Re: [Info-mtools] What does it take to support FAT12 dialects?

[Pali Rohár]

On Wednesday 19 May 2021 12:13:59 Alexander Huemer wrote:
> Hi
> 
> I own an IBM 3174 controller. For those unfamiliar with the device, it's 
> a machine that uses one or two 5.25" floppy drives to load software into 
> it. These drives are either of the common 1.2M variant or a pretty 
> uncommon 2.4M variant.
> In the very small community of people who operate such machines today, 
> several people have replaced the floppy drives with GoTek emulators, to 
> make it easier to interchange data with modern computers via USB flash 
> drives.
> The disk image files coming from an 3174 are recognized by minfo with 
> correct results as far as I can tell, though e.g. mdir cannot list the 
> content, see output below. Hence i figure that the image format is not 
> an issue, but the filesystem layout.
> Somebody (not me) had a look at the format, here is what they found:
> 
> > Has anyone else tried making sense of the filesystem present on the 
> > disk images here?
> > I'be spent a couple days looking at it now and and it's very clearly 
> > based on FAT12. There's only a couple of things that have thrown me 
> > off. First, the number of reserved sectors at offset 0xE is 32, even 
> > though I can see the FAT starting at the second sector. Second, on 2.4 
> > Mb disks I'm convinced the first cluster starts at 0x3C00 as opposed 
> > to right after the root directory which is at 0x3A00. Part of the 
> > reason I think this is because the file here is usually VOLUME.LAB 
> > which is mostly EBCDIC text and only 512 bytes long. So starting at 
> > 0x3A00 wouldn't include this. Starting at 0x3C00, makes the volume.lab 
> > files consistent across all the images I've looked at, 1.2 and 2.4 Mb.  
> > My best guess is that the clusters are aligned to start at offsets 
> > which are multiples of the cluster size, which is two 512 byte 
> > sectors. Perhaps this is expected behavior? I will confess I did not 
> > know any implementation details of FAT12 before a couple days ago, but 
> > I have not seen this phenomenon mentioned anywhere.
> 
> Assuming these findings are correct, I wonder what would be involved in 
> making mtools support this dialect of FAT12.
> I did a quick grep in the source code but couldn't find the offsets that 
> are mentioned in the quote above.
> 
> Any suggestions how to approach adding support for this dialect would be 
> appreciated.

Hello! Is linux kernel able to mount image and access content of this
filesystem? Can you provide example of such image? I can look at it...

> > $ minfo -i 12.img
> > device information:
> > ===================
> > filename="12.img"
> > sectors per track: 15
> > heads: 2
> > cylinders: 80
> > 
> > media byte: f9
> > 
> > mformat command line: mformat -t 80 -h 2 -s 15 -i "12.img" ::
> > 
> > bootsector information
> > ======================
> > banner:"N3174KGN"
> > sector size: 512 bytes
> > cluster size: 1 sectors
> > reserved (boot) sectors: 1
> > fats: 2
> > max available root directory slots: 224
> > small size: 2400 sectors
> > media descriptor byte: 0xf9
> > sectors per fat: 7
> > sectors per track: 15
> > heads: 2
> > hidden sectors: -2071789568
> > big size: 20132486 sectors
> 
> > $ minfo -i 24.img
> > device information:
> > ===================
> > filename="24.img"
> > sectors per track: 30
> > heads: 2
> > cylinders: 80
> > 
> > media byte: f0
> > 
> > mformat command line: mformat -T 4800 -h 2 -s 30 -H 1710948352 -i "24.img" 
> > ::
> > 
> > bootsector information
> > ======================
> > banner:"N3174KGN"
> > sector size: 512 bytes
> > cluster size: 2 sectors
> > reserved (boot) sectors: 32
> > fats: 2
> > max available root directory slots: 224
> > small size: 4800 sectors
> > media descriptor byte: 0xf0
> > sectors per fat: 7
> > sectors per track: 30
> > heads: 2
> > hidden sectors: 1710948352
> > big size: 21172290 sectors
> 
> > $ mdir -i 12.img
> > Volume in drive : has no label
> > Directory for ::/
> > 
> > No files
> >                             469 504 bytes free
> > 
> 
> > $ mdir -i 24.img
> > Cluster # at 36 too big(0xce9)
> > Probably non MS-DOS disk
> > Cannot initialize '::'
> > $ 
> 
> Thanks,
> -Alex
> 

-- 
Pali Rohár
pali.rohar@gmail.com