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Newsgroups: comp.sys.apple2.programmer
Date: Wed, 29 Nov 2023 22:23:56 -0800 (PST)
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Subject: Re: Using Applesoft Internal Error Messages in a BASIC Error Handler
From: fadden <fadden@fadden.com>
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Xref: csiph.com comp.sys.apple2.programmer:6126

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 e=
rror 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 =3D $d270, which=
 is the string "SYNTAX".  The last letter has the high bit set (Dextral Cha=
racter Inverted format, or DCI), which is how the code knows when to stop p=
rinting characters.

Try this:

 100 BASE =3D 53856: REM $D260
 110 ERRCODE =3D 16: GOSUB 1000: REM SYNTAX ERROR
 120 ERRCODE =3D 53: GOSUB 1000: REM ILLEGAL QUANTITY
 130  END=20
 1000 PTR =3D BASE + ERRCODE
 1010 L =3D  PEEK (PTR): IF L >  =3D 128 THEN  PRINT  CHR$ (L - 128);: PRIN=
T " ERROR": RETURN=20
 1020  PRINT  CHR$ (L);:PTR =3D PTR + 1: GOTO 1010