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Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 1:07 pm
by IFW
http://www.sarnau.info/atari:protection_turrican

There are a few variants, but read the link to understand how it achieves the impossible; reading the ripped MFM clock bits using a WD177x controller. ;)
The use of the NFA is that it's impossible to either copy via software or hardware or replicate unless you have dedicated hardware able to generate the required frequency. Long story short: when you copy it via hardware an NFA area becomes weak bits, writing or generating - no chance :)
Problem: does not work with all drives.

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 2:38 pm
by DrCoolZic
IFW wrote:Problem: does not work with all drives.
Second time I see this remark :o

Is it difficult to write or to read?

What puzzles me is that NFA seems to be extremely widely used as protection on Atari. Would be surprise it can't be read on some drives ????

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 3:03 pm
by IFW
Read.

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 4:53 pm
by mr.vince
Some drives have different filtering; the NFA won't generate no flux there but other patterns. It's the reason this was abandoned by eg Rob Northen.

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 5:38 pm
by DrCoolZic
IFW wrote:Read.
Then lots of games should not work on some machines? Never heard of that
Seems to work well with all "modern drives3 I have tested would that be on old old Atari drives?

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 5:44 pm
by IFW
Atari, Commodore etc. used whichever drive was available and cheapest at any time from many different vendors. While the drives are functionally identical, edge cases like NFA that purely depends on filtering/AGC are obviously don't care... as they don't exist :)

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 6:30 pm
by DrCoolZic
Thanks. Interesting never heard that some games would not work based on floppy drives they have
I will ask on Atari forum to see if people are aware of this problem ;)

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 6:39 pm
by IFW
As the most known example for Amiga, when ESCOM took over production quite a few games and demos failed to work, because they moved to using a newer PC drive type that did not assert the READY signal...

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 7:23 pm
by mr.vince
We have had this. Some A500 owner wrote back some game like Stunt Car Racer/Amiga. It would not work. We tried to find the problem. After many checks, he found the original would not work either. A drive swap solved this.

Re: Visualizing a C64 floppydisk

Posted: Thu Feb 20, 2014 7:55 pm
by DrCoolZic
IFW wrote:As the most known example for Amiga, when ESCOM took over production quite a few games and demos failed to work, because they moved to using a newer PC drive type that did not assert the READY signal...
I have somewhere about 4 or 6 old original Atari drives + I have two very very old original STf 512 drives (the much taller ones extremely hard to find). If I have the courage I may try to use them.
But on the three machines I use for test I never had this problem.

This might be only a problem with Amiga machines ??? What some people refer to as strongbits ???

You are talking about "ESCOM taking over production" in that case isn't it writing rather than reading ?