6 replies to this topic

[Marius1976]

How is this possible:

I have a floppy disk here. It is a copy protected floppy.

It is medium density, and sector $03DE (990) is filled with #$FF. Well... that is the FIRST time you read that sector.

When you RE-READ the sector it reads all #$00!

When you RE-READ it one more time the sector reads sometimes #$FF and sometimes #$FF.

(I mean the sector is filled with 128x the same value; all are #$00 or all are #$FF)

My questions are:

1) Is it possible to reproduce this behavior on a .ATR image?

2) How in the world does this work? Is there something funny going on here?

3) How can I create such a disk myself?

Thanks
M.

[Rybags]

The old "duplicate sector" trick.

Fairly simple - put 2 or more sectors with the same ID in the header, the drive will pick up one or the other depending on what sector was accessed previously.

Such schemes would need a Happy board or similar. ATR doesn't support it, I'd say the protected image formats might.

[Marius1976]

Rybags, on Fri Feb 3, 2012 9:55 AM, said:

The old "duplicate sector" trick.

Fairly simple - put 2 or more sectors with the same ID in the header, the drive will pick up one or the other depending on what sector was accessed previously.

Ehm... I don't understand completely yet. Where is this header and what is the ID of a sector? Isn't this simply hard-coded in the firmware of the drive?

When I want to access sector $03DE and I instruct my drive to read that sector, where does it get that "ID"?
And even when I give 5 times the SAME instruction:

read $03de
read $03de
read $03de
read $03de
read $03de

the results are not all the 5 times the same.

Thanks for your assistance!

[Marius1976]

ah... I found this:

http://www.scribd.co...tion-Techniques

That describes what you suggested. Hmmm. Interesting

[Rybags]

ID as in part of the header data the disk controller needs to find sectors on a track.

Hardcoded ? Kind of. A factory drive will create a cookie-cutter type of layout where each track's layout of sectors is consistent and can't be altered by the user.

A modded drive can let you do fancy stuff like duplicate sector IDs, missing sectors, CRC errors etc etc.

I forget which book, might be one of Sam's computer facts, but the track layout is documented. For each 128 byte sector, there's actually a fair bit more data like sync marks, headers and the like. Datasheets for the popular disk controller chips usually have some description too.

[Mathy]

Hello Marius

You should also try to find "Advanced Atari Protection Techniques". It's the second book of the (two part?) series. (BTW I bought these books from Alpha Systems even before I had a disk drive. And NO that are NOT for sale.)

Your "changing sector" could also be a fuzzy sector. A duplicate sector is like two identical twins that take advantage of the fact that you don't know that they are not one but two individuals. But a fuzzy sector is a sector that's partly unformatted. But in such a way, that the drives thinks, it's a normal sector. (BTW I had to look this up, the info indeed is in AAPT (the second book) and not in ASPT (which is mentioned above).)

Mathy

[Stephen]

Mathy, on Fri Feb 3, 2012 4:20 PM, said:

Hello Marius

You should also try to find "Advanced Atari Protection Techniques". It's the second book of the (two part?) series. (BTW I bought these books from Alpha Systems even before I had a disk drive. And NO that are NOT for sale.)

Your "changing sector" could also be a fuzzy sector. A duplicate sector is like two identical twins that take advantage of the fact that you don't know that they are not one but two individuals. But a fuzzy sector is a sector that's partly unformatted. But in such a way, that the drives thinks, it's a normal sector. (BTW I had to look this up, the info indeed is in AAPT (the second book) and not in ASPT (which is mentioned above).)

Mathy

Those are great books. I met the author at a local computer show, and I had him sign both of mine. He was incredibly shocked to see them on display.