Hard (unstoppable) reset uses both RESET and EXROM. No software can override it. There is a schematic on C64 Wiki. it's using only a switch, 2 resistors, 2 capacitors and a diode. Tried it in my C64C, works great.
Thanks for the tip! It says that the reset routine in kernal checks for cartridge at $8000, and if CBM80 is written at $8000 in RAM, the kernal thinks there is a cartridge..
There is a magic 5-char sequence (off the top of my head I believe it is something like 'CBM80') that can be placed in RAM, along with a custom reset vector, at address $8000 to bypass reset to a user-supplied function. This is used by cartridges. If you pull down EXROM and don't have any cartridge plugged in that memory is then disabled during reset, so the custom reset code does not get called. The problem here is that you need to bring EXROM up pretty soon after the reset, otherwise the C64 will find a big piece of RAM missing.
Thanks! That makes sense. I just saw the kernal's reset routine checking for cartridge (CBM80).. I release EXROM after 100ms and it seems to work fine. not sure if subsequent resets will work CBM80 will still be in memory...?
The memory would be unaffected, it is just banked out. I believe the Ultimate II+ uses a different trick at reset where it actually changes the CBM80 signature in RAM.
Hard (unstoppable) reset uses both RESET and EXROM. No software can override it. There is a schematic on C64 Wiki. it's using only a switch, 2 resistors, 2 capacitors and a diode. Tried it in my C64C, works great.
Thanks for the tip! It says that the reset routine in kernal checks for cartridge at $8000, and if CBM80 is written at $8000 in RAM, the kernal thinks there is a cartridge..
There is a magic 5-char sequence (off the top of my head I believe it is something like 'CBM80') that can be placed in RAM, along with a custom reset vector, at address $8000 to bypass reset to a user-supplied function. This is used by cartridges. If you pull down EXROM and don't have any cartridge plugged in that memory is then disabled during reset, so the custom reset code does not get called. The problem here is that you need to bring EXROM up pretty soon after the reset, otherwise the C64 will find a big piece of RAM missing.
Thanks! That makes sense. I just saw the kernal's reset routine checking for cartridge (CBM80).. I release EXROM after 100ms and it seems to work fine. not sure if subsequent resets will work CBM80 will still be in memory...?
The memory would be unaffected, it is just banked out. I believe the Ultimate II+ uses a different trick at reset where it actually changes the CBM80 signature in RAM.
Multiple Kernals on one C64 - it's still quite amazing! :)
It is very useful to have :) different turbo loaders for different usecases. :) Also turbotape, allthough I don't use tape that much.