Modification of XL-(OS ROM) triggered Memory Test

[Madi]

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

[+bob1200xl]

As you discovered, the OS does a ROM checksum. You can disable the checksum by storing $EA into $C31D-$C31E.

 

Bob

[+David_P]

Or you can calculate the checksum for your modified ROM and put it into the ROM.

[Madi]

@ 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