Question about copy protection

[Konrad]

Ok, stupid question here. Didn't know where to put it.
Since I own my Amigas I always wondered how copy protection on disks work. I mean, THOSE ARE SIMPLE DISKS. While copying you read the tracks (or blocks ? cylinders ? whatever...) of the source disk and write them on the destination disk. Simple. They didn't put bad blocks on disks back then, did they ? Can't be the whole clue, as I know that there were different systems of copy protections.

So can someone please be so kind to explain it to me after all these years?

[jmmijo]

Well you kinda answered part of this yourself when you said:

"They didn't put bad blocks on disks back then, did they ?"

Well actually yes they did Some early forms of copy protection would employ bad blocks or sectors on the disk. That when read a certain way would return an error code, sometimes a special error code. If this code didn't match what the copy protection program would expect then the software would fail to load, a lot of times would give the user a Guru Meditation Error

This of course is a basic explanation as there are many more types of disk based copy protection out there then you could shake a stick at

[Konrad]

Finally things are clearer for me. Thank you for your answer.

[fiath]

Two of the most common forms of copy protection (trying to be as non-technical answer as I can):

1) Store much more data on a track so they Amiga's floppy contoller can *read* it, but cannot *write* it. Therefore you effectively get non-copyable disks.

Better forms of this protection also checked how long it took to read that track or how much was on it - if it was less than expected = copied disk.

2) Normal AmigaDOS disk store data in a certain way. You cannot just write a track of *data* - there is a lot of structure around it. So you need to say, put a "header" after a "certain position" telling me about the "data" I am going to store (checksum, size, etc) - then store the "data" itself in the "position" indicated by the "header".

Now, the Amiga's ROM stores the code to be able to read & write AmigaDOS disk format - programmers do not need to know *how* the disk is written to.

So wouldn't an ideal copy protection be to store data in a different way than the Amiga's ROM code expects? Think of an Amiga trying to read/copy Atari or PC disks - it can't - without special software that knows the format - and that is just one format!

Why not write a custom one?

This is really the most common form of copy protection - because without knowing how the data is stored - you cannot read it. So when a "cracker" removes the copy protection - what they are actually doing is looking at the games "loader" to see how the data is stored - then rewriting that loader so that the game can be stored on a format that the Amiga understands - AmigaDOS.


Hmm, perhaps (2) was not such a great answer. Basically - there is a lot more stuff on a floppy disk than the "user" data - the data that you actually want to store. You have to also inform whatever reads that "user data" details about that "user data".

Okay - tell me if you understood that...

[Konrad]

Yes, I DID understand , although the first variant surprised me. I didn't know that the floppy controller could read more tracks than write.

[fiath]

Cool

But not "more tracks" - more *data* on *a* track. This form of copy protection is called "longs tracks".

You can have a look at the "density" variations here. (look at the graphs)


Another interesting one:

3) Unformatted disks are completely *noise* (random) data. Some protections use this by effectively having a noise track somewhere on the disk. The test usually says read track X a number times if each of the reads yields different data then the game is genuine - otherwise it is a copy (if the data is the same).

[Konrad]

Why does it give random data while reading ? Is this because of the magnetic system of a disk, so there is nothing sorted ? Don't know how to descibe it better so don't laugh :P

I've also had CDs with an empty track on it. You could see it on the CD if you looked at the back, but couldn't read it. Is this variant similiar ? A CD won't give random data, would it ?

[fiath]

I don't know much about CD's so I wouldn't really know. However CD's data is clearly defined. A bit is either 0 or 1. So I would not think it would have a noise "state". At least - nothing like magnetic media...

Noise:

Basically when a disk has not been formatted, all the magnetic particles are in a transient state, i.e. they do not really form a clear 0 or 1.

When you try to read the disk in this state, the floppy logic gets "confused" and gives a best guess estimate on whether a bit read was either 0 or 1. Because of the nature of the magnetic media - this is not a "best guess" at all - it is a completely random guess.

Multiple reads will yield different results - because the state of the bits on the disk is not definable by the floppies circuitry.

When your format a disk, these particles are pulled into order so clear 0's and 1's can be read as normal.


Interesting aside:

When a disk gets old, the magnetic particles lose their "charge" and hence revert back to this "noise" state. This is more commonly called "bit-rot" and it mainly cause by the natural magneic fields in the environment - unless you leave your disks next to a speaker of course, then I guess it could be called "accelerated bit-rot"...

[Konrad]

Hmmm...better check all of my 400 disks. Damn speakers :laugh. But I know you're right, although it takes a long time till the disks take any damage if you take care of them. Thanks for your explanations. Now I got alot of detailed info.

P.S. You're probably right with the cds. I suspected the same.

[fiath]

A long time? Does 15 years count? That is how old quite a few of my collection are!

TDK gives disks 5-10 years reliable life - however I think they are figuring disks will be written to as well as read - which does not really apply here.

But your right, most disks will probably last for years to come... However, we have see a worrying number of bad ones at CAPS so far...

[Mozzy]

only now does the value of TAPES (good ol c64, bless its little cotton socks) become clear agrred they suffer from bit rot too but they last a helluva lot longer.