Madi Posted August 25, 2014 Share Posted August 25, 2014 (edited) I tried do modify the 600XL-800XL (OS ROM) with a disk editor to make it compatible with MyDOS - Ultra-speed utility. I modified the area CC00 - CE98 which is allocated for the International character set. I tested the modified ROM using Altirra. I got the following bad ROM image and never pass it : Are the Atari's XL OS ROMS tested for a checksum identification upon run? or only for some parts/locations of the OS ? Why does copying ROM to ram and then saving them back as ROM succeed although they may be altered ( e.g.The Translator)? madi Edited August 25, 2014 by Madi Quote Link to comment Share on other sites More sharing options...
+bob1200xl Posted August 25, 2014 Share Posted August 25, 2014 As you discovered, the OS does a ROM checksum. You can disable the checksum by storing $EA into $C31D-$C31E. Bob 2 Quote Link to comment Share on other sites More sharing options...
+David_P Posted August 25, 2014 Share Posted August 25, 2014 Or you can calculate the checksum for your modified ROM and put it into the ROM. Quote Link to comment Share on other sites More sharing options...
Madi Posted August 25, 2014 Author Share Posted August 25, 2014 (edited) @ bob1200xl @ David_P Thank you for the clarification. I will "EA" those 2 bytes and continue with my hacking project .. Edited August 25, 2014 by Madi Quote Link to comment Share on other sites More sharing options...
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