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Subject: Re: Using Applesoft Internal Error Messages in a BASIC Error Handler
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From: Michael J. Mahon <mjmahon@aol.com>
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fadden <fadden@fadden.com> wrote:
> The error code, passed in the X register, is just the index of the start
> of the error text (table at $d260).  The error routine prints "?", then
> the error text, then "ERROR" and a bell (from $d350).  If the code was
> running, it prints the line number, and does the Applesoft error handling.
> 
> So for example, SYNTAX ERROR is error code 16.  $d260 + 16 = $d270, which
> is the string "SYNTAX".  The last letter has the high bit set (Dextral
> Character Inverted format, or DCI), which is how the code knows when to
> stop printing characters.
> 
> Try this:
> 
>  100 BASE = 53856: REM $D260
>  110 ERRCODE = 16: GOSUB 1000: REM SYNTAX ERROR
>  120 ERRCODE = 53: GOSUB 1000: REM ILLEGAL QUANTITY
>  130  END 
>  1000 PTR = BASE + ERRCODE
>  1010 L =  PEEK (PTR): IF L >  = 128 THEN  PRINT  CHR$ (L - 128);: PRINT " ERROR": RETURN 
>  1020  PRINT  CHR$ (L);:PTR = PTR + 1: GOTO 1010
> 

When writing the NadaNet ampersand extension for Applesoft, I needed a
“standard” way to signal a data packet error. Since there is no easy way to
extend the error message table, I chose to piggyback on an existing message
by choosing an error number that points to the last word in the “OUT OF
DATA” error, creating a “DATA” error message for NadaNet communication
errors. ;-)

-- 
-michael - NadaNet 3.1 and AppleCrate II:  http://michaeljmahon.com