SEGA CD Temporary offset hex /ISO file patching?

[Former user]

Original issue 385 created by 47iscool on 2014-06-27T06:09:49.000Z:

So how about it?

You know just like the Game Genie does to Genesis/MD games. I know it would never work on a real one but can you consider adding it to this already great port?

Example:

One of Tony Hedstrom's codes/hacks:

Changing the value at 0x022E2A2 in the ISO (binary mode2/2352 ISO) from:
33FC000000FF1512
to
6006000000FF1512

on Sonic CD (US version) allows Sonic not to lose any rings when he's hit.

I have tried this before (022E2A2:6006) on the emulator before but it doesn't work. I was thinking since PAR codes work maybe this would/could too.

[Former user]

Comment # 1 originally posted by ekeeke31 on 2014-06-27T10:45:36.000Z:

This is completely different than ROM patching: ISOs are accessed by sectors (2048 bytes), a new sector being read every 1/75s so temporary patching the wole ISO is not possible, the file being read only partially when required. It would require looking for the sector address vs the address to patch everytime you read a new sectors, which would be very unefficient (even with patching disabled, the code would still need to check at every sectors if patching is enabled or not).

ROM patching won't work since there is no ROM and the memory map with Mega CD is completely different than with a normal cartridge so it makes absolutely no sense trying ROM patches like you did.

RAM patches are still working but will only affect the Mega Drive side RAM.

[Former user]

Comment # 2 originally posted by 47iscool on 2014-06-27T17:08:43.000Z:

Okay. Just wondering.