ROM checksum location? Calculation?

[Calab]

I have a U1MB installed in my 1200XL. I'm making some modifications to my XL OS 1.3 ROM. The Atari boots to the diagnostic test screen because of the changes that I made.

Does anyone know where the checksum location is stored in the OS ROM? Is it simply a sum of all the bytes in the OS ROM? If so, is it 8bit? 16bit?

 

Any help is appreciated!

[tmp]

 

[Calab]

I was able to work it out...

 

$C000, $C001 holds the 16 bit checksum for $C002-$DFFF

$FFF8, $CFFF9 holds the 16 bit checksum for $E000-$FFF7 and $FFFA-$FFFF

[+mytek]

Here's a great little Windows program that will check and correct the OS Rom checksum for you, created by @HiassofT.

 

Hias PatchROM-1.33.zip

 

It'll also patch your OS Rom with High Speed SIO routines if desired. I included a pdf that gives a brief description of how to use it. Visit HiassofT's Atari 8bit world for even more information.

 

[Calab]

8 hours ago, Calab said:

I was able to work it out...

 

$C000, $C001 holds the 16 bit checksum for $C002-$DFFF

$FFF8, $CFFF9 holds the 16 bit checksum for $E000-$FFF7 and $FFFA-$FFFF

Correction...

$FFF8, $FFF9 holds the 16 bit checksum for $E000-$FFF7 and $FFFA-$FFFF