APPENDIX TWELVE



3889           F31            DOS 3
     If you PEEK here and get 76 ($4C), you have an early version of
     DOS 3 (the later version will read 78). To correct some errors in
     the earlier FMS files, type this in:

     10 FOR N=1 TO 9:READ A,B:POKE A,B:NEXT N
     20 DATA 3889,78,3923,78,3943,78,3929,76,389
        5,76
     30 DATA 3901,77,3935,77,3955,77,2117,240

     Better yet, get DOS 2.5 from Atari (supports double-density and
     the 130XE RAMdisk). DOS 3.0 saves in blocks, not sectors--of a
     minimum 1000 bytes per block. If you write a program 1001
     bytes long, it saves 2000 bytes, wasting 999 bytes on your disk.

20480-22527    $5000-$57FF    Self-test ROM
     Self-test ROM when enabled, controlled by bit 7 of PORTB
     (54017; $D301). The self-test code is in a special ROM area
     underneath the GTIA, POKEY, ANTIC chips area (starting at
     53248; $D3000) and is moved (remapped) here when you type
     BYE in BASIC or when you POKE PORTB with the right value and
     JMP (or USR) to the initialization vector (see 58481; $E471 and
     58496-58499, $E480-$E483). RAM when self-test isn't enabled.

39967-40959    9C1F-9FFF      ....
     Display list and screen RAM, moved into lower memory if a
     cartridge is 16K (using RAM from 32767 to 49151 as well).

43234          A8E2           BASIC ROM
     If you PEEK here and get 96 ($60), you have the BASIC Revision
     B ROMs. What you need is Revision C. B stands for Bugs! See
     Appendix 13 on enhancements and bugs. If you get 234 ($EA),
     you have Revision C. From Matt Ratcliff.

     You can turn BASIC off when you go to DOS by POKEing 1016
     ($3F8), then pressing RESET. The problem is to turn it back on
     again from DOS rather than rebooting the system. There is a
     public domain program by Matt Ratcliff on the Gateway BBS
     which does this for you.

Introduction to the OS ROM

Atari modified the new XL/XE ROMs since Revision B. Atari main-
tained the handler and interrupt vectors, although the routines they
point to changed between versions.

Atari did produce a listed source code for the XL OS, although for
some reason it was never published for public sale as it was in-
tended. It may be available now through Atari--write and ask for it.
It is an excellent 500+ page resource document.