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

[Alexander Huemer]

On Thu, May 20, 2021 at 12:32:20AM +0200, Alain Knaff wrote:
> Hi,
> 
> Found it.
> 
> There are actually 2 different reasons for the 1.2M and the 2.4M image.
> 
> 1. For the 1.2M image, everything is fine, but the directory entries all
> have the DOS "hidden" attribute set, and hence are not shown.
> 
> To show them, use the -a flag to mdir:
> 
> > mdir  -i ~/tmp/12.img  -a ::
>  Volume in drive : has no label
> Directory for ::/
> 
> VOLUME   LAB       512 1991-11-22  14:26
> MAINTRK  DIA     15360 1991-11-22  14:26
> MRNLEV   ECN       512 1991-11-22  14:26
> 013179   DSL    390144 1991-11-22  14:27
> 003290   DSL    180224 1980-01-01   0:00
> 023193   DSL    157696 1980-00-00   0:00
>         6 files             744 448 bytes
>                             469 504 bytes free
> 
> 
> 2. For the 2.4M image, there is an actual inconsistency. The number of
> "reserved" sectors (boot sectors) is set to 32, but actually only one
> boot sector is used. Hence everything is shifted by 31 sectors.
> 
> Hardwiring this to 1 in init.c results in a readable directory:
> 
> > ./mdir  -i ~/tmp/24.img  -a ::
>  Volume in drive : has no label
> Directory for ::/
> 
> VOLUME   LAB       512 1980-00-00   0:00
> PLOM     IML    179200 1980-00-00   0:00
> CTLDMC0  IML    110080 1980-00-00   0:00
> USCNI    IML     74638 1980-00-00   0:00
> USCNI2   IML    197538 1980-00-00   0:00
> CONTROL  FIL       181 1980-00-00   0:00
> PIT      TBL     64892 1980-00-00   0:00
> HOST     PUR      6058 1980-00-00   0:00
> SAMOA    PUR     27606 1980-00-00   0:00
> SAMOAA   PUR     26776 1980-00-00   0:00
> APPNODE  PUR     28023 1980-00-00   0:00
> LOC      PUR       128 1980-00-00   0:00
> FILTR    PUR      2937 1980-00-00   0:00
> WAN      PUR       564 1980-00-00   0:00
> APPNCOS  PUR      6524 1980-00-00   0:00
> DVD      PUR      6943 1980-00-00   0:00
> PAST     PUR     22712 1980-00-00   0:00
> AEASTATN PUR      4474 1980-00-00   0:00
> AEAOTHER PUR     19533 1980-00-00   0:00
> TCPIP    PUR      1330 1980-00-00   0:00
> CTLPAT   PUR       480 1980-00-00   0:00
> RPQ      PUR       220 1980-00-00   0:00
> SSDACONF OPR     13312 1980-00-00   0:00
> CDID     DIA      1920 1980-00-00   0:00
> COPYCTL  DUM       728 1980-00-00   0:00
> MODKEYBD KDU     18432 1980-00-00   0:00
> RASDATA  OPR     30720 1980-00-00   0:00
> RASDATAM OPR     30720 1980-00-00   0:00
> LINKMAP  IML      5120 1980-00-00   0:00
> LINKMA51 IML      4096 1980-00-00   0:00
> LINKMA52 IML      4096 1980-00-00   0:00
> SAFSTOR  STA       512 1980-00-00   0:00
> MAINTEC  MEC       662 1980-00-00   0:00
> PATCH001 CUT      6144 1980-00-00   0:00
> LEOTF    IML   1048576 1980-00-00   0:00
> TRSETUP  OPR      2168 1980-00-00   0:00
> VPDDATA  FIL      5874 1980-00-00   0:00
> UDDDATA  FIL     13900 1980-00-00   0:00
> SIOA     AML     47616 1980-00-00   0:00
> CSCMBSL  IML      1024 1980-00-00   0:00
> FCSCMBSL IML       512 1980-00-00   0:00
> MSFPID   DAT       345 1980-00-00   0:00
> PCB38E3  PAT       512 1980-00-00   0:00
>        43 files           2 018 338 bytes
>                             407 552 bytes free
> 
> In the next few days, I'll make a release that adds a flag to optionally
> ignore the number of boot sectors, and always use 1

Amazing! Thanks for taking the time to work on this Alain.

I am relaying a message from Chris, who did the initial investigation.  
He has issues subscribing to the list.

> I'm skeptical that the reserved sectors is the only difference with 
> 2.4 MB media. For context, the floppy drive spins at a slower speed 
> after the first cylinder in order to fit 30 sectors per track. The 
> first cylinder spins at the normal speed and has 15 sectors per track.
> When inspecting the images you can see that the first file is always 
> VOLUME.LAB (and usually resides in the first cluster). These files are 
> very distinct in their formatting. For example, they always start off 
> with three letters that match the purpose of the disk (CTL, DSL, UTL), 
> followed later by EBCIDIC text which matches the print label on the 
> disk itself.
> I can see in the image that this file starts at offset 0x3C00, which 
> is one sector after the expected location. This offset also 
> corresponds to the start of the second cylinder, and I believe this is 
> where the data area actually begins. If the data area did actually 
> start in the typical place then a single sector would be left to spin 
> slower.
>
> Chris

Maybe this is worth a bit further analysis?
Please let me know if I can help.

-Alex