Help with preservation of arcade system

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visitor
Posts: 7
Joined: Sat Aug 13, 2016 8:21 pm

Help with preservation of arcade system

Post by visitor » Wed Aug 17, 2016 3:04 pm

Dear forum, I am currently working on a project to document and preserve the only known unit of an 80s arcade games platform. The ultimate goal is to avoid it form getting lost and to bring emulation into Mame.

One of the rarities about this platform is the fact that it uses DD 3.5" floppies rather than the more common eprom memories found in most arcade systems of the time. Each floppy contains a full game which gets loaded during boot time and further accessed as needed during gameplay and/or demo.

The system uses a drive manufactured by Mitsumi (model D357), and a SAB 2797B (FM - MFM type) chip disk controller, datasheet: http://cygnus.speccy.cz/download/datasheety/sab2793.pdf

Using Kryoflux I have been able to dump the floppies to raw without any errors, but so far haven't able to dump an MFM readable image. I suspect the original developers of the platform somehow altered the format to avoid copies from happening.

I have next to no clue on magnetic media so i'd like to request your collaboration and help me out preserve this system. The following link contains a raw dump done using the original drive in the system attached to Kryoflux: https://drive.google.com/open?id=0B6ZdY ... nFkUkpDOHM

Any feedback, clues or help is welcome, I will credit on the project.

Thanks a lot for reading. Eduardo
http://arcadehacker.blogspot.com

Feltzkrone
Posts: 255
Joined: Sat Jun 18, 2011 8:03 pm

Re: Help with preservation of arcade system

Post by Feltzkrone » Wed Aug 17, 2016 11:44 pm

The sectors can be decoded but there's still something odd with the checksums and/or the track format which needs further investigation. Until this has been done it's unclear if any of the sectors are really erroneous or not as all sectors are found to be bad.

There are 6 sectors of 512 bytes per track (-z2 -n6), track distance is 80 (-k1) and for whatever reason the target rpm is 240 (-v240) instead of 300 or 360. Using these parameters you should be able to decode the disk and take a look at the data using some hex editor.

visitor
Posts: 7
Joined: Sat Aug 13, 2016 8:21 pm

Re: Help with preservation of arcade system

Post by visitor » Thu Aug 18, 2016 1:06 am

Hi, thanks for taking a look!, tried the following but i get MFM unformatted messages on all tracks: dtc -m1 -ftimescannerMAGNET00.0.raw -i0 -ftest.bin -i4 -z2 -n6 -k1 -v240

Am I doing something wrong?

ZrX
Posts: 530
Joined: Tue Dec 06, 2011 9:09 pm

Re: Help with preservation of arcade system

Post by ZrX » Thu Aug 18, 2016 5:10 am

The order of parameters is important:

dtc -m1 -ftimescannerMAGNET00.0.raw -i0 -ftest.bin -v240 -z2 -n6 -k1 -i4

visitor
Posts: 7
Joined: Sat Aug 13, 2016 8:21 pm

Re: Help with preservation of arcade system

Post by visitor » Thu Aug 18, 2016 7:42 am

That worked for me now, thanks!

ZrX
Posts: 530
Joined: Tue Dec 06, 2011 9:09 pm

Re: Help with preservation of arcade system

Post by ZrX » Thu Aug 18, 2016 10:45 am

Hard to find much info about that system. Seems to use standard diskdrive but the FDC has some configuration possibilities.

ZrX
Posts: 530
Joined: Tue Dec 06, 2011 9:09 pm

Re: Help with preservation of arcade system

Post by ZrX » Thu Aug 18, 2016 4:22 pm

The sectorsize is actually 1024. Headers report it as 512. Sector numbering also counts from 200. All likely to prevent copying.
timescannerMAGNET.png

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IFW
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Re: Help with preservation of arcade system

Post by IFW » Thu Aug 18, 2016 4:38 pm

Ok, so I've reverse-engineered the format ;)

The disk is very well copy-protected.

It is written at 240RPM which increases the capacity by 25% compared to a standard disk.
The track contents do not fit a normal capacity disk, at least not with gaps used, however there is quite some extra capacity on each track which seems to be unused - I've only checked a random track for anything written in the unused area (other than default formatting gap value), so this may or may not be accurate.

Each track has 6 sectors, and each sector is 1024 bytes long.
The sector headers are marked as if the sector length was 512 bytes - that's why you get the checksum errors. This also effectively prevents copying the second half of the data in each sector unless you know the real sector size. (plus have a drive running at 240 RPM...)

Sectors are numbered from 200...205.

Only tracks 0...56 are formatted, then you see many unformatted tracks, then tracks 77...79 are formatted again.

The drive that was used the dumps the disk was clearly not capable to read more than 80 tracks (tracks repeat from 80), so it would be advisable to redump the disk with a drive capable of reading 84 tracks.
Given that track content is format default from 77, I don't think there is anything there from 80, but better be safe than sorry :)

The only exception is track 78, side 0, sector 201 which was rewritten with a genuine 512 bytes sector - in place of the 1024 byte sector, so you can see the remnants of that after the 512 bytes.
It's either:
- protection data
- high scores
- someone accidentally overwriting the sector.

Given it's an arcade machine my guess would be high scores, or protection.

Track 79, side 1, sector 205 is also 512 bytes, but it only has default formatting values.

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IFW
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Re: Help with preservation of arcade system

Post by IFW » Thu Aug 18, 2016 4:39 pm

Some real data :)
Attachments
timescanner_decoded.png

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IFW
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Re: Help with preservation of arcade system

Post by IFW » Thu Aug 18, 2016 4:52 pm

...and here is the decoded sector image ;)
Keep in mind what I wrote about the sectors that are actually really 512 bytes above, they are easy to see towards the end of the disk image.
All sectors are error free for the first formatted half of the disk, the 512 byte long sectors (embedded in place of 1024 byte sectors) may or may not be correct - but probably they are.
Attachments
timescanner.zip
(165.37 KiB) Downloaded 85 times

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