTO X where X 
is a valid line number (i.e., LET X = 100). But I know that's not possible with 
Microsoft BASIC. Of course there is the ON X GO TO 100,200 ...
>
> So I thought "I know, I'll try and use a function, could CBM BASIC use this 
as a look up table". The following, I expected to go to the value in the 
function A() which returns 10, but...
>
> 0 DEF FN A(X) = 10
> 1 PRINT FN A(0)
> 2 GO TO FN A(0)
> 3 PRINT "END GRACEFULLY"
> 4 END
> 10 PRINT "STOP"
> 11 STOP
>
> Try it for yourself. What's going on here?
>
> Thanks in advance.
>
> Shaun.

I went the "hard" way and traced what happens using VICE's monitor's 'z' 
command. I recommend that you do the same, consulting a commented BASIC ROM 
listing if necessary.
But there's a much easier way to get the idea: Let your programs start at line 
1 instead of 0.

Chris

Edit: The above should read "Let your programs start _with_ line number 1".
Here's a short version of the "hard" way:
- Start x64 from a console window (otherwise breakpoints might not work)
- Enter a one-liner "0 goto 0"
- RUN the one-liner
- Enter VICE's monitor (Ctrl-h)
- Enter 'bk a971' in VICE's monitor
- Enter 'x' in VICE's monitor
- Not(ic)e that the C-flag is set but every other time the breakpoint is 
reached

Now repeat the above procedure, but let your one-liner be "0 goto a" instead 
this time. Note that the C-flag is set _all_ the time.

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